- In situelectrosynthesis of anthraquinone electrolytes in aqueous flow batteries
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We demonstrate the electrochemical oxidation of an anthracene derivative to a redox-active anthraquinone at room temperature in a flow cell without the use of hazardous oxidants or noble metal catalysts. The anthraquinone, generatedin situ, was used as the active species in a flow battery electrolyte without further modification or purification. This potentially scalable, safe, green, and economical electrosynthetic method is also applied to another anthracene-based derivative and may be extended to other redox-active aromatics.
- Aziz, Michael J.,Fell, Eric M.,Gordon, Roy G.,Jin, Shijian,Jing, Yan,Kerr, Emily F.,Pollack, Daniel A.,Wong, Andrew A.,Wu, Min
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- Organocatalytic oxidative dehydrogenation of dihydroarenes by dioxygen using 2,3-dichloro-5,6-dicyano-benzoquinone (DDQ) and NaNO2
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The oxidative dehydrogenation of dihydroarenes catalyzed by 2,3-dichloro-5,6-dicyano-benzoquinone(DDQ) and NaNO2 with dioxygen is reported. The combination of DDQ and NaNO2 showed high efficiency and high selectivity, compared with other benzoquinones and anthraquinones, e.g., >99% conversion of 9,10-dihydroanthracene with 99% selectivity for anthracene can be obtained at 120 °C under 1.3 MPa O2 for 8 h. Excellent results were achieved in the oxidative dehydrogenation of variety of dihydroarenes.
- Zhang, Wei,Ma, Hong,Zhou, Lipeng,Sun, Zhiqiang,Du, Zhongtian,Miao, Hong,Xu, Jie
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- Photochemistry of 9-Benzoylanthracene
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Photoexcitation of 9-benzoylanthracene (1) in toluene solution under argon results in head-to-tail dimerization by 4? + 4? cycloaddition to give dibenzoyl-substituted dianthracene in about 60percent yield.The concomitant formation of both anthracene and 9,10-dibenzoylanthracene (ca. 4percent yield) suggests that intermolecular benzoyl group/hydrogen exchange may be an inefficient mode of deactivation the intermediate excimer.Irradiation of crystalline 1 gave the head-to-tail dimer, without byproducts, in a maximal yield of 50percent.It was established by X-ray diffraction that theasymmetric unit of 1 consists of two molecules, 1A and 1B, in which the carbonyl group is twisted out of the plane of the anthracene by 67.4 deg and 86.5 deg, respectively.Investigation of the packing pattern revealed that only parallel overlapping head-to-tail oriented molecules of 1A, characterized by an interplanar spacing of 3.35 Angstroem, can undergo photochemical dimerization by 4? + 4? cycloaddition.The spatial relation of adjacent molecules of 1B is such as to preclude their involvement in the photochemical dimerization.
- Becker, Hans-Dieter,Langer, Vratislav,Becker, Hans-Christian
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- Stoichiometric Formation of an Oxoiron(IV) Complex by a Soluble Methane Monooxygenase Type Activation of O2 at an Iron(II)-Cyclam Center
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In soluble methane monooxygenase enzymes (sMMO), dioxygen (O2) is activated at a diiron(II) center to form an oxodiiron(IV) intermediate Q that performs the challenging oxidation of methane to methanol. An analogous mechanism of O2 activation at mono-or dinuclear iron centers is rare in the synthetic chemistry. Herein, we report a mononuclear non-heme iron(II)-cyclam complex, 1-trans, that activates O2 to form the corresponding iron(IV)-oxo complex, 2-trans, via a mechanism reminiscent of the O2 activation process in sMMO. The conversion of 1-trans to 2-trans proceeds via the intermediate formation of an iron(III)-superoxide species 3, which could be trapped and spectroscopically characterized at-50 °C. Surprisingly, 3 is a stronger oxygen atom transfer (OAT) agent than 2-trans; 3 performs OAT to 1-trans or PPh3 to yield 2-trans quantitatively. Furthermore, 2-trans oxidizes the aromatic C-H bonds of 2,6-di-tert-butylphenol, which, together with the strong OAT ability of 3, represents new domains of oxoiron(IV) and superoxoiron(III) reactivities.
- Kass, Dustin,Corona, Teresa,Warm, Katrin,Braun-Cula, Beatrice,Kuhlmann, Uwe,Bill, Eckhard,Mebs, Stefan,Swart, Marcel,Dau, Holger,Haumann, Michael,Hildebrandt, Peter,Ray, Kallol
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- Investigation of the formation reaction and structural characterization of the 'platinum Grignard reagent' [Pt(MgCl)2(THF)(x)] by extended X-ray absorption fine structure (EXAFS) and other methods
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The 'platinum Grignard reagent' [Pt(MgCl)2(THF)(x)] (2), obtained by the reaction of PtCl2 and Et2Mg in a 1:2 molar ratio, as well as finely divided platinum (Pt*) a possible intermediate formed during the preparation of 2-have been investigated by EXAFS spectroscopy at the Pt L(III) edge. Parallel investigations were carried out on Pt* obtained from PtCl2 and (9,10-dihydro-9,10-anthracenediyl)tris(tettrahydrofuran)magnesium (MgA), and on 2 obtained from Pt* MgA, and MgCl2. The EXAFS results suggest that Pt* consists of extremely small particles (? 5-11 A) with strongly reduced Pt-Pt distances compared to bulk Pt (?0.09 A). The EXAFS spectra of 2 indicate the presence of Mg shells in addition to Pt shells in the Pt environment; Mg atoms are at a bonding distance from Pt atoms (2.78-2.80 A). These results suggest that 2 consists of very small Pt-Mg clusters and confirm their formation from organomagnesium reagents and PtCl2 or Pt*.
- Aleandri, Lorraine E.,Bogdanovic, Borislav,Duerr, Christine,Huckett, Sara C.,Jones, Deborah J.,Kolb, Uwe,Lagarden, Martin,Roziere, Jacques,Wilczok, Ursula
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- Indium-catalyzed construction of polycyclic aromatic hydrocarbon skeletons via dehydration
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Polycyclic aromatic compounds can be synthesized from 2-benzylic- or 2-allylbenzaldehydes using a catalytic amount of In(III) or Re(I) complexes. By using this method, polycyclic aza-aromatic compounds can also be prepared efficiently. In these reactions, only water is formed as a side product.
- Kuninobu, Yoichiro,Tatsuzaki, Tomohiro,Matsuki, Takashi,Takai, Kazuhiko
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- Transition metal-free regioselective access to 9,10-dihydroanthracenes via the reaction of anthracenes with elemental phosphorus in the KOH/DMSO system
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Anthracene and its 9- or 9,10-substituted (Me, Ph, Cl, Br) derivatives react with red phosphorus (Pn) in the KOH/DMSO superbase system at 85–120 °C to afford 9,10-dihydroanthracenes in good to excellent yields, thus providing simple and clean access to these extensively used dihydroaromatics.
- Kuimov, Vladimir A.,Gusarova, Nina K.,Malysheva, Svetlana F.,Trofimov, Boris A.
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- Synthesis and Pyrolysis of a Triafulvene Precursor
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In view of retro-Diels-Alder reactions (RDA reactions), the triafulvene precursor 3 has been prepared in a simple three-step synthesis by dibromocarbene addition at dibenzo-barralene (11-->12; 44percent), halogen-Li exchange followed by methylation (12-->14, 100percent) and HBr elimination (14-->3, 62percent) Scheme 3).Reactivity of the so far unknown bridged 1,1-dibromocyclopropane 12 has been explored, including reductions, allylic rearrangements, and "carbene dimerizations" (Scheme 4).First experiments with respect to the thermal behaviour of 3 show that RDA reaction, although occuring in most cases, is not the predominant pathway.When 3 is heated in a sealed tube without solvent, two dimers 26 and 27 are isolated in a total yield of 55percent (Scheme 6).On the other hand, gas-phase pyrolysis of 3 at 400 deg mainly produces rearranged 28 (56percent; Scheme 7).It is assumed that bridged trimethylenemethane 29 is an essential intermediate in thermal rearrangements of 3 (Scheme 8).
- Muehlebach, Michel,Neuenschwander, Markus,Engel, Peter
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- Evaluation of the Transferability of the “Flexible Steric Bulk” Concept from N-Heterocyclic Carbenes to Planar-Chiral Phosphinoferrocenes and their Electronic Modification
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The concept of “flexible steric bulk” is discussed at 2-phenylvinyl-1-phosphinoferrocenes. The introduction of freely rotatable 1'-silyl groups increases the catalytic productivity within the synthesis of tri-ortho-substituted biaryls by Suzuki–Miyaura C,C cross-coupling reactions, giving higher yields with 1/4 of catalyst concentration than for the non-silylated derivatives. Electronic modification of the P and the vinyl donor functionalities was investigated by introducing substituents in the para positions of both groups. Therein, electron-withdrawing phosphines increased the yield from 78 to 91 percent for a given biaryl, by changing from a diphenylphosphino to the P(p-CN-C6H4)2 unit. Opposite results, obtained from electron-donating and sterically demanding phosphines, were in accordance with the 1J(31P,77Se) values. However, the electron density of the ferrocenyl backbone, expressed by the redox potential of the first ferrocenyl-related redox process, cannot be correlated with the donor-properties at the P atom. Changing from a PPh2-substituted ferrocene to a (RA)-1,1'-binaphthyl-containing phosphonite, a complex interaction between the axial- and the planar-chiral motifs occurs, resulting a change of the absolute biaryl configuration.
- Korb, Marcus,Schaarschmidt, Dieter,Grumbt, Martin,K?nig, Matthias,Lang, Heinrich
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- Aerobic Oxidation Catalysis by a Molecular Barium Vanadium Oxide
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Aerobic catalytic oxidations are promising routes to replace environmentally harmful oxidants with O2 in organic syntheses. Here, we report a molecular barium vanadium oxide, [Ba4(dmso)14V14O38(NO3)] (={Ba4V14}) as viable homogeneous catalyst for a series of oxidation reactions in N,N-dimethyl formamide solution under oxygen (8 bar). Starting from the model compound 9,10-dihydroanthracene, we report initial dehydrogenation/ aromatization leading to anthracene formation; this intermediate is subsequently oxidized by stepwise oxygen transfer, first giving the mono-oxygenated anthrone and then the di-oxygenated target product, anthraquinone. Comparative reaction analyses using the Neumann catalyst [PV2Mo10O40]5? as reference show that oxygen diffusion into the reaction mixture is the rate-limiting step, resulting in accumulation of the reduced catalyst species. This allows us to propose improved reactor designs to overcome this fundamental challenge for aerobic oxidation catalysis.
- Lechner, Manuel,Kastner, Katharina,Chan, Chee Jian,Güttel, Robert,Streb, Carsten
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- The effect of oils on PAH, PCDD, PCDF, and PCB emissions from a spark engine fueled with leaded gasoline
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The effect of synthetic and mineral oils on the formation of polyaromatic hydrocarbons (PAHs), polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and biphenyls (PCBs) in emissions from a spark ignition engine was studied on a Skoda Favorit engine fueled with leaded gasoline. The test cycle simulated urban traffic conditions on a chassis dynamometer, in accordance with the ECC 83.00 test. The data for selected PAHs as well as PCDDs, PCDFs, and PCBs congener profiles are presented. PCDD/Fs emissions for an unused oil and the oil after 10 000-km operation varied from 300 to 2000 fmol/m3, PCBs emissions from 75 to 178 pmol/m3, and PAHs emissions from 150 to 420 μg/m3. The content of PCBs in oils varied from 2 to 920 mg/kg.
- Broz, Jan,Grabic, Roman,Kilian, Juraj,Lojkasek, Milan,Marklund, Stellan,Ocelka, Tomas,Pekarek, Vladimir,Pribyl, Josef,Tydlitat, Vratislav,Vyska, Jan
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- Concerted Proton-Electron Transfer Reactivity at a Multimetallic Co4O4Cubane Cluster
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High-valent oxocobalt(IV) species have been invoked as key intermediates in oxidative catalysis, but investigations into the chemistry of proton-coupled redox reactions of such species have been limited. Herein, the reactivity of an established water oxidation catalyst, [Co4O4(OAc)4(py)4][PF6], toward H-atom abstraction reactions is described. Mechanistic analyses and density functional theory (DFT) calculations support a concerted proton-electron transfer (CPET) pathway in which the high energy intermediates formed in stepwise pathways are bypassed. Natural bond orbital (NBO) calculations point to cooperative donor-acceptor σ interactions at the transition state, whereby the H-atom of the substrate is transferred to an orbital delocalized over a Co3(μ3-O) fragment. The mechanistic insights provide design principles for the development of catalytic C-H activation processes mediated by a multimetallic oxo metal cluster.
