- Electron-rich phenoxyl mediators improve thermodynamic performance of electrocatalytic alcohol oxidation with an iridium pincer complex
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Electron-rich phenols, including α-rac-tocopherol Ar1OH, 2,4,6,-tri-tert-butylphenol Ar3OH, and butylated hydroxy-toluene Ar4OH, are effective electrochemical mediators for the electrocatalytic oxidation of alcohols by an iridium amido dihyride complex (PNP)Ir(H)2 (IrN 1, PNP = bis[2-diisopropylphosphino)ethyl]amide). Addition of phenol mediators leads to a decrease in the onset potential of catalysis from -0.65 V vs Fc+/0 under unmediated conditions to -1.07 V vs Fc+/0 in the presence of phenols. Mechanistic analysis suggests that oxidative turnover of the iridium amino trihydride (PNHP)Ir(H)3 (IrH 2, PNHP = bis[2-diisopropylphosphino)ethyl]amine) to IrN 1 proceeds through two successive hydrogen atom transfers (HAT) to 2 equiv of phenoxyl that are generated transiently at the anode. Isotope studies and comparison to known systems are consistent with initial homolysis of an Ir-H bond being rate-determining. Turnover frequencies up to 14.6 s-1 and an average Faradaic efficiency of 93% are observed. The mediated system shows excellent chemoselectivity in bulk oxidations of 2-propanol and 1,2-benzenedimethanol in THF and is also viable in neat 2-propanol.
- Galvin, Conor M.,Waymouth, Robert M.
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- Solvent effects on homolytic bond dissociation energies of hydroxylic acids
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The homolytic bond dissociation energies (BDEs) of the O-H bonds in DMSO solution for (a) phenol and a number of its derivatives, (b) three oximes, (c) three alcohols, (d) three hydroxylamines, and (e) two hydroxamic acids have been estimated by eq 1: BDE(HA) = 1.37pK(HA) + 23.1E(ox)(A-) + 73.3 kcal/mol. For most of these hydroxylic acids, the BDEs of the O-H bonds estimated by eq 1 are within ±2 kcal/mol of the literature values in nonpolar solvents or in the gas phase. There is no reason to believe, therefore, that these BDEs are 'seriously in error because of failure to correct for solvent effects' as has been claimed on the basis that BDEs in highly polar solvents estimated for the O-H bond in phenol by photoacoustic calorimetry must be so corrected.
- Bordwell, Frederick G.,Liu, Wei-Zhong
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- Visible-light unmasking of heterocyclic quinone methide radicals from alkoxyamines
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In nature, the unmasking of heterocyclic quinones to form stabilized quinone methide radicals is achieved using reductases (bioreduction). Herein, an alternative controllable room-temperature, visible-light activated protocol using alkoxyamines and bis-alkoxyamines is provided. Selective synthetic modification of the bis-alkoxyamine, allowed chromophore deactivation to give one labile alkoxyamine moiety.
- Kielty, Patrick,Farràs, Pau,Mcardle, Patrick,Smith, Dennis A.,Aldabbagh, Fawaz
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- Iron(III)-promoted hydrofunctionalization/bicyclization of 1,7-enynes toward benzo[a]fluoren-5-ones
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A new hydrofunctionalization/bicyclization of C(sp3)-tethered 1,7-enynes toward benzo[a]fluoren-5-ones with good to excellent yields has been developed by using Iron(III)/PhSiH3 system as a hydride donor. This protocol involves a reductive 1,7-enyne-carboannulation cascade, leading to C(sp3)[sbnd]H and C[sbnd]C bond-forming events to rapidly build up tetracarbocyclic complexity. A wide range of functional groups relative to 1,7-enyne substrates were tolerated well under mild neutral-redox conditions.
- Huang, Min-Hua,Hong, Yu,Hu, Jian-Qiang,Yang, Shi-Zhao,Zhu, Yi-Long,Wang, De-Cai,Jiang, Bo
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- Synthesis and characterization of novel type poly (4-chloromethyl styrene-grft-4-vinylpyridine)/TiO2 nanocomposite via nitroxide-mediated radical polymerization
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This paper describes the synthesis and characterization of novel type poly (4-chloromethyl styrene-graft-4-vinylpyridine)/TiO2 nanocomposite. Firstly, poly (4-chloromethyl styrene)/TiO2 nanocomposite was synthesized by in situ free radical polymerizing of 4-chloromethyl styrene monomers in the presence of 3-(trimethoxysilyl) propylmethacrylate (MPS) modified nano-TiO2. Thereafter, 1-hydroxy-2,2,6,6-tetramethyl-1- piperidinyloxy (TEMPO-OH) was synthesized by the reduction of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). This functional nitroxyl compound was covalently attached to the poly (4-chloromethyl styrene)/TiO2 with replacement of chlorine atoms in the poly (4-chloromethyl styrene) chains. The controlled graft copolymerization of 4-vinylpyridine was initiated by poly (4-chloromethyl styrene)/TiO2 nanocomposite carrying TEMPO groups as a macroinitiators. The coupling of TEMPO with poly (4-chloromethyl styrene)/TiO2 was verified using 1H nuclear magnetic resonance (NMR) spectroscopy. The obtained nanocomposites were studied using transmission electron microscopy (TEM), Fourier-transform infrared (FTIR) spectra, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and the optical properties of the nanocomposites were studied using ultraviolet-visible (UV-Vis) spectroscopy.
- Jaymand, Mehdi
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- Co(i) complexes with a tetradentate phenanthroline-based PNNP ligand as a potent new metal-ligand cooperation platform
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A series of low spin cobalt(i) complexes bearing a tetradentate phenanthroline-based PNNP ligand (2,9-bis((diphenylphosphanyl)methyl)-1,10-phenanthroline), [CoCl(PNNP)] (1), [CoMe(PNNP)] (2) and [Co(CH2SiMe3)(PNNP)] (3), were synthesized and structurally identified. Complex 3 underwent a structural rearrangement of the PNNP skeleton upon heating to form [Co(PNNP′)] (4), which is supported by an asymmetrical PNNP′ ligand with a dearomatized phenanthroline backbone. Mechanistic studies supported that the transformation from 3 to 4 was initiated by the homolysis of either a Co-CH2SiMe3 bond or a benzylic C-H bond. Complex 4 achieved H-H bond cleavage of H2 (1 atm) at ambient temperature, to form [Co(PNNP″)] (6), in which two H atoms were incorporated into the endocyclic double bond of the PNNP″ ligand backbone. This journal is
- Ishizaka, Yusuke,Jheng, Nai-Yuan,Naganawa, Yuki,Nakajima, Yumiko,Sekiguchi, Akira
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- Mechanistic studies of the reaction of Ir (III) porphyrin hydride with 2,2,6,6-tetramethylpiperidine-1-oxyl to an unsupported Ir-Ir porphyrin dimer
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Reaction of hydrido[5,10,15,20-tetrakis(p-tolyl)porphyrinato]iridium(III) (Ir(ttp)H) (1) with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) (2) at room temperature gave a 90% yield of the unsupported iridium(II) porphyrin dimer, IrII2(ttp)2 (3). Kinetic measurements revealed that the oxidation followed overall second-order kinetics: rate = k[Ir(ttp)H][TEMPO], k(25 °C) = 6.65 × 10-4M-1. The entropy of activation (ΔS? = -25.3 ± 2.5 calmol-1 K-1)and the kinetic isotope effect of 7.2 supported a bimolecular associative mechanism in the rate-determining hydrogen atom transfer from Ir(ttp)H to TEMPO.