- Amtawong, Jaruwan,Balcells, David,Skjelstad, Bastian B.,Tilley, T. Don
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- OXIDATION OF ARENES BY MOLTEN GALLIUM(III) CHLORIDE
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Molten, anhydrous Ga2Cl6 at 100-130 deg C readily oxidizes pyrene, anthracene and naphthacene with the concomitant reduction of Ga(III) to Ga(I).Dilute solutions of these arenes (ArH) are oxidized to their radical cations (ArH+.), wich decay very slowly in Ga2Cl6, while more concentrated solutions are oxidized to stable arenium ions (ArH2+) with hydrogen derived from aryl-aryl coupling reactions.
- Dworkin, A. S.,Brown, L. L.,Buchanan, A. C.,Smith, G. P.
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- A Two-Coordinate Cyclic (Alkyl)(amino)silylene: Balancing Thermal Stability and Reactivity
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A crystalline two-coordinate cyclic (alkyl)(amino)silylene (1) was successfully synthesized and isolated. Its29Si NMR and UV/Vis spectra indicate that the electronic properties of 1 fall between those of cyclic dialkylsilylenes and diaminosilylenes. At very low temperature, the color of a solution of 1 turned from colorless to yellow, which was monitored by UV/Vis spectroscopy. DFT calculations supported the hypothesis that head-to-head dimers (disilenes) with a very long Si–Si distance are formed at such low temperatures. Although 1 is thermally stable, it readily undergoes cycloadditions, Si?H insertions, and photochemical reactions with benzene similar to dialkylsilylenes. At higher temperatures, 1 is also susceptible to intermolecular benzylic C?H insertion reactions, as well as unprecedented dehydrogenation reactions with cyclohexa-1,4-diene and 9,10-dihydroanthracene to afford benzene and anthracene, respectively.
- Kosai, Tomoyuki,Ishida, Shintaro,Iwamoto, Takeaki
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- Metal organic frameworks as efficient heterogeneous catalysts for the oxidation of benzylic compounds with t-butylhydroperoxide
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Iron and copper metal organic frameworks of 1,3,5-benzenetricarboxylate [Fe(BTC) and Cu3(BTC)2] are efficient and reusable solid catalysts for the oxidation of benzylic compounds with t-butylhydroperoxide as oxidant in acetonitrile with moderate to good yields. The solids were stable under the reaction conditions as confirmed by comparison of the powder XRD of the fresh and reused catalysts. EPR spectroscopy using diphenyl-N-t-butyl nitrone as spin trap has allowed to detect C-centered radicals as reaction intermediates.
- Dhakshinamoorthy, Amarajothi,Alvaro, Mercedes,Garcia, Hermenegildo
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- Isolation and characterization of a dihydroxo-bridged iron(III,III)(μ- OH)2 diamond core derived from dioxygen
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Dioxygen addition to coordinatively unsaturated [Fe(II)(O Me2N4(6-Me-DPEN))](PF6) (1) is shown to afford a complex containing a dihydroxo-bridged Fe(III)2(μ-OH) 2 diamond core, [FeIII(OMe2N 4(6-Me-DPEN))]2(μ-OH)2(PF6) 2·(CH3CH2CN)2 (2). The diamond core of 2 resembles the oxidized methane monooxygenase (MMOox) resting state, as well as the active site product formed following H-atom abstraction from Tyr-OH by ribonucleotide reductase (RNR). The Fe-OH bond lengths of 2 are comparable with those of the MMOHox suggesting that MMOHox contains a Fe(III)2(μ-OH)2 as opposed to Fe(III) 2(μ-OH)(μ-OH2) diamond core as had been suggested. Isotopic labeling experiments with 18O2 and CD 3CN indicate that the oxygen and proton of the μ-OH bridges of 2 are derived from dioxygen and acetonitrile. Deuterium incorporation (from CD3CN) suggests that an unobserved intermediate capable of abstracting a H-atom from CH3CN forms en route to 2. Given the high C-H bond dissociation energy (BDE = 97 kcal/mol) of acetonitrile, this indicates that this intermediate is a potent oxidant, possibly a high-valent iron oxo. Consistent with this, iodosylbenzene (PhIO) also reacts with 1 in CD 3CN to afford the deuterated Fe(III)2(μ-OD)2 derivative of 2. Intermediates are not spectroscopically observed in either reaction (O2 and PhIO) even at low-temperatures (-80 C), indicating that this intermediate has a very short lifetime, likely due to its highly reactive nature. Hydroxo-bridged 2 was found to stoichiometrically abstract hydrogen atoms from 9,10-dihydroanthracene (C-H BDE = 76 kcal/mol) at ambient temperatures.
- Coggins, Michael K.,Toledo, Santiago,Kovacs, Julie A.
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- 9,10-Dehydroanthracene: p-Benzyne-type biradicals abstract hydrogen unusually slowly
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The 9,10-dehydroanthracene biradical, a model for the p-benzyne-type biradicals implicated in DNA cleavage by the enediyne antitumor antibiotics, was prepared by photodissociation of a propellane in solution. Trapping products characteristic of biradicals, e.g. anthracene-d2, are found. The rates of hydrogen abstraction by the biradical from acetonitrile and isopropyl alcohol are measured directly by flash photolysis/transient absorption spectroscopy, giving second-order rate constants of k(MeCN,abstr) = (1.1 ± 0.2) x 103 M-1 s-1 and k(i-PrOH,abstr) = (6.5 ± 0.6) x 103 M-1 s-1 at room temperature, which are 100-200 times lower than the corresponding rate constants for phenyl or 9-anthryl radical. A second decay route for the biradical is found, and assigned, based on thermochemical, kinetic, and trapping arguments to a retro-Bergman reaction that converts the 9,10-dehydroanthracene biradical into the ring-opened 3,4-benzocyclodeca-3,7,9-triene-1,5-diyne. Although the retro-Bergman reaction is relatively fast, k ~ 4 x 105 s-1 at room temperature, it is competitive with hydrogen abstraction by the biradical only because the hydrogen abstraction is slower than expected. Through-bond coupling in the 1,4-biradical is discussed as a rationalization for the 100- to 200-fold reduction in the abstraction rate.
- Schottelius, Marc J.,Chen, Peter
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- Facile Synthesis of Polycyclic Aromatic Hydrocarbons: Br?nsted Acid Catalyzed Dehydrative Cycloaromatization of Carbonyl Compounds in 1,1,1,3,3,3-Hexafluoropropan-2-ol
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The cycloaromatization of aromatic aldehydes and ketones was readily achieved by using a Br?nsted acid catalyst in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP). In the presence of a catalytic amount of trifluoromethanesulfonic acid, biaryl-2-ylacetaldehydes and 2-benzylbenzaldehydes underwent sequential intramolecular cationic cyclization and dehydration to afford phenacenes and acenes, respectively. Furthermore, biaryl-2-ylacetaldehydes bearing a cyclopentene moiety at the α-position underwent unprecedented cycloaromatization including ring expansion to afford triphenylenes. HFIP effectively promoted the cyclizations by suppressing side reactions presumably as a result of stabilization of the cationic intermediates.
- Fujita, Takeshi,Takahashi, Ikko,Hayashi, Masaki,Wang, Jingchen,Fuchibe, Kohei,Ichikawa, Junji
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- Photochemical Debromination of Meso-Substituted Bromoanthracenes Studied by Steady-State Photolysis and Laser Photolysis
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Debrominations of 9-bromoanthracene (BA) and 9,10-dibromoanthracene (DBA) in acetonitrile containing triethylamine (TEA) or N,N-dimethylaniline (DMA) have been studied by means of steady-state photolysis and laser photolysis.By the addition of TEA, the decay constants of the lowest excited singlet states of BA and DBA increase and the maximum yields of the triplet states decrease.The singlet quenching rate constants by TEA are calculated to be of the order of 1E10 M-1 s-1, showing that the reactions are diffusion controlled.Compared with the result of γ-radiolysis and pulse radiolysis, it is suggested that the photochemical debrominations of BA and DBA in the presence of amines take place via the anion radicals which are produced through exciplexes between amines and the lowest excited singlet states of bromoanthracenes.
- Hamanoue, Kumao,Tai, Shigeyoshi,Hidaka, Toshiharu,Nakayama, Toshihiro,Kimoto, Masaki,Teranishi, Hiroshi
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- CuCl2/[hmim]Br: A new recyclable catalytic system for aromatization of cyclic dienes
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The combination of copper(II) chloride and 1-hexyl-3-methylimidazolium bromide ([hmim]Br) as a new recyclable catalytic system in the presence of TBHP for aromatization of cyclic dienes is disclosed. The system could furnish the corresponding aromatics in moderate to excellent yield. The competition between aromatization and oxidation of benzylic substrates were also studied. This developed catalytic system could be reused and recycled at least nine times without significant loss of the producitivity.
- Taboonpong, Piyada,Chavasiri, Warinthorn
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- Reactivity of bio-inspired Cu(II) (N2/Py2) complexes with peroxide at room temperature
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Developing coordination complexes of earth abundant metals that can perform substrate oxidations under benign conditions is an ongoing challenge. Herein, the reactivity of two mononuclear Cu-complexes toward the oxidant H2O2 is reported. Both complexes displayed ligand oxidation upon reaction with the oxidant. Analysis of spectroscopic data established that the respective product complexes contained mononuclear Cu(II) centers. Moreover, treatment of these Cu-complexes with oxidant in the presence of substrate resulted in the interception of ligand oxidation with preferential oxidation of the substrate. Computational studies identified plausible mechanistic pathways, suggesting a copper-oxyl intermediate as the likely reactive intermediate responsible for substrate and ligand oxidation. To our knowledge, this is the first Cu-mediated system that showed ligand oxidation, oxo-transfer capability, and external hydrocarbon oxidation under stoichiometric conditions.
- Singh, Nirupama,Botcha, Niharika Krishna,Jones, Thomas M.,Ertem, Mehmed Z.,Niklas, Jens,Farquhar, Erik R.,Poluektov, Oleg G.,Mukherjee, Anusree
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- Unprecedented rate enhancements of hydrogen-atom transfer to a manganese(v)-oxo corrolazine complex
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Chemical equation presented Runaway reactivity: A manganese(V)-oxo porphyrinoid complex displays an unprecedented increase in reaction rate for a hydrogen-atom abstraction upon addition of anionic axial ligands (X=F - and CN-; see scheme). Density functional theory calculations are in excellent agreement with experiment, and provide insight into the origins of these remarkable axial ligand effects.
- Prokop, Katharine A.,De Visser, Sam P.,Goldberg, David P.
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- Metal-Free Selective Borylation of Arenes by a Diazadiborinine via C-H/C-F Bond Activation and Dearomatization
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A newly developed annulated 5-chlorinated 1,3,2,5-diazadiborinine derivative (4) selectively activates a C-H bond of benzene (C6H6) and 1,3-di(trifluoromethyl)benzene, as well as a C-F bond in partially fluorinated arenes, to furnish borylation products under catalyst-, metal-, and irradiation-free conditions. Moreover, 4 readily undergoes a reversible dearomative coupling reaction with polycyclic aromatic hydrocarbons to afford diboration products. The latter represents the first reversible intermolecular dearomative diboration of arenes.
- Su, Yuanting,Huan Do, Dinh Cao,Li, Yongxin,Kinjo, Rei
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- A High Spin Mn(IV)-Oxo Complex Generated via Stepwise Proton and Electron Transfer from Mn(III)-Hydroxo Precursor: Characterization and C-H Bond Cleavage Reactivity
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The oxomanganese(IV) complex [(dpaq)MnIV(O)]+-Mn+ (1-Mn+, Mn+ = redox-inactive metal ion, H-dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-ylacetamide), generated in the reaction of the precursor hydroxomanganese(III) complex 1 with iodosylbenzene (PhIO) in the presence of redox-inactive metal triflates, has recently been reported. Herein the generation of the same oxomanganese(IV) species from 1 using various combinations of protic acids and oxidants at 293 K is reported. The reaction of 1 with triflic acid and the one-electron-oxidizing agent [RuIII(bpy)3]3+ leads to the formation of the oxomanganese(IV) complex. The putative species has been identified as a mononuclear high-spin (S = 3/2) nonheme oxomanganese(IV) complex (1-O) on the basis of mass spectrometry, Raman spectroscopy, EPR spectroscopy, and DFT studies. The optical absorption spectrum is well reproduced by theoretical calculations on an S = 3/2 ground spin state of the complex. Isotope labeling studies confirm that the oxygen atom in the oxomanganese(IV) complex originates from the MnIII-OH precursor and not from water. A mechanistic investigation reveals an initial protonation step forming the MnIII-OH2 complex, which then undergoes one-electron oxidation and subsequent deprotonations to form the oxomanganese(IV) transient, avoiding the requirements of either oxo-transfer agents or redox-inactive metal ions. The MnIV-oxo complex cleaves the C-H bonds of xanthene (k2 = 5.5 M-1 s-1), 9,10-DHA (k2 = 3.9 M-1 s-1), 1,4-CHD (k2 = 0.25 M-1 s-1), and fluorene (k2 = 0.11 M-1 s-1) at 293 K. The electrophilic character of the nonheme MnIV-oxo complex is demonstrated by a large negative ρ value of 2.5 in the oxidation of para-substituted thioanisoles. The complex emerges as the "most reactive" among the existing MnIV/V-oxo complexes bearing anionic ligands.
- Biswas, Sachidulal,Mitra, Amritaa,Banerjee, Sridhar,Singh, Reena,Das, Abhishek,Paine, Tapan Kanti,Bandyopadhyay, Pinaki,Paul, Satadal,Biswas, Achintesh N.
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- Dioxygen Activation by a Hexagonal SrMnO3 Perovskite Catalyst for Aerobic Liquid-Phase Oxidation
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Heterogeneous catalysts, which allow a reductive activation of dioxygen (O2) under mild reaction conditions, are promising candidates for highly efficient aerobic oxidation. An effective hexagonal SrMnO3 (SMO) perovskite catalyst for liquid-phase selective oxidation with O2 was successfully synthesized by the polymerized complex method. The activity of SMO for the aerobic oxidation of alcohols was higher than those of typical manganese oxide-based catalysts and was heterogeneous, so that the recovered catalyst could be reused without heat treatment under oxidative conditions while keeping its high catalytic performance. The reversible reaction on the surface Mn species on SMO with O2 leads to the formation of Mn–superoxo species, which catalyze the selective oxidative transformation of various types of organic substrates into the desired oxygenated or dehydrogenated products.
- Kawasaki, Shuma,Kamata, Keigo,Hara, Michikazu
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- Iron-catalyzed oxidation of unreactive C-H bonds: Utilizing bio-inspired axial ligand modification to increase catalyst stability
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Three different bio-inspired Fe(II) complexes are applied as powerful catalysts for the oxidation of unreactive C-H bonds under ambient conditions. Cyclohexane as the main model substrate is oxidized to cyclohexanol, cyclohexyl hydroperoxide, and cyclohexanone. Alcohol + cyclohexyl hydroperoxide to ketone ratios ((A + H)/K) of up to 26 are obtained with comparatively high turnovers of up to 43. Bio-inspired modification of the Fe(II) complexes in the axial positions is used to increase catalyst stability toward hydrogen peroxide, leading to an increase in turnovers of up to 34%. Several parameters for the catalytic oxidation are investigated, e.g., the amount and type of oxidant, reaction temperature, and the relative catalyst concentration. Among others, 9,10-dihydroantracene and 2,3-dimethylbutane are used as substrates for the catalytic C-H bond oxidation.
- Haslinger, Stefan,Raba, Andreas,Cokoja, Mirza,P?thig, Alexander,Kühn, Fritz E.
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- Stabilizing Terminal Ni(III)-Hydroxide Complex Using NNN-Pincer Ligands: Synthesis and Characterization
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The reaction of [Ni(COD)2] (COD; cyclooctadiene) in THF with the NNN-pincer ligand bis(imino)pyridyl (L1) reveals a susceptibility to oxidation in an inert atmosphere ([O2] level III(L1··)2-(OH)] (1). Various isotopically labeled experiments (16O/18O) assertively endorse the origin of terminal oxygen based ligand in 1 due to the activation of molecular dioxygen. The presence of proton bound to the terminal oxygen in 1 is well supported by NMR, IR spectroscopy, DFT calculations, and hydrogen atom transfer (HAT) reactions promoted by 1. The observation of shakeup satellite peaks for the primary photoelectron lines of Ni(2p) in the X-ray photoelectron spectroscopy (XPS) unambiguously confirms the paramagnetic signature associated with the distorted square planar nickel ion, which is consistent with the trivalent oxidation state assigned for the nickel ion in 1. The variable temperature magnetic susceptibility data of 1 shows dominant antiferromagnetic interactions exist among the paramagnetic centers, resulting in an overall S = 1/2 ground state. Variable temperature X-band EPR studies performed on 1 show evidence for the S = 1/2 ground state, which is consistent with magnetic data. The unusual g-tensor extracted for the ground state S = 1/2 is analyzed under a strong exchange limit of spin-coupled centers. The electronic structure predicted for 1 is in good agreement with theoretical calculations.
- Shanmugam, Maheswaran,Rajpurohit, Jitendrasingh,Shukla, Pragya,Kumar, Pardeep,Das, Chinmoy,Vaidya, Shefali,Sundararajan, Mahesh,Shanmugam, Muralidharan
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- Highly electron-deficient hexaazapentacenes and their dihydro precursors
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Novel silylethynylated N-heteropentacenes that have three adjacent pyrazine rings at the center of a pentacene backbone are reported. These hexaazapentacenes exhibit a record low energy level of lowest unoccupied molecular orbital (LUMO) for N-heteropentacenes and thus are able to oxidize dihydroanthracene to anthracene. Their synthetic precursors are the corresponding dihydrohexaazapentacenes, which exhibit interesting H-bonding.
- He, Zikai,Mao, Renxin,Liu, Danqing,Miao, Qian
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- Pd(II) Coordination Sphere Engineering: Pyridine Cages, Quinoline Bowls, and Heteroleptic Pills Binding One or Two Fullerenes
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Fullerenes and their derivatives are of tremendous technological relevance. Synthetic access and application are still hampered by tedious purification protocols, peculiar solubility, and limited control over regioselective derivatization. We present a modular self-assembly system based on a new low-molecular-weight binding motif, appended by two palladium(II)-coordinating units of different steric demands, to either form a [Pd2L14]4+ cage or an unprecedented [Pd2L23(MeCN)2]4+ bowl (with L1 = pyridyl, L2 = quinolinyl donors). The former was used as a selective induced-fit receptor for C60. The latter, owing to its more open structure, also allows binding of C70 and fullerene derivatives. By exposing only a fraction of the bound guests' surface, the bowl acts as fullerene protecting group to control functionalization, as demonstrated by exclusive monoaddition of anthracene. In a hierarchical manner, sterically low-demanding dicarboxylates were found to bridge pairs of bowls into pill-shaped dimers, able to host two fullerenes. The hosts allow transferring bound fullerenes into a variety of organic solvents, extending the scope of possible derivatization and processing methodologies.
- Chen, Bin,Holstein, Julian J.,Horiuchi, Shinnosuke,Hiller, Wolf G.,Clever, Guido H.
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- MnIV-Oxo complex of a bis(benzimidazolyl)-containing N5 ligand reveals different reactivity trends for MnIV-oxo than FeIV-oxo species
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Using the pentadentate ligand (N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine, 2pyN2B), presenting two pyridyl and two (N-methyl)benzimidazolyl donor moieties in addition to a central tertiary amine, new MnII and MnIV-oxo complexes were generated and characterized. The [MnIV(O)(2pyN2B)]2+ complex showed spectroscopic signatures (i.e., electronic absorption band maxima and intensities, EPR signals, and Mn K-edge X-ray absorption edge and near-edge data) similar to those observed for other MnIV-oxo complexes with neutral, pentadentate N5 supporting ligands. The near-IR electronic absorption band maximum of [MnIV(O)(2pyN2B)]2+, as well as DFT-computed metric parameters, are consistent with the equatorial (N-methyl)benzimidazolyl ligands being stronger donors to the MnIV center than the pyridyl and quinolinyl ligands found in analogous MnIV-oxo complexes. The hydrogen- and oxygen-atom transfer reactivities of [MnIV(O)(2pyN2B)]2+ were assessed through reactions with hydrocarbons and thioanisole, respectively. When compared with related MnIV-oxo adducts, [MnIV(O)(2pyN2B)]2+ showed muted reactivity in hydrogen-atom transfer reactions with hydrocarbons. This result stands in contrast to observations for the analogous FeIV-oxo complexes, where [FeIV(O)(2pyN2B)]2+ was found to be one of the more reactive members of its class.
- Denler, Melissa C.,Massie, Allyssa A.,Singh, Reena,Stewart-Jones, Eleanor,Sinha, Arup,Day, Victor W.,Nordlander, Ebbe,Jackson, Timothy A.
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- A Highly Reactive Seven-Coordinate Osmium(V) Oxo Complex: [OsV(O)(qpy)(pic)Cl]2+
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Seven-coordinate ruthenium oxo species have been proposed as active intermediates in catalytic water oxidation by a number of highly active ruthenium catalysts, however such species have yet to be isolated. Reported herein is the first example of a seven-coordinate group 8 metal-oxo species, [OsV(O)(qpy)(pic)Cl]2+ (qpy=2,2′:6′,2″:6″,2-quaterpyridine, pic=4-picoline). The X-ray crystal structure of this complex shows that it has a distorted pentagonal bipyramidal geometry with an Os=O distance of 1.7375 ?. This oxo species undergoes facile O-atom and H-atom-transfer reactions with various organic substrates. Notably it can abstract H atoms from alkylaromatics with C-H bond dissociation energy as high as 90 kcal mol-1. This work suggests that highly active oxidants may be designed based on group 8 seven-coordinate metal oxo species.
- Liu, Yingying,Ng, Siu-Mui,Lam, William W. Y.,Yiu, Shek-Man,Lau, Tai-Chu
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- Formation mechanism for polycyclic aromatic hydrocarbons in methane flames
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A laminar diffusion flame of methane was investigated using time-of-flight mass spectroscopy with two-photon UV laser ionization. Benzenoid polycyclic aromatic hydrocarbons (PAHs) up to 788 amu (C64H20) were detected in the combustion gases. Only the most compact PAHs are formed in the flame. The observed groups of PAH peaks with 24 amu spacings belong to PAHs with constant hydrogen content and are separated by 26 amu gaps. The sequences of PAH peaks with 24 amu spacing are explained by a repetitive bay closure mechanism. The first PAH of a constant H-sequence is proposed to form by a dimerization process. The PAHs observed can be arranged in a repetitive pattern in Dias's formula periodic system.
- Siegmann,Sattler
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- Efficient heterogeneous oxidation of alkylarenes with molecular oxygen
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(Chemical Equation Presented) Ru(OH)x/Al2O 3 efficiently catalyzes the heterogeneous aerobic oxygenation or oxidative dehydrogenation of alkylarenes to give the corresponding oxygenated or dehydrogenated products. Catalyst/product separation is very easy, and the recovered catalyst is reusable with retention of the high catalytic performance.
- Kamata, Keigo,Kasai, Jun,Yamaguchi, Kazuya,Mizuno, Noritaka
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- Hydrogen Transfer between Anthracene Structures
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This work reports results of kinetic studies of hydrogen migration between 9,10-dihydro positions in anthracene structures.The transfer of two H atoms from 9,10-dimethyl-9,10-dihydroanthracene (AnH2) to 2-ethylanthracene (EAn) follows simple bimolecular kinetics with k/M-1 s-1=109.64+/-0.14exp (250-375 deg C) At 300-350 deg C, H transfer to 9,10-dimethylanthracene led to nearly equimolar mixtures of cis- and trans-9,10-dihydroanthracene, consistent with a free radical mechanism.The rate-limiting step appears to be transfer of a single benzylic H atom from a donor molecule to an acceptor molecule, resulting in the formation of two highly stabilized free radicals.Reactions of this nature are likely to serve as major sources of free radicals in condensed-phase thermolysis reactions.Measurements of their rate constants offer a new, relatively direct means of determining bond strengths.From the above rate expression, we derive an AnH-H bond strength of 78.4+/- 1.8 kcal mol-1.From literature data for a similar reaction (Halpern et al.) we obtain an H-Mn(CO)5 bond strength of 63 kcal mol-1.Based on an observed lowering of the reaction rate with added anthracene, a rate constant was derived for β-H transfer from a2-ethyl-9-hydroanthryl radical to anthracene.At 350 deg C, this value was 120 M-1 s-1, indicating a high activation barrier (ca. 18 kcal mol-1) for this seldom reported process.