- Lee, Siu Yin,Cheung, Chi Wai,Hsu, I-Jui,Chan, Kin Shing
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- Extremely Fast Hydrogen Atom Transfer between Nitroxides and HOO · Radicals and Implication for Catalytic Coantioxidant Systems
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We report a novel coantioxidant system based on TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) that, in biologically relevant model systems, rapidly converts chain-carrying alkylperoxyl radicals to HOO·. Extremely efficient quenching of HOO· by TEMPO blocks the oxidative chain. Rate constants in chlorobenzene were measured to be 1.1 × 109 M-1 s-1 for the reductive reaction TEMPO + HOO· → TEMPOH + O2 and 5.0 × 107 M-1 s-1 for the oxidative reaction TEMPOH + HOO· → TEMPO + H2O2. These rate constants are significantly higher than that associated with the reaction of HOO· with α-tocopherol, Nature's best lipid soluble antioxidant (k = 1.6 × 106 M-1 s-1). These data show that in the presence of ROO·-to-HOO· chain-transfer agents, which are common in lipophilic environments, the TEMPO/TEMPOH couple protects organic molecules from oxidation by establishing an efficient reductive catalytic cycle. This catalytic cycle provides a new understanding of the efficacy of the antioxidant capability of TEMPO in nonaqueous systems and its potential to act as a chemoprotective against radical damage.
- Baschieri, Andrea,Valgimigli, Luca,Gabbanini, Simone,Dilabio, Gino A.,Romero-Montalvo, Eduardo,Amorati, Riccardo
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- Titration of nitroxide free radicals by nuclear magnetic relaxometry
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An alternative to electron spin resonance spectroscopy is proposed for the quantitative analysis of nitroxide free radicals in solution. The method is based on the chemical reduction of the paramagnetic compounds followed by NMR measurements of the longitudinal relaxation rate of the solvent protons. This titration of the nitroxides has been carried out in ethanolic solutions by reaction with known amounts of phenylhydrazine. The paramagnetic fraction of the solvent relaxation rate is precisely related to the concentration of the free radical which can be measured without prior knowledge of its specific influence on the protons relaxation rate (relaxivity). Oxygen has to be eliminated from the solutions in order to avoid reoxidation of the hydroxylamine formed. The precision of the method, tested on 11 diversely subsituted derivatives of piperidine-1-oxyl, pyrrolidine-1-oxyl, and 3-oxazolidine-1-oxyl, offers a precision of about 3%.
- Muller, Robert N.,Van Haverbeke, Yves,Bonnet, Pierre A.,Chapat, Jean-Pierre,Vallet, Patrick
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- CO-Photolysis-Induced H-Atom Transfer from MnIO-H Bonds
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The formation of TEMPOH from a mixture of [Mn(CO)3(μ3-OH)]4 (1) and (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) is shown to occur through a light-initiated CO photolysis from 1 (illumination at 300-375 nm). One hypothesis is that the loss of carbon monoxide (CO) causes significant O-H bond weakening to render proton-coupled electron transfer (PCET) to TEMPO favorable. For instance, the ground-state O-H bond dissociation free energy (BDFEO-H) of 1 (computed with density functional theory and estimated using effective BDFE reagents) is too high to transfer an H-atom to TEMPO. We also demonstrate that TEMPO and 1 interact in the dark through a hydrogen-bonded "precomplex" (1···TEMPO). We suggest that the PCET reaction that forms TEMPOH is the result of a H-atom-transfer reaction that occurs immediately after photolysis of a CO ligand(s).
- Kadassery, Karthika J.,Sethi, Komal,Fanara, Paul M.,Lacy, David C.
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- Single crystal to single crystal transformation and hydrogen-atom transfer upon oxidation of a cerium coordination compound
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Trivalent and tetravalent cerium compounds of the octamethyltetraazaannulene (H2omtaa) ligand have been synthesized. Electrochemical analysis shows a strong thermodynamic preference for the formal cerium(IV) oxidation state. Oxidation of the cerium(III) congener Ce(Homtaa)(omtaa) occurs by hydrogen-atom transfer that includes a single crystal to single crystal transformation upon exposure to an ambient atmosphere.
- Williams, Ursula J.,Mahoney, Brian D.,Lewis, Andrew J.,Degregorio, Patrick T.,Carroll, Patrick J.,Schelter, Eric J.
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- Access to formally Ni(i) states in a heterobimetallic NiZn system
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Heterobimetallic NiZn complexes featuring metal centers in distinct coordination environments have been synthesized using diimine-dioxime ligands as binucleating scaffolds. A tetramethylfuran-containing ligand derivative enables a stable one-electron-reduced S = 1/2 species to be accessed using Cp 2Co as a chemical reductant. The resulting pseudo-square planar complex exhibits spectroscopic and crystallographic characteristics of a ligand-centered radical bound to a Ni(ii) center. Upon coordination of a π-acidic ligand such as PPh3, however, a five-coordinate Ni(i) metalloradical is formed. The electronic structures of these reduced species provide insight into the subtle effects of ligand structure on the potential and reversibility of the NiII/I couple for complexes of redox-active tetraazamacrocycles.
- Uyeda, Christopher,Peters, Jonas C.
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- Homolytic Reactivity of Group 14 Organometallic Hydrides toward Nitroxides
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The spontaneous reactions of Et3SiH, Ph3GeH, Bu3SnH, and (Me3Si)3SiH with 2,2,6,6-tetramethyl1-piperidinoxyl (TEMPO) and related nitroxides have been investigated. Metal hydrides characterized by relatively weak metal-hydrogen bonds, like Ph3GeH and Bu3SnH, reduce TEMPO to the corresponding hydroxylamine while no reaction is observed with Et3SiH and Ph3SiH. Tris(trimethylsilyDsilane, on the other hand, is able to reduce the nitroxide to the corresponding hydroxylamine and amine in a ca. 1:1 ratio. By repeating the above reactions in the presence of thermal or photochemical radical initiators, deoxygenation of TEMPO was obtained in high yield with (Me3SD)3SiH and (Me3Si)2Si(H)Me, but not with other silanes, germanes, and stannanes. A mechanism for the reduction of TEMPO by the two trimethylsilyl-substituted silanes is proposed, and kinetic data for the various steps of the overall reaction are reported. In particular rate constants and activation parameters have been measured for the hydrogen transfer reaction from several silanes to the hindered aminyl radical 2,2,6,6-tetramethylpiperidinyl.