- Billmers, R.,Griffith, L. L.,Stein, S. E.
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- Base-Free Palladium-Catalyzed Borylation of Aryl Chlorides with Diborons
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The base-free palladium-catalyzed borylation of aryl chlorides with diborons was achieved. The base-free conditions offered acceptable functional group compatibility. Based on experimental and computational studies, it was shown that smooth boryl transfer from the diborons to the arylpalladium chloride was promoted by strong interaction between the Lewis acidic boron and the chlorine atom on palladium.
- Yamamoto, Yutaro,Matsubara, Hiroshi,Yorimitsu, Hideki,Osuka, Atsuhiro
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- Supercritical water oxidation of 2-chlorophenol effected by Li+ and CuO/zeolites
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Catalytic oxidation of 2-chlorophenol (2CP) in supercritical water was investigated. Experimentally, conversion of 2CP in supercritical water oxidation (SCWO) process is effectively enhanced in the presence of Li+ that also reduces the formation of higher chlorinated phenols and PAHs. The global reaction rate of SCWO of 2CP in the presence of Li+ is expressed as: 92.5 exp (-10.5/RT)[2CP]0.95[O2]0.56[H2O]0.45. The undesired by-products in the SCWO of 2CP are also extensively reduced in the channels of zeolite catalysts. By EXAFS spectroscopy, Cu-O and Cu-Cu are identified as the main oxidation active species in the zeolite channels. Cl-bonded CuO species in zeolite Y are not observed.
- Lin, Kuen-Song,Wang, H. Paul,Yang
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- C(sp3)-H Fluorination with a Copper(II)/(III) Redox Couple
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Despite the growing interest in the synthesis of fluorinated organic compounds, few reactions are able to incorporate fluoride ions directly into alkyl C-H bonds. Here, we report the C(sp3)-H fluorination reactivity of a formally copper(III) fluoride complex. The C-H fluorination intermediate, LCuF, along with its chloride and bromide analogues, LCuCl and LCuBr, were prepared directly from halide sources with a chemical oxidant and fully characterized with single-crystal X-ray diffraction, X-ray absorption spectroscopy, UV-vis spectroscopy, and 1H nuclear magnetic resonance spectroscopy. Quantum chemical calculations reveal significant halide radical character for all complexes, suggesting their ability to initiate and terminate a C(sp3)-H halogenation sequence by sequential hydrogen atom abstraction (HAA) and radical capture. The capability of HAA by the formally copper(III) halide complexes was explored with 9,10-dihydroanthracene, revealing that LCuF exhibits rates 2 orders of magnitude higher than LCuCl and LCuBr. In contrast, all three complexes efficiently capture carbon radicals to afford C(sp3)-halogen bonds. Mechanistic investigation of radical capture with a triphenylmethyl radical revealed that LCuF proceeds through a concerted mechanism, while LCuCl and LCuBr follow a stepwise electron transfer-halide transfer pathway. The capability of LCuF to perform both hydrogen atom abstraction and radical capture was leveraged to enable fluorination of allylic and benzylic C-H bonds and α-C-H bonds of ethers at room temperature.
- Bower, Jamey K.,Cypcar, Andrew D.,Henriquez, Brenda,Stieber, S. Chantal E.,Zhang, Shiyu
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- The Reaction of 9-Bromoanthracene with Benzenethiolate Anion in Tetraglyme: Evidence against a Competing Electron-Transfer Mechanism
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The reaction of 9-bromoanthracene (1) with benzenethiol(ate) in tetraglyme proceeds by a SNAr mechanism.The concurrent formation of anthracene is not due to a competing single-electron-transfer pathway involving either benzenethiol or benzenethiolate anion.
- Baumgarner, Charles D.,Malen, Alyssa H.,Pastor, Stephen D.,NabiRahni, M. Ali
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- Non-heme iron(ii/iii) complexes that model the reactivity of lipoxygenase with a redox switch
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Three ferrous/ferric complexes of a N6 hexadentate ligand, N,N,N′,N′-tetrakis(2-benzimidazolyl-methyl)ortho-diamine-trans- cyclohexane (ctb), [FeII(ctb)](ClO4)2· EtOH (1), [FeIII(OEt)(Hctb)](ClO4)3·EtOH (2), and [FeIII(OMe)(Hctb)](ClO4)3· 3MeOH·4.5H2O (3), were synthesized and characterized as models of lipoxygenase. The lipoxygenase activities of the complexes were checked and the results indicate that ferrous complex 1 is inactive while ferric alkoxide complexes 2 and 3 show catalytic activity via the hydrogen atom abstraction reaction mechanism.
- Mei, Fusheng,Ou, Chunping,Wu, Guiling,Cao, Li,Han, Fang,Meng, Xianggao,Li, Ji,Li, Dongfeng,Liao, Zhanru
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- Aromatization of highly alkyl-substituted dihydroanthracenes using n-BuLi/TMEDA/MeI
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Aromatization of highly alkyl-substituted dihydroanthracene was performed with the n-BuLi/TMEDA/MeI. Treatment of 1,2,3,4,5,6,7,8-octapropyl-9, 10-dihydroanthracene with 2.2 equiv of n-BuLi, TMEDA at 50 °C in hexane for 3 h, followed by 1.1 equiv of MeI at room temperature for 1 h gave the corresponding anthracene in 98% yield.
- Kitamura, Masanori,Shen, Baojian,Liu, Yanjun,Zheng, Hegen,Takahashi, Tamotsu
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- Spectroscopic capture and reactivity of a low-spin cobalt(IV)-oxo complex stabilized by binding redox-inactive metal ions
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High-valent cobalt-oxo intermediates are proposed as reactive intermediates in a number of cobalt-complex-mediated oxidation reactions. Herein we report the spectroscopic capture of low-spin (S=1/2) CoIV-oxo species in the presence of redox-inactive metal ions, such as Sc3+, Ce3+, Y3+, and Zn2+, and the investigation of their reactivity in C-H bond activation and sulfoxidation reactions. Theoretical calculations predict that the binding of Lewis acidic metal ions to the cobalt-oxo core increases the electrophilicity of the oxygen atom, resulting in the redox tautomerism of a highly unstable [(TAML)CoIII(O.)]2- species to a more stable [(TAML)CoIV(O)(Mn+)] core. The present report supports the proposed role of the redox-inactive metal ions in facilitating the formation of high-valent metal-oxo cores as a necessary step for oxygen evolution in chemistry and biology. What is the metal's role? Cobalt(IV)-oxo complexes binding redox-inactive metal ions, such as Sc3+, Ce3+, Y3+, and Zn2+, are investigated in oxygenation reactions. Theory predicts that the binding of metal ions to the cobalt-oxo core increases the electrophilicity of the oxygen atom. This result supports the role of redox-inactive metal ions in facilitating the formation of high-valent metal-oxo cores as a necessary step for oxygen evolution in chemistry and biology. CAN=cerium ammonium nitrate.
- Hong, Seungwoo,Pfaff, Florian F.,Kwon, Eunji,Wang, Yong,Seo, Mi-Sook,Bill, Eckhard,Ray, Kallol,Nam, Wonwoo
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- Synthesis of 1,8-bis(cyclam) and 1,8-bis(azacrown) substituted anthracenes by palladium-catalyzed arylation of cyclam
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The Pd-catalyzed arylation of cyclam, cyclen, and azacrown ethers by aryl halides was studied. Cofacial biscyclam and bisazacrown were synthesized by direct bonding of the anthracenyl spacer to a nitrogen atom of macrocycles.
- Beletskaya, Irina P,Averin, Alexei D,Bessmertnykh, Alla G,Denat, Franck,Guilard, Roger
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- Reduced Phenalenyl in Catalytic Dehalogenative Deuteration and Hydrodehalogenation of Aryl Halides
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Dehalogenative deuteration reactions are generally performed through metal-mediated processes. This report demonstrates a mild protocol for hydrodehalogenation and dehalogenative deuteration of aryl/heteroaryl halides (39 examples) using a reduced odd alternant hydrocarbon phenalenyl under transition metal-free conditions and has been employed successfully for the incorporation of deuterium in various biologically active compounds. The combined approach of experimental and theoretical studies revealed a single electron transfer-based mechanism.
- Singh, Bhagat,Ahmed, Jasimuddin,Biswas, Amit,Paira, Rupankar,Mandal, Swadhin K.
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p. 7242 - 7255
(2021/05/29)
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- Investigating reactivity and electronic structure of copper(II)-polypyridyl complexes and hydrogen peroxide
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This work presents a detailed study of the reactivity of three mononuclear CuII complexes supported by derivatives of the tetradentate ligand N,N'-bis(2-pyridylmethyl)-1,2-ethylenediamine (bispicen). The CuII complexes are capable of performing C[sbnd]H bond activation in the presence of NEt3 and H2O2 through what has been proposed computationally to be a [CuO]+ intermediate. A wavefunction-based quantum chemical investigation into the electronic structure of the proposed [CuO]+ intermediate reveals a triplet ground state predominantly consistent with an S = ? CuII center ferromagnetically coupled to an oxyl radical, though contributions from the corresponding biradicaloid CuI-oxene resonance structure may be nontrivial. Furthermore, correlation of the electronic structure of the proposed intermediate with analogous high-valent metal-oxo species capable of olefin epoxidation suggests that the CuII complexes might be also capable of olefin epoxidation in the presence of NEt3 and H2O2. To test this hypothesis experimentally, the CuII complexes are treated with NEt3 and H2O2 in the presence of alkene substrates, resulting in the formation of epoxides.
- Khazanov, Thomas M.,Botcha, Niharika Krishna,Yergeshbayeva, Sandugash,Shatruk, Michael,Mukherjee, Anusree
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- C-H Activation by RuCo3O4Oxo Cubanes: Effects of Oxyl Radical Character and Metal-Metal Cooperativity
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High-valent multimetallic-oxo/oxyl species have been implicated as intermediates in oxidative catalysis involving proton-coupled electron transfer (PCET) reactions, but the reactive nature of these oxo species has hindered the development of an in-depth understanding of their mechanisms and multimetallic character. The mechanism of C-H oxidation by previously reported RuCo3O4 cubane complexes bearing a terminal RuV-oxo ligand, with significant oxyl radical character, was investigated. The rate-determining step involves H atom abstraction (HAA) from an organic substrate to generate a Ru-OH species and a carbon-centered radical. Radical intermediates are subsequently trapped by another equivalent of the terminal oxo to afford isolable radical-trapped cubane complexes. Density functional theory (DFT) reveals a barrierless radical combination step that is more favorable than an oxygen-rebound mechanism by 12.3 kcal mol-1. This HAA reactivity to generate organic products is influenced by steric congestion and the C-H bond dissociation energy of the substrate. Tuning the electronic properties of the cubane (i.e., spin density localized on terminal oxo, basicity, and redox potential) by varying the donor ability of ligands at the Co sites modulates C-H activations by the RuV-oxo fragment and enables construction of structure-activity relationships. These results reveal a mechanistic pathway for C-H activation by high-valent metal-oxo species with oxyl radical character and provide insights into cooperative effects of multimetallic centers in tuning PCET reactivity.
- Amtawong, Jaruwan,Balcells, David,Handford, Rex C.,Skjelstad, Bastian Bjerkem,Suslick, Benjamin A.,Tilley, T. Don
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supporting information
p. 12108 - 12119
(2021/08/20)
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- A Bottleable Imidazole-Based Radical as a Single Electron Transfer Reagent
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Reduction of 1,3-bis(2,6-diisopropylphenyl)-2,4-diphenyl-1H-imidazol-3-ium chloride (1) resulted in the formation of the first structurally characterized imidazole-based radical 2. 2 was established as a single electron transfer reagent by treating it with an acceptor molecule tetracyanoethylene. Moreover, radical 2 was utilized as an organic electron donor in a number of organic transformations such as in activation of an aryl-halide bond, alkene hydrosilylation, and in catalytic reduction of CO2 to methoxyborane, all under ambient temperature and pressure.
- Das, Arpan,Ahmed, Jasimuddin,Rajendran,Adhikari, Debashis,Mandal, Swadhin K.
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p. 1246 - 1252
(2020/12/21)
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- Metal-Free Heterogeneous Semiconductor for Visible-Light Photocatalytic Decarboxylation of Carboxylic Acids
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A suitable protocol for the photocatalytic decarboxylation of carboxylic acids was developed with metal-free ceramic boron carbon nitrides (BCN). With visible light irradiation, BCN oxidize carboxylic acids to give carbon-centered radicals, which were trapped by hydrogen atom donors or employed in the construction of the carbon-carbon bond. In this system, both (hetero)aromatic and aliphatic acids proceed the decarboxylation smoothly, and C-H, C-D, and C-C bonds are formed in moderate to high yields (35 examples, yield up to 93%). Control experiments support a radical process, and isotopic experiments show that methanol is employed as the hydrogen atom donor. Recycle tests and gram-scale reaction elucidate the practicability of the heterogeneous ceramic BCN photoredox system. It provides an alternative to homogeneous catalysts in the valuable carbon radical intermediates formation. Moreover, the metal-free system is also applicable to late-stage functionalization of anti-inflammatory drugs, such as naproxen and ibuprofen, which enrich the chemical toolbox.
- Shi, Jiale,Yuan, Tao,Zheng, Meifang,Wang, Xinchen
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p. 3040 - 3047
(2021/03/09)
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- A Mononuclear Non-heme Iron(III)-Peroxo Complex with an Unprecedented High O-O Stretch and Electrophilic Reactivity
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A mononuclear non-heme iron(III)-peroxo complex, [Fe(III)(O2)(13-TMC)]+ (1), was synthesized and characterized spectroscopically; the characterization with electron paramagnetic resonance, M?ssbauer, X-ray absorption, and resonance Raman spectroscopies and mass spectrometry supported a high-spin S = 5/2 Fe(III) species binding an O2 unit. A notable observation was an unusually high νO-O at ～1000 cm-1 for the peroxo ligand. With regard to reactivity, 1 showed electrophilic reactivity in H atom abstraction (HAA) and O atom transfer (OAT) reactions. In the HAT reaction, a kinetic isotope effect (KIE) value of 5.8 was obtained in the oxidation of 9,10-dihydroanthracene. In the OAT reaction, a negative ρ value of -0.61 in the Hammett plot was determined in the oxidation of p-X-substituted thioanisoles. Another interesting observation was the electrophilic reactivity of 1 in the oxidation of benzaldehyde derivatives, such as a negative ρ value of -0.77 in the Hammett plot and a KIE value of 2.2. To the best of our knowledge, the present study reports the first example of a mononuclear non-heme iron(III)-peroxo complex with an unusually high νO-O value and unprecedented electrophilic reactivity in oxidation reactions.
- Ezhov, Roman,Guo, Yisong,Jang, Semin,Kim, Taeyeon,Lee, Yong-Min,Li, Xiao-Xi,Nam, Wonwoo,Pushkar, Yulia,Sarangi, Ritimukta,Seo, Mi Sook,Xiong, Jin,Zhu, Wenjuan
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supporting information
p. 15556 - 15561
(2021/10/01)
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- Mn(III) active site in hydrotalcite efficiently catalyzes the oxidation of alkylarenes with molecular oxygen
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Developing efficient heterogeneous catalytic systems based on easily available materials and molecular oxygen for the selective oxidation of alkylarenes is highly desirable. In the present research, NiMn hydrotalcite (Ni2Mn-LDH) has been found as an efficient catalyst in the oxidation of alkylarenes using molecular oxygen as the sole oxidant without any additive. Impressive catalytic performance, excellent stability and recyclability, broad applicable scope and practical potential for the catalytic system have been observed. Mn3+ species was proposed to be the efficient active site, and Ni2+ played an important role in stabilizing the Mn3+ species in the hydrotalcite structure. The kinetic study showed that the aerobic oxidation of diphenylmethane is a first-order reaction over Ni2Mn-LDH with the activation energy (Ea) and pre-exponential factor (A0) being 85.7 kJ mol?1 and 1.8 × 109 min?1, respectively. The Gibbs free energy (ΔG≠) was determined to be -10.4 kJ mol-1 K-1 for the oxidation based on Eyring-Polanyi equation, indicating the reaction is exergonic. The mechanism study indicated that the reaction proceeded through both radical and carbocation intermediates. The two species were then trapped by molecular oxygen and H2O or hydroxyl species, respectively, to yield the corresponding products. The present research might provide information for constructing highly efficient and stable active site for the catalytic aerobic oxidation based on available and economic material.
- Wang, Anwei,Zhou, WeiYou,Sun, Zhonghua,Zhang, Zhong,Zhang, Zhihui,He, MingYang,Chen, Qun
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- A bioinspired oxoiron(iv) motif supported on a N2S2macrocyclic ligand
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A mononuclear oxoiron(iv) complex1-transbearing two equatorial sulfur ligations is synthesized and characterized as an active-site model of the elusive sulfur-ligated FeIVO intermediates in non-heme iron oxygenases. The introduction of sulfur ligands weakens the Fe-O bond and enhances the oxidative reactivity of the FeIVO unit with a diminished deuterium kinetic isotope effect, thereby providing a compelling rationale for nature's use of thecis-thiolate ligated oxoiron(iv) motif in key metabolic transformations.
- Deutscher, Jennifer,Gerschel, Philipp,Warm, Katrin,Kuhlmann, Uwe,Mebs, Stefan,Haumann, Michael,Dau, Holger,Hildebrandt, Peter,Apfel, Ulf-Peter,Ray, Kallol
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supporting information
p. 2947 - 2950
(2021/03/24)
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- Radical Anion Promoted Chemoselective Cleavage of Csp2-S Bond Enables Formal Cross-Coupling of Aryl Methyl Sulfones with Alcohols
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A novel formal cross-coupling of aryl methyl sulfones and alcohols affording alkyl aryl ethers via an SRN1 pathway is developed. Two marketed antitubercular drugs were efficiently prepared employing this approach as the key step. A dimsyl-anion initiated radical chain process was revealed as the major pathway. DFT calculations indicate that the formation of a radical anion via nucleophilic addition of alkoxide to the aryl radical is the key step in determining the observed chemoselectivity.
- Bai, Jixiang,Wang, Tianxin,Dai, Botao,Liu, Qingchao,Yu, Peiyuan,Jia, Tiezheng
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supporting information
p. 5761 - 5765
(2021/08/16)
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- Formation of cobalt-oxygen intermediates by dioxygen activation at a mononuclear nonheme cobalt(ii) center
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A mononuclear nonheme cobalt(ii) complex, [(TMG3tren)CoII(OTf)](OTf) (1), activates dioxygen in the presence of hydrogen atom donor substrates, such as tetrahydrofuran and cyclohexene, resulting in the generation of a cobalt(ii)-alkylperoxide intermediate (2), which then converts to the previously reported cobalt(iv)-oxo complex, [(TMG3tren)CoIV(O)]2+-(Sc(OTf)3)n(3), in >90% yield upon addition of a redox-inactive metal ion, Sc(OTf)3. Intermediates2and3represent the cobalt analogues of the proposed iron(ii)-alkylperoxide precursor that converts to an iron(iv)-oxo intermediateviaO-O bond heterolysis in pterin-dependent nonheme iron oxygenases. In reactivity studies,2shows an amphoteric reactivity in electrophilic and nucleophilic reactions, whereas3is an electrophilic oxidant. To the best of our knowledge, the present study reports the first example showing the generation of cobalt-oxygen intermediates by activating dioxygen at a cobalt(ii) center and the reactivities of the cobalt-oxygen intermediates in oxidation reaction.
- Chandra, Anirban,Dau, Holger,Farquhar, Erik R.,Lee, Yong-Min,Malik, Deesha D.,Mebs, Stefan,Nam, Wonwoo,Ray, Kallol,Seo, Mi Sook
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supporting information
p. 11889 - 11898
(2021/09/06)
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- Combining Structural with Functional Model Properties in Iron Synthetic Analogue Complexes for the Active Site in Rabbit Lipoxygenase
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Iron complexes that model the structural and functional properties of the active iron site in rabbit lipoxygenase are described. The ligand sphere of the mononuclear pseudo-octahedral cis-(carboxylato)(hydroxo)iron(III) complex, which is completed by a tetraazamacrocyclic ligand, reproduces the first coordination shell of the active site in the enzyme. In addition, two corresponding iron(II) complexes are presented that differ in the coordination of a water molecule. In their structural and electronic properties, both the (hydroxo)iron(III) and the (aqua)iron(II) complex reflect well the only two essential states found in the enzymatic mechanism of peroxidation of polyunsaturated fatty acids. Furthermore, the ferric complex is shown to undergo hydrogen atom abstraction reactions with O-H and C-H bonds of suitable substrates, and the bond dissociation free energy of the coordinated water ligand of the ferrous complex is determined to be 72.4 kcal·mol-1. Theoretical investigations of the reactivity support a concerted proton-coupled electron transfer mechanism in close analogy to the initial step in the enzymatic mechanism. The propensity of the (hydroxo)iron(III) complex to undergo H atom abstraction reactions is the basis for its catalytic function in the aerobic peroxidation of 2,4,6-tri(tert-butyl)phenol and its role as a radical initiator in the reaction of dihydroanthracene with oxygen.
- Bonck, Thorsten,De Waal Malefijt, Matina Elo?se,Dobbelaar, Emiel,Kelm, Harald,Klein, Johannes E. M. N.,Krüger, Hans-J?rg,Rauber, Christian,Schmitz, Markus
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supporting information
p. 13145 - 13155
(2021/09/03)
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- Tin(II)–Nitrene Radical Complexes Formed by Electron Transfer from Redox-Active Ligand to Organic Azides and Their Reactivity in C(sp3)–H Activation
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A tin(II) complex coordinated by a sterically demanding o-phenylenediamido ligand is synthesized. The ligand is redox-active to reach a tin(II) complex with the diiminobenzosemiquinone radial anion in the oxidation by AgPF6. The tin(II) complex reacts with a series of nosylazides (x-NO2C6H4–SO2–N3; x = o, m, or p) at ?30 °C to yield the corresponding nitrene radical bound tin(II) complexes. The nitrene radical complexes exhibit C(sp3)–H activation and amination reactivity.
- Sugimoto, Hideki,Yano, Mayuka,Sato, Kazunobu,Miyanishi, Mayuko,Sugisaki, Kenji,Shiota, Yoshihito,Kaga, Akira,Yoshizawa, Kazunari,Itoh, Shinobu
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supporting information
p. 18603 - 18607
(2021/12/01)
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- Rapid Iron(III)?Fluoride-Mediated Hydrogen Atom Transfer
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We anticipate high-valent metal–fluoride species will be highly effective hydrogen atom transfer (HAT) oxidants because of the magnitude of the H?F bond (in the product) that drives HAT oxidation. We prepared a dimeric FeIII(F)?F?FeIII(F) complex (1) by reacting [FeII(NCCH3)2(TPA)](ClO4)2 (TPA=tris(2-pyridylmethyl)amine) with difluoro(phenyl)-λ3-iodane (difluoroiodobenzene). 1 was a sluggish oxidant, however, it was readily activated by reaction with Lewis or Br?nsted acids to yield a monomeric [FeIII(TPA)(F)(X)]+ complex (2) where X=F/OTf. 1 and 2 were characterized using NMR, EPR, UV/Vis, and FT-IR spectroscopies and mass spectrometry. 2 was a remarkably reactive FeIII reagent for oxidative C?H activation, demonstrating reaction rates for hydrocarbon HAT comparable to the most reactive FeIII and FeIV oxidants.
- Doyle, Lorna M.,Gericke, Robert,McDonald, Aidan R.,Panda, Chakadola
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supporting information
p. 26281 - 26286
(2021/11/12)
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- g-C3N4/metal halide perovskite composites as photocatalysts for singlet oxygen generation processes for the preparation of various oxidized synthons
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g-C3N4/metal halide perovskite composites were prepared and used for the first time as photocatalysts forin situ1O2generation to perform hetero Diels-Alder, ene and oxidation reactions with suitable dienes and alkenes. The standardized methodology was made applicable to a variety of olefinic substrates. The scope of the method is finely illustrated and the reactions afforded desymmetrized hydroxy-ketone derivatives, unsaturated ketones and epoxides. Some limitations were also observed, especially in the case of the alkene oxidations, and poor chemoselectivity was somewhere observed in this work which is the first application of MHP-based composites forin situ1O2generation. The experimental protocol can be used as a platform to further expand the knowledge and applicability of MHPs to organic reactions, since perovskites offer a rich variety of tuning strategies which may be explored to improve reaction yields and selectivities.