- Lucarini, Marco,Marchesi, Emanuela,Pedulli, Gian Franco,Chatgilialoglu, Chryssostomos
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- Protonation and proton-coupled electron transfer at S-ligated [4Fe-4S] clusters
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Biological [Fe-S] clusters are increasingly recognized to undergo proton-coupled electron transfer (PCET), but the site of protonation, mechanism, and role for PCET remains largely unknown. Here we explore this reactivity with synthetic model clusters. Protonation of the arylthiolate-ligated [4Fe-4S] cluster [Fe4S4(SAr)4]2- (1, SAr=S-2,4-6-(iPr)3C6H2) leads to thiol dissociation, reversibly forming [Fe4S4(SAr)3L]1- (2) and ArSH (L=solvent, and/or conjugate base). Solutions of 2+ArSH react with the nitroxyl radical TEMPO to give [Fe4S4(SAr)4]1- (1ox) and TEMPOH. This reaction involves PCET coupled to thiolate association and may proceed via the unobserved protonated cluster [Fe4S4(SAr)3(HSAr)]1- (1-H). Similar reactions with this and related clusters proceed comparably. An understanding of the PCET thermochemistry of this cluster system has been developed, encompassing three different redox levels and two protonation states. Transfer target: S-Ligated [4Fe-4S] clusters undergo proton-coupled electron transfer involving the reassociation of a thiol ligand (see figure). An understanding of the proton-coupled electron-transfer (PCET) thermochemistry of this cluster system has been developed, encompassing three different redox levels and two protonation states.
- Saouma, Caroline T.,Morris, Wesley D.,Darcy, Julia W.,Mayer, James M.
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- Homoleptic Rhodium Pyridine Complexes for Catalytic Hydrogen Oxidation
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The homoleptic rhodium pyridine complex [Rh(py)4]+([1]+) is prepared from simple precursors. Lacking good π-acceptor ligands but being sterically protected, [1]+reversibly oxidizes to colorless [Rh(py)4(thf)2]2+. This monomericS= 1/2 Rh(II) complex activates H2to give [HRh(py)4L]2+, which can also be generated by protonation of [1]+. The Rh(III)-H bond is weak, being susceptible to H atom abstraction as well as deprotonation. These results underpin a novel catalytic system for the oxidation of H2by ferrocenium.
- Zhang, Yu,Woods, Toby J.,Rauchfuss, Thomas B.
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- Metal-Free Tandem Oxidative Coupling of Primary Alcohols with Azoles for the Synthesis of Hemiaminal Ethers
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A novel metal-free tandem oxidative coupling process for the synthesis of hemiaminal ethers has been developed. This protocol could be applied for the C-N bond formation of electron-deficient trizoles, tetrazoles, carbazoles and indazoles with primary alcohols.
- Sun, Jinwei,Zhang, Yu,Mathan, Sankaran,Wang, Yi,Pan, Yi
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- Non-Heme-Iron-Mediated Selective Halogenation of Unactivated Carbon?Hydrogen Bonds
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Oxidation of the iron(II) precursor [(L1)FeIICl2], where L1 is a tetradentate bispidine, with soluble iodosylbenzene (sPhIO) leads to the extremely reactive ferryl oxidant [(L1)(Cl)FeIV=O]+ with a cis disposition of the chlorido and oxido coligands, as observed in non-heme halogenase enzymes. Experimental data indicate that, with cyclohexane as substrate, there is selective formation of chlorocyclohexane, the halogenation being initiated by C?H abstraction and the result of a rebound of the ensuing radical to an iron-bound Cl?. The time-resolved formation of the halogenation product indicates that this primarily results from sPhIO oxidation of an initially formed oxido-bridged diiron(III) resting state. The high yield of up to >70 % (stoichiometric reaction) as well as the differing reactivities of free Fe2+ and Fe3+ in comparison with [(L1)FeIICl2] indicate a high complex stability of the bispidine-iron complexes. DFT analysis shows that, due to a large driving force and small triplet-quintet gap, [(L1)(Cl)FeIV=O]+ is the most reactive small-molecule halogenase model, that the FeIII/radical rebound intermediate has a relatively long lifetime (as supported by experimentally observed cage escape), and that this intermediate has, as observed experimentally, a lower energy barrier to the halogenation than the hydroxylation product; this is shown to primarily be due to steric effects.
- Bleher, Katharina,Comba, Peter,Faltermeier, Dieter,Gupta, Ashutosh,Kerscher, Marion,Krieg, Saskia,Martin, Bodo,Velmurugan, Gunasekaran,Yang, Shuyi
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supporting information
(2021/12/09)
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- Visible-Light-Driven Isocyanide Insertion to o-Alkenylanilines: A Route to Isoindolinone Synthesis
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A visible-light-mediated intermolecular radical insertion of isocyanides to electron-deficient o-alkenylanilines leading to isoindolinone is reported. Deuterium (D2O) and H2O18 labelling experiments suggest H and O incorporation in the product. The formation of an N-centered radical (NCR) via stepwise PT/ET process was confirmed by radical trapping experiments, photoluminescence, cyclic voltammetry and DFT studies. This photo cascade methodology is overall a redox neutral process featuring metal-free condition and broad substrate scope (32 examples). The synthesis of analogue of GABA receptor antagonist shows the practical utility of this method. (Figure presented.).
- Dahiya, Anjali,Das, Bubul,Sahoo, Ashish Kumar,Patel, Bhisma K.
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supporting information
p. 966 - 973
(2022/02/07)
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- Visible light enables catalytic formation of weak chemical bonds with molecular hydrogen
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The synthesis of weak chemical bonds at or near thermodynamic potential is a fundamental challenge in chemistry, with applications ranging from catalysis to biology to energy science. Proton-coupled electron transfer using molecular hydrogen is an attractive strategy for synthesizing weak element–hydrogen bonds, but the intrinsic thermodynamics presents a challenge for reactivity. Here we describe the direct photocatalytic synthesis of extremely weak element–hydrogen bonds of metal amido and metal imido complexes, as well as organic compounds with bond dissociation free energies as low as 31 kcal mol?1. Key to this approach is the bifunctional behaviour of the chromophoric iridium hydride photocatalyst. Activation of molecular hydrogen occurs in the ground state and the resulting iridium hydride harvests visible light to enable spontaneous formation of weak chemical bonds near thermodynamic potential with no by-products. Photophysical and mechanistic studies corroborate radical-based reaction pathways and highlight the uniqueness of this photodriven approach in promoting new catalytic chemistry. [Figure not available: see fulltext.].
- Park, Yoonsu,Kim, Sangmin,Tian, Lei,Zhong, Hongyu,Scholes, Gregory D.,Chirik, Paul J.
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p. 969 - 976
(2021/07/25)
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- AIBN-initiated direct thiocyanation of benzylic sp3C-H with: N -thiocyanatosaccharin
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Direct thiocyanations of benzylic compounds have been implemented. Here, a new strategy, involving a free radical reaction pathway initiated by AIBN, was used to construct the benzylic sp3 C-SCN bond. In this way, the disadvantage of other strategies involving introducing leaving groups in advance to synthesize benzyl thiocyanate compounds was overcome. The currently developed protocol also involved the use of readily available raw materials and resulted in high product yields (up to 100%), both being great advantages for synthesizing benzyl thiocyanates.
- Wu, Di,Duan, Yongjie,Liang, Kun,Yin, Hongquan,Chen, Fu-Xue
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p. 9938 - 9941
(2021/10/12)
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- Visible-light-promoted decarboxylative addition cyclization of: N -aryl glycines and azobenzenes to access 1,2,4-triazolidines
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Methods for the synthesis of 1,2,4-triazolidines are scarce. Herein, we report a visible-light-promoted decarboxylative addition cyclization of N-aryl glycines and azobenzenes to access such important compounds. Using commercially available methylene blue (MB) as an organic photocatalyst, the reaction proceeded smoothly in the absence of transition-metal catalysts at ambient temperature, affording the corresponding products, 1,2,4-triaryl 1,2,4-triazolidines, in good to excellent yields. This work demonstrates a new synthetic application of readily available azobenenes and provides a novel strategy for constructing 1,2,4-triazolidines.
- Ma, Ben,Qi, Yanfang,Song, Menghui,Wang, Xi-Cun,Yang, Jingya,Zhou, Hongyan
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supporting information
p. 5806 - 5811
(2021/08/23)
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- Fe-catalyzed Fukuyama-type indole synthesis triggered by hydrogen atom transfer
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Fe, Co, and Mn hydride-initiated radical olefin additions have enjoyed great success in modern synthesis, yet the extension of other hydrogen radicalophiles instead of olefins remains largely elusive. Herein, we report an efficient Fe-catalyzed intramolec
- Huang, Hanmin,Yu, Min,Zhang, Tianze
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p. 10501 - 10505
(2021/08/20)
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- Catalytic CO2 hydrosilylation with [Mn(CO)5Br] under mild reaction conditions
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Carbon dioxide hydrosilylation with earth-abundant transition-metal catalysts is an attractive alternative for the design of greener and cost-effective synthetic strategies. Herein, simple [Mn(CO)5Br] is an efficient precatalyst in the hydrosilylation of carbon dioxide with Et3SiH under mild reaction conditions. Using THF as a solvent, triethylsilylformate Et3SiCH(O)O was obtained in 67% yield after 1 h at 50 °C and 4 bar of CO2 pressure. The selectivity of the reaction was tuned by changing the solvent to a mixture of THF and toluene producing bis(triethylsilyl)acetal (Et3SiO)2CH2 in 86% yield. The CO2 hydrosilylation was also effective at room temperature and atmospheric pressure using either THF or the mixture THF/toluene as the solvent resulting in high Et3SiH conversion (92%–99%) but with a decrease in the selectivity. Radical trapping experiments indicated the participation of radical species in the catalytic mechanism. To the best of our knowledge, this is the first report on CO2 hydrosilylation catalyzed by transition-metal radical intermediates.
- García, Juventino J.,González, Tania
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- A Bioinspired Disulfide/Dithiol Redox Switch in a Rhenium Complex as Proton, H Atom, and Hydride Transfer Reagent
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The transfer of multiple electrons and protons is of crucial importance in many reactions relevant in biology and chemistry. Natural redox-active cofactors are capable of storing and releasing electrons and protons under relatively mild conditions and thus serve as blueprints for synthetic proton-coupled electron transfer (PCET) reagents. Inspired by the prominence of the 2e-/2H+ disulfide/dithiol couple in biology, we investigate herein the diverse PCET reactivity of a Re complex equipped with a bipyridine ligand featuring a unique SH···-S moiety in the backbone. The disulfide bond in fac-[Re(S-Sbpy)(CO)3Cl] (1, S-Sbpy = [1,2]dithiino[4,3-b:5,6-b′]dipyridine) undergoes two successive reductions at equal potentials of-1.16 V vs Fc+|0 at room temperature forming [Re(S2bpy)(CO)3Cl]2- (12-, S2bpy = [2,2′-bipyridine]-3,3′-bis(thiolate)). 12- has two adjacent thiolate functions at the bpy periphery, which can be protonated forming the S-H···-S unit, 1H-. The disulfide/dithiol switch exhibits a rich PCET reactivity and can release a proton (G°H+ = 34 kcal mol-1, pKa = 24.7), an H atom (? G°H = 59 kcal mol-1), or a hydride ion (G°H- = 60 kcal mol-1) as demonstrated in the reactivity with various organic test substrates.
- Hua, Shao-An,Paul, Lucas A.,Oelschlegel, Manuel,Dechert, Sebastian,Meyer, Franc,Siewert, Inke
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supporting information
p. 6238 - 6247
(2021/05/07)
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- Cu-Catalyzed Oxidative Allylic C-H Arylation of Inexpensive Alkenes with (Hetero)Aryl Boronic Acids
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Herein, we present a regioselective Cu-catalyzed oxidative allylic C(sp3)-H arylation by radical relay using a broad range of heteroaryl boronic acids with inexpensive and readily available unactivated terminal and internal olefins. This C(sp2)-C(sp3) allyl coupling has the advantage of using cheap, abundant, and nontoxic Cu2O without the need to use prefunctionalized alkenes, thus offering an alternative method to allylic arylation reactions that employ more traditional coupling partners with preinstalled leaving groups (LGs) at the allylic position.
- Pal, Suman,Cotard, Marine,Gérardin, Baptiste,Hoarau, Christophe,Schneider, Cédric
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supporting information
p. 3130 - 3135
(2021/05/05)
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- Photocatalyzed cross-dehydrogenative coupling of silanes with alcohols and water
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An efficient method for the dehydrogenative coupling of silanes with alcohols under photocatalysis was developed. The reaction proceeded in the presence of Ru(bpy)3Cl2(0.5 mol%) under visible light irradiation in acetonitrile at room temperature. The developed methodology was also applicable for the synthesis of silanols using water as a coupling partner.
- Lv, Haiping,Laishram, Ronibala Devi,Chen, Jingchao,Khan, Ruhima,Zhu, Yuanbin,Wu, Shiyuan,Zhang, Jianqiang,Liu, Xingyuan,Fan, Baomin
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supporting information
p. 3660 - 3663
(2021/04/16)
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- Visible-Light-Promoted Diboron-Mediated Transfer Hydrogenation of Azobenzenes to Hydrazobenzenes
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A visible-light-promoted transfer hydrogenation of azobenzenes has been developed. In the presence of B2pin2 and upon visible-light irradiation, the reactions proceeded smoothly in methanol at ambient temperature. The azobenzenes with diverse functional groups have been reduced to the corresponding hydrazobenzenes with a yield of up to 96%. Preliminary mechanistic studies indicated that the hydrogen atom comes from the solvent and the transformation is achieved through a radical pathway.
- Song, Menghui,Zhou, Hongyan,Wang, Ganggang,Ma, Ben,Jiang, Yajing,Yang, Jingya,Huo, Congde,Wang, Xi-Cun
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supporting information
p. 4804 - 4811
(2021/04/06)
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- TEMPO-Mediated Cross-Dehydrogenative Coupling of Indoles and Imidazo[1,2- a]pyridines with Fluorinated Alcohols
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A simple and highly efficient metal-free method has been developed for hydroxyfluoroalkylation of indoles and imidazo[1,2-a]pyridines via TEMPO-mediated C(sp3)-H and C(sp2)-H bond cross-dehydrogenative coupling of fluorinated alcohols and indoles. The protocol showed broad substrate scope, afforded good yields of hydroxyfluoroalkylated products, and was amenable for scale-up. Mechanistic investigation indicated involvement of the radical pathway.