- Corti, Marco,Chiara, Rossella,Romani, Lidia,Mannucci, Barbara,Malavasi, Lorenzo,Quadrelli, Paolo
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p. 2292 - 2298
(2021/04/12)
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- Insight into the chemoselective aromatic: Vs. side-chain hydroxylation of alkylaromatics with H2O2catalyzed by a non-heme imine-based iron complex
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The oxidation of a series of alkylaromatic compounds with H2O2 catalyzed by an imine-based non-heme iron complex prepared in situ by reaction of 2-picolylaldehyde, 2-picolylamine, and Fe(OTf)2 in a 2?:?2?:?1 ratio leads to a marked chemoselectivity for aromatic ring hydroxylation over side-chain oxidation. This selectivity is herein investigated in detail. Side-chain/ring oxygenated product ratio was found to increase upon decreasing the bond dissociation energy (BDE) of the benzylic C-H bond in line with expectation. Evidence for competitive reactions leading either to aromatic hydroxylation via electrophilic aromatic substitution or side-chain oxidation via benzylic hydrogen atom abstraction, promoted by a metal-based oxidant, has been provided by kinetic isotope effect analysis. This journal is
- Ticconi, Barbara,Capocasa, Giorgio,Cerrato, Andrea,Di Stefano, Stefano,Lapi, Andrea,Marincioni, Beatrice,Olivo, Giorgio,Lanzalunga, Osvaldo
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p. 171 - 178
(2021/01/28)
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- Ligand-Constraint-Induced Peroxide Activation for Electrophilic Reactivity
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μ-1,2-peroxo-bridged diiron(III) intermediates P are proposed as reactive intermediates in various biological oxidation reactions. In sMMO, P acts as an electrophile, and performs hydrogen atom and oxygen atom transfers to electron-rich substrates. In cyanobacterial ADO, however, P is postulated to react by nucleophilic attack on electrophilic carbon atoms. In biomimetic studies, the ability of μ-1,2-peroxo-bridged dimetal complexes of Fe, Co, Ni and Cu to act as nucleophiles that effect deformylation of aldehydes is documented. By performing reactivity and theoretical studies on an end-on μ-1,2-peroxodicobalt(III) complex 1 involving a non-heme ligand system, L1, supported on a Sn6O6 stannoxane core, we now show that a peroxo-bridged dimetal complex can also be a reactive electrophile. The observed electrophilic chemistry, which is induced by the constraints provided by the Sn6O6 core, represents a new domain for metal?peroxide reactivity.
- Ansari, Mursaleem,Chandra, Anirban,Kundu, Subrata,Monte-Pérez, Inés,Rajaraman, Gopalan,Ray, Kallol
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supporting information
p. 14954 - 14959
(2021/06/01)
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- A facile and versatile electro-reductive system for hydrodefunctionalization under ambient conditions
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A general electrochemical system for reductive hydrodefunctionalization is described, employing the inexpensive and easily available triethylamine (Et3N) as a sacrificial reductant. This protocol is characterized by facile operation, sustainable conditions, and exceptionally wide substrate scope covering the cleavage of C-halogen, N-S, N-C, O-S, O-C, C-C and C-N bonds. Notably, the selectivity and capability of reduction can be conveniently switched by simple incorporation or removal of an alcohol as a co-solvent.
- Huang, Binbin,Guo, Lin,Xia, Wujiong
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supporting information
p. 2095 - 2103
(2021/03/26)
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- Ni-catalyzed reductive decyanation of nitriles with ethanol as the reductant
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A nickel-catalyzed reductive decyanation of aromatic nitriles has been developed, in which the readily available and abundant ethanol was applied as the hydride donor. Various functional groups on the aromatic rings, such as alkoxyl, amino, imino and amide, were compatible in this catalytic protocol. Heteroaryl, benzylic and alkenyl nitriles were also tolerated. Mechanistic investigation indicated that ethanol provided hydride efficientlyviaβ-hydride elimination in this reductive decyanation.
- Wu, Ke,Ling, Yichen,Sun, Nan,Hu, Baoxiang,Shen, Zhenlu,Jin, Liqun,Hu, Xinquan
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supporting information
p. 2273 - 2276
(2021/03/09)
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- Dual Reactivity of 1,2,3,4-Tetrazole: Manganese-Catalyzed Click Reaction and Denitrogenative Annulation
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A general catalytic method using a Mn-porphyrin-based catalytic system is reported that enables two different reactions (click reaction and denitrogenative annulation) and affords two different classes of nitrogen heterocycles, 1,5-disubstituted 1,2,3-triazoles (with a pyridyl motif) and 1,2,4-triazolo-pyridines. Mechanistic investigations suggest that although the click reaction likely proceeds through an ionic mechanism, which is different from the traditional click reaction, the denitrogenative annulation reaction likely proceeds via an electrophilic metallonitrene intermediate rather than a metalloradical intermediate. Collectively, this method is highly efficient and offers several advantages over other methods. For example, this method excludes a multi-step synthesis of the N-heterocyclic molecules described and produces only environmentally benign N2 gas a by-product.
- Chattopadhyay, Buddhadeb,Das, Sandip Kumar,Khatua, Hillol,Roy, Satyajit
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supporting information
p. 304 - 312
(2020/10/29)
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- Small-Molecule Investigation of Diels-Alder Complexes for Thermoreversible Crosslinking in Polymeric Applications
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Combinations of dienes and dienophiles were examined in order to elicit possible combinations for thermoreversible crosslinking units. Comparison of experimental results and quantum calculations indicated that reaction kinetics and activation energy were much better prediction factors than change in enthalpy for the prediction of successful cycloaddition. Further testing on diene-dienophile pairs that underwent successful cycloaddition determined the feasibility of thermoreversibility/retro-reaction of each of the Diels-Alder compounds. Heating and testing of the compounds in the presence of a trapping agent allowed for experimental determination of reverse kinetics and activation energy for the retro-reaction. The experimental values were in good agreement with quantum calculations. The combination of chemical calculations with experimental results provided a strong insight into the structure-property relationships and how quantum calculations can be used to examine the feasibility of the thermoreversibility of new Diels-Alder complexes in potential polymer systems or to fine-tune thermoreversible Diels-Alder systems already in use.
- Rowlett, Jarrett R.,Deglmann, Peter,Sprafke, Johannes,Roy, Nabarun,Mülhaupt, Rolf,Bruchmann, Bernd
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p. 8933 - 8944
(2021/07/20)
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- Decarboxylation of Aromatic Carboxylic Acids by the Prenylated-FMN-dependent Enzyme Phenazine-1-carboxylic Acid Decarboxylase
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Phenazine-1-carboxylic acid decarboxylase (PhdA) is a member of the expanding class of prenylated-FMN-dependent (prFMN) decarboxylase enzymes. These enzymes have attracted interest for their ability to catalyze (de)carboxylation reactions on aromatic rings and conjugated double bonds. Here we describe a method to reconstitute PhdA with prFMN that produces an active and stable form of the holo-enzyme that does not require prereduction with dithionite for activity. We establish that oxidized phenazine-1-carboxylate (PCA) is the substrate for decarboxylation, withkcat= 2.6 s-1andKM= 53 μM. PhdA also catalyzes the much slower exchange of solvent deuterium into the product, phenazine, with an apparent turnover number of 0.8 min-1. The enzyme was found to catalyze the decarboxylation of a broad range of polyaromatic carboxylic acids, including anthracene-1-carboxylic acid. Previously described prFMN-dependent aromatic (de)carboxylases have utilized electron-rich phenolic or heterocyclic molecules as substrates. PhdA extends the substrate range of prFMN-dependent (de)carboxylases to electron-poor and unfunctionalized aromatic systems, suggesting that it may prove a useful catalyst for the regioselective (de)carboxylation of otherwise unreactive aromatic molecules.
- Datar, Prathamesh M.,Marsh, E. Neil G.
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p. 11723 - 11732
(2021/09/28)
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- Substituted 9-Anthraldehydes from Dibenzocycloheptanol Epoxides via Acid-Catalyzed Epoxide Opening/Semipinacol Rearrangement
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Starting from benzaldehyde derivatives, the corresponding dibenzocycloheptenol could be prepared in five steps. Under both substrate (secondary vs tertiary alcohol and the substituents on the aromatic ring(s)) and condition control, the subsequent epoxidation and acid-catalyzed epoxide opening/semipinacol rearrangement/aromatization afforded the corresponding 9-anthraldehydes in good yields, up to 88% over two steps. The presence of the electron-withdrawing group(s) on the aromatic ring(s) suppressed the rate of the epoxidation while the subsequent semipinacol rearrangement step required heating; the presence of the electron-donating group(s), on the other hand, frequently led to the decomposition during the epoxidation. From the mechanistic studies, the semipinacol rearrangement of the epoxide could precede the ionization at the bisbenzylic position, yielding the aldehyde intermediate. The ensuing dehydrative aromatization led to the formation of 9-anthraldehyde. Conversely, nucleophilic addition to the aldehyde and dehydrative aromatization with concomitant loss of formic acid led to anthracene.
- Phumjan, Tanawat,Songthammawat, Poramate,Jongcharoenkamol, Jira,Batsomboon, Paratchata,Ruchirawat, Somsak,Ploypradith, Poonsakdi
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p. 13322 - 13349
(2021/09/13)
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- Frustrated Lewis Pair Stabilized Phosphoryl Nitride (NPO), a Monophosphorus Analogue of Nitrous Oxide (N2O)
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Phosphoryl nitride (NPO) is a highly reactive intermediate, and its chemistry has only been explored under matrix isolation conditions so far. Here we report the synthesis of an anthracene (A) and phosphoryl azide based molecule (N3P(O)A) that acts as a molecular synthon of NPO. Experimentally, N3P(O)A dissociates thermally with a first-order kinetic half-life that is associated with an activation enthalpy of ΔH? = 27.5 ± 0.3 kcal mol–1 and an activation entropy of ΔS? = 10.6 ± 0.3 cal mol–1 K–1 that are in good agreement with calculated DLPNO-CCSD(T)/cc-pVTZ//PBE0-D3(BJ)/cc-pVTZ energies. In solution N3P(O)A undergoes Staudinger reactivity with tricyclohexylphosphine (PCy3) and subsequent complexation with tris(pentafluorophenyl)borane (B(C6F5)3, BCF) to form Cy3P?NP(A)O–B(C6F5)3. Anthracene is cleaved off photochemically to form the frustrated Lewis pair (FLP) stabilized NPO complex Cy3P⊕–N═P–O–B?(C6F5)3. An intrinsic bond orbital (IBO) analysis suggests that the adduct is zwitterionic, with a positive and negative charge localized on the complexing Cy3P and BCF, respectively.
- Eckhardt, André K.,Riu, Martin-Louis Y.,Müller, Peter,Cummins, Christopher C.
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p. 21252 - 21257
(2021/12/27)
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- A Synthetic Model for the Possible FeIV2(μ-O)2Core of Methane Monooxygenase Intermediate Q Derived from a Structurally Characterized FeIIIFeIV(μ-O)2Complex
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A bis(μ-oxo)diiron(IV,IV) complex as a model for intermediate Q in the methane monooxygenase reaction cycle has been prepared. The precursor complex with a [FeIIIFeIV(μ-O)2] core was fully characterized by X-ray crystallography and other spectroscopic analyses and was converted to the [FeIV2(μ-O)2] complex via electrochemical oxidation at 1000 mV (vs Ag/Ag+) in acetone at 193 K. The UV-vis spectral features, M?ssbauer parameters (ΔEQ = 2.079 mm/s and δ= -0.027 mm/s), and EXAFS analysis (Fe-O/N = 1.73/1.96 ? and Fe···Fe = 2.76 ?) support the structure of the low-spin (S = 1, for each Fe) [FeIV2(μ-O)2] core. The rate constants of the hydrogen abstraction reaction from 9,10-dihydroanthracene at 243 K suggest the high reactivity of these synthetic bis(μ-oxo)diiron complexes supported by simple N4 tripodal ligand.