- Nipate, Dhananjay S.,Jaspal, Sonam,Shinde, Vikki N.,Rangan, Krishnan,Kumar, Anil
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supporting information
p. 1373 - 1377
(2021/02/20)
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- Photoinduced homolytic decarboxylative acylation/cyclization of unactivated alkenes with α-keto acid under external oxidant and photocatalyst free conditions: access to quinazolinone derivatives
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A novel and green strategy for the synthesis of acylated quinazolinone derivativesviaphoto-induced decarboxylative cascade radical acylation/cyclization of quinazolinone bearing unactivated alkenes has been developed. The protocol provides a novel route to access acyl radicals from α-keto acids through a self-catalyzed energy transfer process. Most importantly, the reaction proceeded smoothly without any external photocatalyst, additive or oxidant, and could be easily scaled-up in flow conditions with sunlight irradiation.
- Sun, Bin,Shi, Rongcheng,Zhang, Kesheng,Tang, Xiaoli,Shi, Xiayue,Xu, Jiayun,Yang, Jin,Jin, Can
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supporting information
p. 6050 - 6053
(2021/06/21)
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- Concerted Multiproton-Multielectron Transfer for the Reduction of O2to H2O with a Polyoxovanadate Cluster
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The concerted transfer of protons and electrons enables the activation of small-molecule substrates by bypassing energetically costly intermediates. Here, we present the synthesis and characterization of several hydrogenated forms of an organofunctionalized vanadium oxide assembly, [V6O13(TRIOLNO2)2]2-, and their ability to facilitate the concerted transfer of protons and electrons to O2. Electrochemical analysis reveals that the fully reduced cluster is capable of mediating 2e-/2H+ transfer reactions from surface hydroxide ligands, with an average bond dissociation free energy (BDFE) of 61.6 kcal/mol. Complementary stoichiometric experiments with hydrogen-atom-accepting reagents of established bond strengths confirm that the electrochemically established BDFE predicts the 2H+/2e- transfer reactivity of the assembly. Finally, the reactivity of the reduced polyoxovanadate toward O2 reduction is summarized; our results indicate a stepwise reduction of the substrate, proceeding through H2O2 en route to the formation of H2O. Kinetic isotope effect experiments confirm the participation of hydrogen transfer in the rate-determining step of both the reduction of O2 and H2O2. This work constitutes the first example of hydrogen atom transfer for small-molecule activation with reduced polyoxometalates, where both electron and proton originate from the cluster.
- Brennessel, William W.,Fertig, Alex A.,Matson, Ellen M.,McKone, James R.
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p. 15756 - 15768
(2021/10/02)
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- Simple manganese carbonyl catalyzed hydrogenation of quinolines and imines
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Manganese-catalyzed hydrogenation of unsaturated molecules has made tremendous progresses recently benefiting from non-innocent pincer or bidentate ligands for manganese. Herein, we describe the hydrogenation of quinolines and imines catalyzed by simple manganese carbonyls, Mn2(CO)10 or MnBr(CO)5, thus eliminating the prerequisite pincer-type or bidentate ligands.
- Wang, Zelong,Chen, Lei,Mao, Guoliang,Wang, Congyang
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p. 1890 - 1894
(2020/03/04)
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- Visible-Light-Induced Decarboxylative Cyclization/Hydrogenation Cascade Reaction to Access Phenanthridin-6-yl(aryl)methanol by an Electron Donor-Acceptor Complex
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A novel and efficient visible-light-induced decarboxylative cyclization/hydrogenation cascade reaction of α-oxocarboxylic acids and 2-isocyanobiaryls has been developed. Without the need of any external photosensitizer, oxidant, and reductant, this method offers a mild and green approach for the synthesis of diverse alcohols in moderate to good yields. A mechanism indicated that an electron donor-acceptor complex-driven decarboxylation, radical addition/cyclization, and in situ photochemical reduction of ketones to alcohols could be involved in the reaction.
- Shi, Wei,Ma, Fang,Li, Pinhua,Wang, Lei,Miao, Tao
-
supporting information
p. 13808 - 13817
(2020/11/20)
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- Electrocatalytic reactivity of imine/oxime-type cobalt complex for direct perfluoroalkylation of indole and aniline derivatives
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Imine/Oxime-type cobalt complexes, regarded as simple vitamin B12model complexes, were utilized as catalysts for direct C-H perfluoroalkylations of indole and aniline derivatives with nonafluorobutyl iodide (n-C4F9I) as the readily available perfluoroalkyl source. The synthetic approach described herein was performed under mild reaction conditions driven by controlled-potential electrolysis at ?0.8 Vvs.Ag/AgCl in organic solvents. The mechanistic investigations suggest that a nonafluorobutyl radical is mediated by homolytic cleavage of the cobalt(iii)-carbon bond in the catalytic cycle. This is the first report concerning a fluoroalkylation reaction of (hetero)aromatics catalyzed by the simple vitamin B12model complex. The convenient electrocatalytic method employing a simple cobalt complex provides a facile synthesis method toward novel fluoroalkylated compounds, demonstrating potential applications in the fields of pharmaceutical chemistry and materials science.
- Cui, Luxia,Hisaeda, Yoshio,Morita, Yoshitsugu,Ono, Toshikazu
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supporting information
p. 7546 - 7551
(2020/06/21)
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- Visible-light-induced photocatalyst-free C-3 functionalization of indoles with diethyl bromomalonate
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A visible-light induced green and efficient method is developed for the synthesis of α-indolyl diethyl malonates. The reaction proceeds without any photocatalysts or ligands in a green solvent in a short time. Moreover, the reaction mechanism has been clearly studied by control experiments, spectrophotometric studies and density functional theory (DFT) calculations. The results showed that the photocatalyst-free transformation may proceed via an XB-promoted radical process. The EDA complex formation of diethyl bromomalonate with a base is the main reason for the reaction initiation.
- Gu, Guangmiao,Huang, Mengmeng,Kim, Jung Keun,Li, Yabo,Wu, Yangjie,Zhang, Jianye
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supporting information
p. 2543 - 2548
(2020/05/13)
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- Direct Synthesis of Alkenylboronates from Alkenes and Pinacol Diboron via Copper Catalysis
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We report an efficient approach for the direct synthesis of alkenylboronates using copper catalysis. The Cu/TEMPO catalyst system (where TEMPO = (2,2,6,6-tetramethylpiperidin-1-yl)oxyl) exhibits both excellent reactivity and selectivity for the synthesis of alkenylboronates, starting from inexpensive and abundant alkenes and pinacol diboron. This approach allows for the direct functionalization of both aromatic and aliphatic terminal alkenes. Mechanistic experiments suggest that the alkenylboronates arise from oxyboration intermediates.
- Lu, Wenkui,Shen, Zengming
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supporting information
p. 142 - 146
(2019/01/11)
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- The Influence of Alkali Metal Ions on the Stability and Reactivity of Chromium(III) Superoxide Moieties Spanned by Siloxide Ligands
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In recent years, it has become clear that the presence of redox-inactive Lewis acidic metal ions can decisively influence the reactivity of metal–dioxygen moieties that are formed in the course of O2 activation, in molecular complexes, and metalloenzymes. Superoxide species are often formed as the primary intermediates but they are mostly too unstable for a thorough investigation. We report here a series of chromium(III) superoxide complexes [L2Cr]M2O2(THF)y (L=?OSiPh2OSiPh2O?, M+=Li+, Na+, K+ and y=4, 5), which could be accessed, studied spectroscopically and partly crystallized at low temperatures. They only differ in the two incorporated Lewis acidic alkali metal counterions (M+) and it could thus be shown that the nature of M+ determines considerably its interaction with the superoxide ligand. This interaction, in turn, has a significant influence on the stability and reactivity of these complexes towards substrates with OH groups. Furthermore, we show that stability and reactivity are also highly solvent dependent (THF versus nitriles), as donor solvents coordinate to the alkali metal ions and thus also influence their interaction with the superoxide moiety. Altogether, these results provide a comprehensive and detailed picture concerning the correlation between spectroscopic properties, structure, and behavior of such superoxides, that may be exemplary for other systems.
- Wind, Marie-Louise,Hoof, Santina,Herwig, Christian,Braun-Cula, Beatrice,Limberg, Christian
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p. 5743 - 5750
(2019/03/26)
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- Hydrogen and Sulfonyl Radical Generation for the Hydrogenation and Arylsulfonylation of Alkenes Driven by Photochemical Activity of Hydrogen Bond Donor-Acceptor Complexes
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An efficient photoinduced strategy for the hydrogenation and arylsulfonylation of alkenes has been developed. The reaction afforded a range of hydrogenated products and sulfonated oxindoles in high yields under external photocatalyst-free, oxidant- and reductant-free conditions. Mechanistic investigations suggested this transformation is driven by the photochemical activity of hydrogen bond donor-acceptor complex, generated from the substrates of arylsulfinic acids and C6-(vinyl sulfone)phenanthridines or N-arylacrylamides via hydrogen bond interaction. (Figure presented.).
- Li, Yang,Ma, Fang,Li, Pinhua,Miao, Tao,Wang, Lei
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supporting information
p. 1606 - 1616
(2019/02/16)
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- TEMPO-Mediated Catalysis of the Sterically Hindered Hydrogen Atom Transfer Reaction between (C5Ph5)Cr(CO)3H and a Trityl Radical
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We have demonstrated the ability of TEMPO to catalyze H· transfer from (C5Ph5)Cr(CO)3H to a trityl radical (tris(p-tert-butylphenyl)methyl radical). We have measured the rate constant and activation parameters for the direct reaction, and for each step in the catalytic process: H· transfer from (C5Ph5)Cr(CO)3H to TEMPO and H· transfer from TEMPO-H to the trityl radical. We have compared the measured rate constants with the differences in bond strength, and with the changes in the Global Electrophilicity Index determined with high accuracy for each radical using state of the art quantum chemical methods. We conclude that neither is a major factor in determining the rates of these H· transfer reactions and that the effectiveness of TEMPO as a catalyst is largely the result of its relative lack of steric congestion compared to the trityl radical.
- Gunasekara, Thilina,Abramo, Graham P.,Hansen, Andreas,Neugebauer, Hagen,Bursch, Markus,Grimme, Stefan,Norton, Jack R.
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supporting information
p. 1882 - 1886
(2019/02/14)
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- The Effect of Viscosity on the Diffusion and Termination Reaction of Organic Radical Pairs
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The effect of viscosity on the diffusion efficiency (Fdif) of an organic radical pair in a solvent cage and the termination mechanism, that is, the selectivity of disproportionation (Disp) and combination (Comb) of the geminated caged radical pair and the diffused radicals encountered, were investigated quantitatively by following the photolysis of dimethyl 2,2′-azobis(2-methylpropionate) (V-601) in the absence and presence of PhSD. Fdif and Disp/Comb selectivity outside the cage [Disp(dif)/Comb(dif)] are highly sensitive to the viscosity. In contrast, the Disp/Comb selectivity inside the cage [Disp(cage)/Comb(cage)] is rather insensitive. The difference in viscosity dependence between Disp(cage)/Comb(cage) and Disp(dif)/Comb(dif) is explained by the spin state of the radical pair inside and outside the cage and the spin state dependent configurational changes of the radical pair upon their collision. Given that the configurational change of the radicals associates the displacement and reorganization of solvents around the radicals, the termination outside the cage, which requires larger change than that inside the cage, is highly viscosity dependent. Furthermore, while the bulk viscosity of each solvent shows good correlation with Fdif and Disp/Comb selectivity, microviscosity is the better parameter predicting Fdif and Disp(dif)/Comb(dif) selectivity regardless of the solvents.
- Li, Xiaopei,Ogihara, Tasuku,Abe, Manabu,Nakamura, Yasuyuki,Yamago, Shigeru
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supporting information
p. 9846 - 9850
(2019/07/10)
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- The Photodynamic Covalent Bond: Sensitized Alkoxyamines as a Tool to Shift Reaction Networks Out-of-Equilibrium Using Light Energy
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We implement sensitized alkoxyamines as "photodynamic covalent bonds" - bonds that, while being stable in the dark at ambient temperatures, upon photoexcitation efficiently dissociate and recombine to the bound state in a fast thermal reaction. This type of bond allows for the photochemically induced exchange of molecular building blocks and resulting constitutional variation within dynamic reaction networks. To this end, alkoxyamines are coupled to a xanthone unit as triplet sensitizer enabling their reversible photodissociation into two radical species. By studying the photochemical properties of three generations of sensitized alkoxyamines it became clear that the nature and efficiency of triplet energy transfer from the sensitizer to the alkoxyamine bond as well as the reversibility of photodissociation crucially depends on the structure of the nitroxide terminus. By employing the thus designed photodynamic covalent bonding motif, we demonstrate how to use light energy to shift a dynamic covalent reaction network away from its thermodynamic minimum into a photostationary state. The network could be repeatedly switched between its minimum and kinetically trapped out-of-equilibrium state by thermal scrambling and selective photoactivation of sensitized alkoxyamines, respectively.
- Herder, Martin,Lehn, Jean-Marie
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p. 7647 - 7657
(2018/06/26)
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- 2,2,6,6-Tetramethylpiperidin-1-yloxycarbonyl: A Protecting Group for Primary, Secondary, and Heterocyclic Amines
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The 2,2,6,6-tetramethylpiperidin-1-yloxycarbonyl (Tempoc) protecting group is readily introduced by the reaction of amines with a new acyl transfer reagent, 4-nitrophenyl (2,2,6,6-tetramethylpiperidin-1-yl) carbonate (NPTC). Tempoc has a reactivity profile that complements the commonly used t-butyloxycarbonyl (Boc) and benzyloxycarbonyl (Cbz) protecting groups. Deprotection can be achieved under mild reductive conditions with in situ generated Cu(I) species or by thermolytic cleavage at 135 °C. Mechanistic studies on the deprotection of Tempoc-indole suggest a combination of ionic and radical fragmentation pathways under thermal conditions.