- Aono, Yuri,Harada, Masafumi,Kajiwara, Atsushi,Katano, Hajime,Kobayashi, Yoshio,Kodera, Masahito,Kotegawa, Fukue,Kubo, Minoru,Matsumoto, Arimasa,Mikata, Yuji,Nakayama, Hiromi,Yamamoto, Chihiro,Yanagisawa, Sachiko
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supporting information
(2021/12/09)
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- Molybdenum-Catalyzed Deoxygenation Coupling of Lignin-Derived Alcohols for Functionalized Bibenzyl Chemicals
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With the growing demand for sustainability and reducing CO2 footprint, lignocellulosic biomass has attracted much attention as a renewable, carbon-neutral and low-cost feedstock for the production of chemicals and fuels. To realize efficient utilization of biomass resource, it is essential to selectively alter the high degree of oxygen functionality of biomass-derivates. Herein, we introduced a novel procedure to transform renewable lignin-derived alcohols to various functionalized bibenzyl chemicals. This strategy relied on a short deoxygenation coupling pathway with economical molybdenum catalyst. A well-designed H-donor experiment was performed to investigate the mechanism of this Mo-catalyzed process. It was proven that benzyl carbon-radical was the most possible intermediate to form the bibenzyl products. It was also discovered that the para methoxy and phenolic hydroxyl groups could stabilize the corresponding radical intermediates and then facilitate to selectively obtain bibenzyl products. Our research provides a promising application to produce functionalized aromatics from biomass-derived materials.
- Jiang, Huifang,Lu, Rui,Luo, Xiaolin,Si, Xiaoqin,Xu, Jie,Lu, Fang
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p. 1292 - 1296
(2020/12/09)
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- Highly Active and Robust Ruthenium Complexes Based on Hemilability of Hybrid Ligands for C-H Oxidation
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Evaluation of the hemilability of hybrid ligands provides a key to understand the metal-ligand cooperation in transition metal catalysis. Here, we design and synthesize a type of RuII complexes based on the hemilability of N-heterocyclic carbenes (NHCs), pyridine, and pyrazole, to compare their activity with other reported Ru catalysts in benzylic C-H oxidation. The RuII catalysts showed ultrastrong catalytic activity in water at room temperature and achieved a turnover frequency (TOF) of 114 h-1, which is the highest TOF value ever reported for Ru-catalyzed benzylic C-H oxidation. The addition of tridentate hybrid ligands in the Ru central position has two beneficial effects: NHCs with a stronger donor ability stabilize the Ru center; however, nitrogen ligands with a relatively weaker donor ability release from the Ru center, so that they induce a reaction. UV-vis, high-resolution electrospray ionization mass spectrometry (ESI-MS), electron paramagnetic resonance (EPR) spectrometry, the trapping of radicals, and the density functional theory calculations (DFT) suggested that a cation catalyst L-RuII-tBuO2H is formed via the reaction between starting RuII catalysts and tert-butyl hydroperoxide, which further undergoes a cleavage of the O-O bond to generate a radical and a cation L-RuIII-OH active intermediate.
- Bo, Chun-Bo,Bu, Qingqing,Li, Xue,Ma, Ge,Wei, Donghui,Guo, Cheng,Dai, Bin,Liu, Ning
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p. 4324 - 4334
(2020/03/05)
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- Visible-Light-Induced, Base-Promoted Transition-Metal-Free Dehalogenation of Aryl Fluorides, Chlorides, Bromides, and Iodides
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We report a simple and efficient visible-light-induced transition-metal-free hydrogenation of aryl halides. The combined visible light and base system is used to initiate the desired radical-mediated hydrogenation. A variety of aryl fluorides, chlorides, bromides, and iodides could be reduced to the corresponding (hetero)arenes with excellent yields under mild conditions. Various functional groups and other heterocyclic compounds are tolerated.
- Ding, Ting-Hui,Qu, Jian-Ping,Kang, Yan-Biao
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supporting information
p. 3084 - 3088
(2020/04/10)
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- Exploiting the radical reactivity of diazaphosphinanes in hydrodehalogenations and cascade cyclizations
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The remarkable reducibility of diazaphosphinanes has been extensively applied in various hydrogenations, based on and yet limited by their well-known hydridic reactivity. Here we exploited their unprecedented radical reactivity to implement hydrodehalogenations and cascade cyclizations originally inaccessible by hydride transfer. These reactions feature a broad substrate scope, high efficiency and simplicity of manipulation. Mechanistic studies suggested a radical chain process in which a phosphinyl radical is generated in a catalytic cycle via hydrogen-atom transfer from diazaphosphinanes. The radical reactivity of diazaphosphinanes disclosed here differs from their well-established hydridic reactivity, and hence, opens a new avenue for diazaphosphinane applications in organic syntheses.
- Cheng, Jin-Pei,Yang, Jin-Dong,Zhang, Jingjing
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p. 4786 - 4790
(2020/06/18)
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- Catalytic Reductions Without External Hydrogen Gas: Broad Scope Hydrogenations with Tetrahydroxydiboron and a Tertiary Amine
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Facile reduction of aryl halides with a combination of 5% Pd/C, B2(OH)4, and 4-methylmorpholine is reported. Aryl bromides, iodides, and chlorides were efficiently reduced. Aryl dihalides containing two different halogen atoms underwent selective reduction: I over Br and Cl, and Br over Cl. Beyond these, aryl triflates were efficiently reduced. This combination was broadly general, effectuating reductions of benzylic halides and ethers, alkenes, alkynes, aldehydes, and azides, as well as for N-Cbz deprotection. A cyano group was unaffected, but a nitro group and a ketone underwent reduction to a low extent. When B2(OD)4 was used for aryl halide reduction, a significant amount of deuteriation occurred. However, H atom incorporation competed and increased in slower reactions. 4-Methylmorpholine was identified as a possible source of H atoms in this, but a combination of only 4-methylmorpholine and Pd/C did not result in reduction. Hydrogen gas has been observed to form with this reagent combination. Experiments aimed at understanding the chemistry led to the proposal of a plausible mechanism and to the identification of N,N-bis(methyl-d3)pyridin-4-amine (DMAP-d6) and B2(OD)4 as an effective combination for full aromatic deuteriation. (Figure presented.).
- Korvinson, Kirill A.,Akula, Hari K.,Malinchak, Casina T.,Sebastian, Dellamol,Wei, Wei,Khandaker, Tashrique A.,Andrzejewska, Magdalena R.,Zajc, Barbara,Lakshman, Mahesh K.
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supporting information
p. 166 - 176
(2020/01/02)
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- Nickel-Catalyzed Electrosynthesis of Aryl and Vinyl Phosphinates
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A mild and useful nickel-catalyzed electrochemical phosphonylation of aryl and vinyl bromides is described. We show that alkyl H-phenylphosphinates can be coupled electrochemically with functionalized aryl and vinyl bromides using very simple conditions (Fe/Ni anode, bench-stable nickel pre-catalyst, undivided cell, galvanostatic electrolysis) to furnish the corresponding aryl and vinyl phosphinates in satisfactory to good yields. Couplings can also be applied to heteroaromatic bromides with some limitations like increased propensity to hydro-dehalogenation.
- Daili, Farah,Ouarti, Abdelhakim,Pinaud, Marine,Kribii, Ibtihal,Sengmany, Stéphane,Le Gall, Erwan,Léonel, Eric
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supporting information
p. 3452 - 3455
(2020/05/25)
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- The Electronic Properties of Ni(PNN) Pincer Complexes Modulate Activity in Catalytic Hydrodehalogenation Reactions
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Three chloronickel(II) complexes of PNN- pincer ligands with pyrazolyl and diphenylphosphino donors appended to different arms of diarylamido anchors were prepared and fully characterized. The three derivatives (1-OMe, 1-Me, 1-CF3) differ only by the identity of the para-aryl substituent on the pyrazolyl arm with 1-OMe being 310 mV easier to oxidize than 1-CF3. All three complexes are competent catalysts for hydrodehalogenation reactions of 1-bromooctane and a variety of aryl halides in dimethylacetamide using NaBH4 as both base and hydride source. Comparative studies using diverse substrates showed that catalytic activity correlates with electron donor properties; 1-OMe was superior to the other two. Deuterium labeling studies verified NaBD4 as the deuteride source and excluded solvent-assisted radical pathways.
- Wang, Denan,Gardinier, James R.
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p. 4425 - 4434
(2020/10/19)
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- Transition-Metal-Free and Visible-Light-Mediated Desulfonylation and Dehalogenation Reactions: Hantzsch Ester Anion as Electron and Hydrogen Atom Donor
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Novel approaches for N- and O-desulfonylation under room temperature (rt) and transition-metal-free conditions have been developed. The first methodology involves the transformation of a variety of N-sulfonyl heterocycles and phenyl benzenesulfonates to the corresponding desulfonylated products in good to excellent yields using only KOtBu in dimethyl sulfoxide (DMSO) at rt. Alternately, a visible light method has been used for deprotection of N-methyl-N-arylsulfonamides with Hantzsch ester (HE) anion serving as the visible-light-absorbing reagent and electron and hydrogen atom donor to promote the desulfonylation reaction. The HE anion can be easily prepared in situ by reaction of the corresponding HE with KOtBu in DMSO at rt. Both protocols were further explored in terms of synthetic scope as well as mechanistic aspects to rationalize key features of desulfonylation processes. Furthermore, the HE anion induces reductive dehalogenation reaction of aryl halides under visible light irradiation.
- Heredia, Micaela D.,Guerra, Walter D.,Barolo, Silvia M.,Fornasier, Santiago J.,Rossi, Roberto A.,Budén, Mariá E.
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p. 13481 - 13494
(2020/12/15)
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- The first crystallographically characterised ruthenium(vi) alkylimido porphyrin competent for aerobic epoxidation and hydrogen atom abstraction
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The syntheses of [RuVI(Por)(NAd)(O)] and [RuVI(2,6-F2-TPP)(NAd)2] have been described. [RuVI(2,6-F2-TPP)(NAd)(O)] capable of catalysing aerobic epoxidation of alkenes has been characterised by X-ray crystallography with RuNAd and RuO bond distances being 1.778(5) ? and 1.760(4) ? (∠O-Ru-NAd: 174.37(19)°), respectively. Its first reduction potential is 740 mV cathodically shifted from that of [RuVI(2,6-F2-TPP)(O)2].
- Chang, Xiao-Yong,Che, Chi-Ming,Shing, Ka-Pan,Wan, Qingyun
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supporting information
p. 4428 - 4431
(2020/05/05)
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- Manganese(Ⅲ)-iodosylbenzene complex, preparation method thereof and oxidant comprising the same
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The present invention relates to a manganese(III)-iodosylbenzene complex, a preparation method thereof, and an oxidant comprising the same. The manganese(III)-iodosylbenzene complex provided in one aspect of the present invention has an effect of inducing a hydrogen atom abstraction (HAA) reaction of cyclohexadiene, dihydroanthracene and xanthine, and an oxygen atom transfer (OAT) reaction of thioanisole and stilbene with excellent electrophilic reactivity. The manganese(III)-iodosylbenzene complex is represented by a compound of formula 1: [Mn^III(L)(OIPh)(OH)]^2+.COPYRIGHT KIPO 2020
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Paragraph 0130-0136; 0166
(2020/09/22)
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- Effect of Ligand Fields on the Reactivity of O2-Activating Iron(II)-Benzilate Complexes of Neutral N5 Donor Ligands
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Three new iron(II)-benzilate complexes [(N4Py)FeII(benzilate)]ClO4 (1), [(N4PyMe2)FeII(benzilate)]ClO4 (2) and [(N4PyMe4)FeII(benzilate)]ClO4 (3) of neutral pentadentate nitrogen donor ligands have been isolated and characterized to study their dioxygen reactivity. Single-crystal X-ray structures reveal a mononuclear six-coordinate iron(II) center in each case, where benzilate binds to the iron center in monodentate mode via one carboxylate oxygen. Introduction of methyl groups in the 6-positions of the pyridine rings makes the N4PyMe2 and N4PyMe4 ligand fields weaker compared to that of the parent N4Py ligand. All the complexes (1–3) react with dioxygen to decarboxylate the coordinated benzilate to benzophenone quantitatively. The decarboxylation is faster for the complex of the more sterically hindered ligand and follows the order 3>2>1. The complexes display oxygen atom transfer reactivity to thioanisole and also exhibit hydrogen atom transfer reactions with substrates containing weak C?H bonds. Based on interception studies with external substrates, labelling experiments and Hammett analysis, a nucleophilic iron(II)-hydroperoxo species is proposed to form upon two-electron reductive activation of dioxygen by each iron(II)-benzilate complex. The nucleophilic oxidants are converted to the corresponding electrophilic iron(IV)-oxo oxidant upon treatment with a protic acid. The high-spin iron(II)-benzilate complex with the weakest ligand field results in the formation of a more reactive iron-oxygen oxidant.