- Lizza, Joseph R.,Bremerich, Maximilian,McCabe, Stephanie R.,Wipf, Peter
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supporting information
p. 6760 - 6764
(2018/10/25)
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- Unique Stereoselective Homolytic C?O Bond Activation in Diketopiperazine-Derived Alkoxyamines by Adjacent Amide Pyramidalization
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Simple monocyclic diketopiperazine (DKP)-derived alkoxyamines exhibit unprecedented activation of a remote C?O bond for homolysis by amide distortion. The combination of strain-release-driven amide planarization and the persistent radical effect (PRE) enables a unique, irreversible, and quantitative trans→cis isomerization under much milder conditions than typically observed for such homolysis-limited reactions. This isomerization is shown to be general and independent of the steric and electronic nature of both the amino acid side chains and the substituents at the DKP nitrogen atoms. Homolysis rate constants are determined, and they significantly differ for both the labile trans diastereomers and the stable cis diastereomers. To reveal the factors influencing this unusual process, structural features of the kinetic trans diastereomers and thermodynamic cis diastereomers are investigated in the solid state and in solution. X-ray crystallographic analysis and computational studies indicate substantial distortion of the amide bond from planarity in the trans-alkoxyamines, and this is believed to be the cause for the facile and quantitative isomerization. Thus, these amino-acid-derived alkoxyamines are the first examples that exhibit a large thermodynamic preference for one diastereomer over the other upon thermal homolysis, and this allows controlled switching of configurations and configurational cycling.
- Amatov, Tynchtyk,Jangra, Harish,Pohl, Radek,Cisa?ová, Ivana,Zipse, Hendrik,Jahn, Ullrich
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p. 15336 - 15345
(2018/09/25)
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- Visible-Light-Triggered, Metal- and Photocatalyst-Free Acylation of N-Heterocycles
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A photoinduced acylation of N-heterocycles is explored. This visible-light triggered reaction occurs not only under extremely mild reaction conditions, but also does not require the presence of a photosensitizer. The mechanistic studies suggest formation of EDA complexes prompt to harness the energy from visible-light. Compatibility with a large panel of α-keto acids as acyl precursors and an array of N-heterocycles clearly showcase the synthetic potential of this handy and green acylation protocol. (Figure presented.).
- Guillemard, Lucas,Colobert, Fran?oise,Wencel-Delord, Joanna
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supporting information
p. 4184 - 4190
(2018/09/25)
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- Substituted Hantzsch Esters as Versatile Radical Reservoirs in Photoredox Reactions
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Substituted Hantzsch esters can act as radical reservoirs in photoredox reactions, steadily releasing a carbon radical and a hydrogen atom radical in the absence of an additional electron acceptor. We propose that radical release by substituted Hantzsch esters occurs via a mechanism involving an internal redox cycle. Cinnamidecinnamides, styrenes, α,β-unsaturated acids, and diarylethenes could be alkylated smoothly with these reagents. (Figure presented.).
- Gu, Fangjun,Huang, Wenhao,Liu, Xu,Chen, Wenxin,Cheng, Xu
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supporting information
p. 925 - 931
(2018/01/04)
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- Eight-Step Enantioselective Total Synthesis of (?)-Cycloclavine
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The first enantioselective total synthesis of (?)-cycloclavine was accomplished in 8 steps and 7.1 % overall yield. Key features include the first catalytic asymmetric cyclopropanation of allene, mediated by the dirhodium catalyst Rh2(S-TBPTTL)4, and the enone 1,2-addition of a new TEMPO carbamate methyl carbanion. An intramolecular strain-promoted Diels–Alder methylenecyclopropane (IMDAMC) reaction provided a pivotal tricyclic enone intermediate with more than 99 % ee after crystallization. The synthesis of (?)-1 was completed by a late-stage intramolecular Diels–Alder furan (IMDAF) cycloaddition to install the indole.
- McCabe, Stephanie R.,Wipf, Peter
-
supporting information
p. 324 - 327
(2016/12/30)
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- Thermodynamics of a μ-oxo Dicopper(II) Complex for Hydrogen Atom Abstraction
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The mono-μ-hydroxo complex {[Cu(tmpa)]2-(μ-OH)}3+ (1) can undergo reversible deprotonation at -30 °C to yield {[Cu(tmpa)]2-(μ-O)}2+ (2). This species is basic with a pKa of 24.3. 2 is competent for concerted proton-electron transfer from TEMPOH, but is an intrinsically poor hydrogen atom abstractor (BDFE(OH) of 77.2 kcal/mol) based on kinetic and thermodynamic analyses. Nonetheless, DFT calculations experimentally calibrated against 2 reveal that [Cu2O]2+ is likely thermodynamically viable in copper-dependent methane monoxygenase enzymes.
- Ali, Ghazanfar,Vannatta, Peter E.,Ramirez, David A.,Light, Kenneth M.,Kieber-Emmons, Matthew T.
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supporting information
p. 18448 - 18451
(2018/01/08)
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- Separating Proton and Electron Transfer Effects in Three-Component Concerted Proton-Coupled Electron Transfer Reactions
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Multiple-site concerted proton-electron transfer (MS-CPET) reactions were studied in a three-component system. 1-Hydroxy-2,2,6,6-tetramethylpiperidine (TEMPOH) was oxidized to the stable radical TEMPO by electron transfer to ferrocenium oxidants coupled to proton transfer to various pyridine bases. These MS-CPET reactions contrast with the usual reactivity of TEMPOH by hydrogen atom transfer (HAT) to a single e-/H+ acceptor. The three-component reactions proceed by pre-equilibrium formation of a hydrogen-bonded adduct between TEMPOH and the pyridine base, and the adduct is then oxidized by the ferrocenium in a bimolecular MS-CPET step. The second-order rate constants, measured using stopped-flow kinetic techniques, spanned 4 orders of magnitude. An advantage of this system is that the MS-CPET driving force could be independently varied by changing either the pKa of the base or the reduction potential (E°) of the oxidant. Changes in ΔG°MS-CPET from either source had the same effect on the MS-CPET rate constants, and a combined Br?nsted plot of ln(kMS-CPET) vs ln(Keq) was linear with a slope of 0.46. These results imply a synchronous concerted mechanism, in which the proton and electron transfer components of the CPET process make equal contributions to the rate constants. The only outliers to the Br?nsted correlation are the reactions with sterically hindered pyridines, which apparently hinder the close approach of proton donor and acceptor that facilitates MS-CPET. These three-component reactions are compared with a related HAT reaction of TEMPOH, with the 2,4,6-tri-tert-butylphenoxyl radical. The MS-CPET and HAT oxidations of TEMPOH at the same driving force occurred with similar rate constants. While this is an imperfect comparison, the data suggest that the separation of the proton and electron to different reagents does not significantly inhibit the proton-coupled electron transfer process.
- Morris, Wesley D.,Mayer, James M.
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p. 10312 - 10319
(2017/08/09)
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- Iron Catalysis for Modular Pyrimidine Synthesis through β-Ammoniation/Cyclization of Saturated Carbonyl Compounds with Amidines
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An efficient method for the modular synthesis of various pyrimidine derivatives by means of the reactions of ketones, aldehydes, or esters with amidines in the presence of an in situ prepared recyclable iron(II)-complex was developed. This operationally simple reaction proceeded with broad functional group tolerance in a regioselective manner via a remarkable unactivated β-C-H bond functionalization. Control experiments were performed to gain deep understanding of the mechanism, and the reactions are likely to proceed through a designed TEMPO complexation/enamine addition/transient α-occupation/β-TEMPO elimination/cyclization sequence.