- Bhattacharya, Shrabanti,Paine, Tapan Kanti,Singh, Reena
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- Iridium(III)-Catalyzed Intermolecular C(sp3)?H Insertion Reaction of Quinoid Carbene: A Radical Mechanism
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Described herein is an IrIII/porphyrin-catalyzed intermolecular C(sp3)?H insertion reaction of a quinoid carbene (QC). The reaction was designed by harnessing the hydrogen-atom transfer (HAT) reactivity of a metal-QC species with aliphatic substrates followed by a radical rebound process to afford C?H arylation products. This methodology is efficient for the arylation of activated hydrocarbons such as 1,4-cyclohexadienes (down to 40 min reaction time, up to 99 % yield, up to 1.0 g scale). It features unique regioselectivity, which is mainly governed by steric effects, as the insertion into primary C?H bonds is favored over secondary and/or tertiary C?H bonds in the substituted cyclohexene substrates. Mechanistic studies revealed a radical mechanism for the reaction.
- Wang, Hai-Xu,Richard, Yann,Wan, Qingyun,Zhou, Cong-Ying,Che, Chi-Ming
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supporting information
p. 1845 - 1850
(2019/12/27)
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- Concerted proton-electron transfer oxidation of phenols and hydrocarbons by a high-valent nickel complex
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The high-valent nickel(iii) complex Ni(pyalk)2+ (2) was prepared by oxidation of a nickel(ii) complex, Ni(pyalk)2 (1) (pyalk = 2-pyridyl-2-propanoate). 2 and derivatives were fully characterized by mass spectrometry and X-ray crystallography. Electron paramagnetic resonance spectroscopy and X-ray photoelectron spectroscopy confirm that the oxidation is metal-centered. 2 was found to react with a variety of phenolic and hydrocarbon substrates. A linear correlation between the measured rate constant and the substrate bond dissociation enthalpy (BDE) was found for both phenolic and hydrocarbon substrates. Large H/D kinetic isotope effects were also observed for both sets of substrates. These results suggest that 2 reacts through concerted proton-electron transfer (CPET). Analysis of measured thermodynamic parameters allows us to calculate a bond dissociation free energy (BDFE) of ～91 kcal mol-1 for the O-H bond of the bound pyalk ligand. These findings may shed light onto CPET steps in oxidative catalysis and have implications for ligand design in catalytic systems.
- Fisher, Katherine J.,Feuer, Margalit L.,Lant, Hannah M. C.,Mercado, Brandon Q.,Crabtree, Robert H.,Brudvig, Gary W.
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p. 1683 - 1690
(2020/02/25)
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- Erratum: Experimental Evidence for p Ka-Driven Asynchronicity in C-H Activation by a Terminal Co(III)-Oxo Complex (Journal of the American Chemical Society (2019) 141:9 (4051?4062) DOI: 10.1021/jacs.8b13490)
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It has been brought to our attention that our previous analysis of kinetic data for C-H activation was incomplete. For the reaction of PhB(tBuIm)3CoIIIO (3) with various substrates, the observed rate constants, kobs, were originally determined by plotting the natural log of the UV-vis absorbance at 470 nm at time t (At) divided by the initial absorbance (A0) versus time in seconds to give kobs as the slope of the linear fit of the data. However, this method implicitly assumes that the absorbance at infinite time, Ainf, is equal to 0. This is not true for these reactions due to absorbances from the reaction products, resulting in an underestimation of the true kobs values for each substrate. In order to obtain more accurate kobs values, the data were reanalyzed by fitting the absorbance versus time data at 470 nm to an exponential function, At = Ainf + (A0 - Ainf) e-kt. This method explicitly accounts for Ainf in the fitting parameters, providing more accurate values for the kobs for each substrate. For the Eyring analysis of the reaction between 3 and 9,10-dihydroanthracene, the noise and subsequent error in the measurements precluded good exponential fits of the variable-temperature data. Therefore, these data were analyzed by finding kobs as the slope of the linear fit of a plot of ln((At - Ainf)/(A0 - Ainf)) versus t in seconds, where Ainf was estimated from reactions carried out in a cuvette at room temperature. These new analyses result in the following corrections to the original Article.
- Anderson, John S.,Goetz, McKenna K.
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supporting information
(2020/04/09)
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- Hydrogen Atom Transfer Oxidation by a Gold-Hydroxide Complex
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AuIII-oxygen adducts have been implicated as intermediates in homogeneous and heterogeneous Au oxidation catalysis, but their reactivity is under-explored. Complex 1, ([AuIII(OH)(terpy)](ClO4)2, (terpy = 2,2′:6′,2-terpyridine), readily oxidized substrates bearing C-H and O-H bonds. Kinetic analysis revealed that the oxidation occurred through a hydrogen atom transfer (HAT) mechanism. Stable radicals were detected and quantified as products of almost quantitative HAT oxidation of alcohols by 1. Our findings highlight the possible role of AuIII-oxygen adducts in oxidation catalysis and the capability of late transition metal-oxygen adducts to perform proton coupled electron transfer.
- Lovisari, Marta,McDonald, Aidan R.
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supporting information
(2020/03/13)
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- Oxo-Free Hydrocarbon Oxidation by an Iron(III)-Isoporphyrin Complex
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Metal-halides that perform proton coupled electron-transfer (PCET) oxidation are an important new class of high-valent oxidant. In investigating metal-dihalides, we reacted [FeIII(Cl)(T(OMe)PP)] (1, T(OMe)PP = meso-tetra(4-methoxyphenyl)porphyrinyl) with (dichloroiodo)benzene. An FeIII-meso-chloro-isoporphyrin complex [FeIII(Cl)2(T(OMe)PP-Cl)] (2) was obtained. 2 was characterized by electronic absorption, 1H NMR, EPR, and X-ray absorption spectroscopies and mass spectrometry with support from computational analyses. 2 was reacted with a series of hydrocarbon substrates. The measured kinetic data exhibited a nonlinear behavior, whereby the oxidation followed a hydrogen-atom-transfer (HAT) PCET mechanism. The meso-chlorine atom was identified as the HAT agent. In one case, a halogenated product was identified by mass spectrometry. Our findings demonstrate that oxo-free hydrocarbon oxidation with heme systems is possible and show the potential for iron-dihalides in oxidative hydrocarbon halogenation.
- Doyle, Lorna M.,Farquhar, Erik R.,Gericke, Robert,Mcdonald, Aidan R.
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supporting information
p. 13952 - 13961
(2020/10/09)
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- Completing the Redox-Series of Silicon Trisdioxolene: ortho-Quinone and Lewis Superacid Make a Powerful Redox Catalyst
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Quinones are mild oxidants, the redox potentials of which can be increased by supramolecular interactions. Whereas this goal has been achieved by hydrogen bonding or molecular encapsulation, a Lewis acid-binding strategy for redox amplification of quinones is unexplored. Herein, the redox chemistry of silicon tris(perchloro)dioxolene 1 was studied, which is the formal adduct of ortho-perchloroquinone QCl with the Lewis superacid bis(perchlorocatecholato)silane 2. By isolating the anionic monoradical 1.?, the redox-series of a century-old class of compounds was completed. Cyclic voltammetry measurements revealed that the redox potential in 1 was shifted by more than 1 V into the anodic direction compared to QCl, reaching that of “magic blue” or NO+. It allowed oxidation of challenging substrates such as aromatic hydrocarbons and could be applied as an efficient redox catalyst. Remarkably, this powerful reagent formed in situ by combining the two commercially available precursors SiI4 and QCl.
- Maskey, Rezisha,Bendel, Christoph,Malzacher, Jonas,Greb, Lutz
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supporting information
p. 17386 - 17389
(2020/11/30)
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- Evaluation of the Synthetic Scope and the Reaction Pathways of Proton-Coupled Electron Transfer with Redox-Active Guanidines in C?H Activation Processes
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Proton-coupled electron transfer (PCET) is currently intensively studied because of its importance in synthetic chemistry and biology. In recent years it was shown that redox-active guanidines are capable PCET reagents for the selective oxidation of organic molecules. In this work, the scope of their PCET reactivity regarding reactions that involve C?H activation is explored and kinetic studies carried out to disclose the reaction mechanisms. Organic molecules with potential up to 1.2 V vs. ferrocenium/ferrocene are efficiently oxidized. Reactions are initiated by electron transfer, followed by slow proton transfer from an electron-transfer equilibrium.
- Hübner, Olaf,Himmel, Hans-J?rg,Kaifer, Elisabeth,Walter, Petra,Wild, Ute
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supporting information
p. 16504 - 16513
(2020/12/01)
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- Fast Hydrocarbon Oxidation by a High-Valent Nickel–Fluoride Complex
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In the search for highly reactive oxidants we have identified high-valent metal–fluorides as a potential potent oxidant. The high-valent Ni–F complex [NiIII(F)(L)] (2, L=N,N′-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate) was prepared from [NiII(F)(L)]? (1) by oxidation with selectfluor. Complexes 1 and 2 were characterized by using 1H/19F NMR, UV-vis, and EPR spectroscopies, mass spectrometry, and X-ray crystallography. Complex 2 was found to be a highly reactive oxidant in the oxidation of hydrocarbons. Kinetic data and products analysis demonstrate a hydrogen atom transfer mechanism of oxidation. The rate constant determined for the oxidation of 9,10-dihydroanthracene (k2=29 m?1 s?1) compared favorably with the most reactive high-valent metallo-oxidants. Complex 2 displayed reaction rates 2000–4500-fold enhanced with respect to [NiIII(Cl)(L)] and also displayed high kinetic isotope effect values. Oxidative hydrocarbon and phosphine fluorination was achieved. Our results provide an interesting direction in designing catalysts for hydrocarbon oxidation and fluorination.
- Lovisari, Marta,McDonald, Aidan R.,Mondal, Prasenjit,Twamley, Brendan
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supporting information
p. 13044 - 13050
(2020/06/05)
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- Practical Transition-Metal-Free Protodeboronation of Arylboronic Acids in Aqueous Sodium Hypochlorite
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A concise and practical method was developed for the protodeboronation of arylboronic acids under mild conditions in aqueous NaClO at 100 °C. The strategy is low-cost, transition-metal-free, and base-free.
- Li, Minxin,Tang, Yanling,Gao, Jinchun,Rao, Gaoxiong,Mao, Zewei
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p. 2039 - 2042
(2020/10/21)
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- Palladium-Catalyzed Three-Component Coupling Reaction of o-Bromobenzaldehyde, N-Tosylhydrazone, and Methanol
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A ligand-controlled palladium-catalyzed three-component reaction of o-bromobenzaldehyde, N-tosylhydrazone, and methanol is described. This reaction uses readily available compounds as starting materials while displaying a broad substrate scope and good functional group compatibility.
- Zhu, Lei,Ren, Xiaojian,Yu, Yinghua,Ou, Pengcheng,Wang, Zhi-Xiang,Huang, Xueliang
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p. 2087 - 2092
(2020/03/04)
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- Cobalt(I)-Catalyzed Borylation of Unactivated Alkyl Bromides and Chlorides
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A cobalt-complex-catalyzed borylation of a wide range of alkyl halides with a diboron reagent (B2pin2 or B2neop2) has been developed under mild reaction conditions, demonstrating the first cobalt-mediated cross-coupling with alkyl electrophiles. This protocol allows alkyl boronic esters to be accessed from alkyl halides, including alkyl chlorides, which were used rarely as coupling partners. Mechanistic studies reveal the possible involvement of an alkyl radical intermediate in this cobalt-mediated catalytic cycle.
- Geetharani, K.,Prasad, K. Sujit,Varghese, Dominic,Verma, Piyush Kumar
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p. 1431 - 1436
(2020/03/13)
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