- Chu, Xue-Qiang,Cao, Wen-Bin,Xu, Xiao-Ping,Ji, Shun-Jun
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supporting information
p. 1145 - 1154
(2018/06/18)
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- Cu-Catalyzed Cyanation of Arylboronic Acids with Acetonitrile: A Dual Role of TEMPO
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The cyanation of arylboronic acids by using acetonitrile as the "CN" source has been achieved under a Cu(cat.)/TEMPO system (TEMPO=2,2,6,6-tetramethylpiperidine N-oxide). The broad substrate scope includes a variety of electron-rich and electron-poor arylboronic acids, which react well to give the cyanated products in high to excellent yields. Mechanistic studies reveal that TEMPO-CH2CN, generated in situ, is an active cyanating reagent, and shows high reactivity for the formation of the CN- moiety. Moreover, TEMPO acts as a cheap oxidant to enable the reaction to be catalytic in copper. The cyanation of arylboronic acids by using acetonitrile as the "CN" source has been achieved under a Cu(cat.)/TEMPO system. Electron-rich and electron-poor arylboronic acids react well to give the cyanated products in high to excellent yields. Mechanistic studies reveal that TEMPO-CH2CN, generated in situ, is an active cyanating reagent. Moreover, TEMPO, a cheap oxidant, enables the reaction to be catalytic in copper (see scheme; TEMPO=2,2,6,6-tetramethylpiperidine N-oxide).
- Zhu, Yamin,Li, Linyi,Shen, Zengming
-
supporting information
p. 13246 - 13252
(2015/09/15)
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- Stoichiometric C6-oxidation of hyaluronic acid by oxoammonium salt TEMPO+Cl- in an aqueous alkaline medium
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This paper reports the selective oxidation of hyaluronic acid (HA) by stoichiometric quantity of 2,2,6,6-tetramethylpiperidine-1-oxoammonium chloride (TEMPO+) in aqueous alkaline medium. High efficiency of the HA oxidation and quantitative yield of carboxy-HA per starting TEMPO+, as well as unusual behavior of the oxidation system generating an oxygen upon alkali-induced oxoammonium chloride decomposition are demonstrated. The scheme for HA oxidation involving both TEMPO+ and oxygen produced upon the TEMPO+Cl- decomposition and/or air oxygen is proposed. For comparison, the data on stoichiometric oxidation of such substrates as dermatan sulfate, water-soluble potato starch, methyl 2-acetamido-2-deoxy-β-d-glucopyranoside and ethanol are presented.
- Ponedel'Kina, Irina Yu,Khaibrakhmanova, Elvira A.,Tyumkina, Tatyana V.,Romadova, Irina V.,Odinokov, Victor N.
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- Ammonia Synthesis by Hydrogenolysis of Titanium-Nitrogen Bonds Using Proton Coupled Electron Transfer
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The catalytic hydrogenolysis of the titanium-amide bond in (η5-C5Me4SiMe3)2Ti(Cl)NH2 to yield free ammonia is described. The rhodium hydride, (η5-C5Me5)(py-Ph)RhH (py-Ph = 2-phenylpyridine), serves as the catalyst and promotes N-H bond formation via hydrogen atom transfer. The N-H bond dissociation free energies of ammonia ligands have also been determined for titanocene and zirconocene complexes and reveal a stark dependence on metal identity and oxidation state. In all cases, the N-H BDFEs of coordinated NH3 decreases by >40 kcal/mol from the value in the free gas phase molecule.
- Pappas, Iraklis,Chirik, Paul J.
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supporting information
p. 3498 - 3501
(2015/03/30)
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- Kinetics and thermodynamics of H-/H?/H+ transfer from a rhodium(III) hydride
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The thermodynamics and kinetics of all three cleavage modes for Rh-H, the transfer of H-, H+, or H?, have been studied for the Rh(III) hydride complex Cp*Rh(2-(2-pyridyl)phenyl)H (1a). The thermodynamic hydricity, ΔG°H-, for 1a has been measured (49.5(1) kcal/mol) by heterolytic cleavage of H2 with Et3N in CH3CN. The transfer of H- from 1a to 1-(1-phenylethylidene)pyrrolidinium is remarkably fast (k H- = 3.5(1) × 105 M-1 s -1), making 1a a very efficient catalyst for the ionic hydrogenation of iminium cations. The pKa of 1a in CH3CN has been measured as 30.3(2) with (tert-butylimino)tris(pyrrolidino)phosphorane (12), and the rate constant for H+ transfer from 1a to 12 has been estimated (kH+ = 5(1) × 10-4 M-1 s -1) from the half-life of the equilibration. Thus, 1a is a poor H+ donor both thermodynamically and kinetically. However, 1a transfers H? to TEMPO smoothly, forming a stable Rh(II) radical Cp*Rh(2-(2-pyridyl)phenyl)? (14a) that can activate H2 at room temperature and 1 atm. The metalloradical 14a has a g value of 2.0704 and undergoes reversible one-electron reduction at -1.85 V vs Fc+/Fc in benzonitrile, implying a bond-dissociation enthalpy for the Rh-H bond of 1a of 58.2(3) kcal/mol-among the weakest Rh(III)-H bonds reported. The transfer of H? from 1a to Ar3C? (Ar = p-tBuC 6H4) is fast, with kH? = 1.17(3) × 103 M-1 s-1. Thus, 1a is a good H- and H? donor but a poor H+ donor, a combination that reflects the high energy of the Rh(I) anion [Cp*Rh(2-(2-pyridyl)phenyl)] -.
- Hu, Yue,Norton, Jack R.
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p. 5938 - 5948
(2014/05/20)
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- Saturation kinetics in phenolic O-H bond oxidation by a mononuclear Mn(III)-OH complex derived from dioxygen
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The mononuclear hydroxomanganese(III) complex, [MnIII(OH)(dpaq)] +, which is supported by the amide-containing N5 ligand dpaq (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate) was generated by treatment of the manganese(II) species, [MnII(dpaq)] (OTf), with dioxygen in acetonitrile solution at 25 °C. This oxygenation reaction proceeds with essentially quantitative yield (greater than 98% isolated yield) and represents a rare example of an O2-mediated oxidation of a manganese(II) complex to generate a single product. The X-ray diffraction structure of [MnIII(OH)(dpaq)]+ reveals a short Mn-OH distance of 1.806(13) A, with the hydroxo moiety trans to the amide function of the dpaq ligand. No shielding of the hydroxo group is observed in the solid-state structure. Nonetheless, [MnIII(OH)(dpaq)]+ is remarkably stable, decreasing in concentration by only 10% when stored in MeCN at 25 °C for 1 week. The [MnIII(OH)(dpaq)]+ complex participates in proton-coupled electron transfer reactions with substrates with relatively weak O-H and C-H bonds. For example, [Mn III(OH)(dpaq)]+ oxidizes TEMPOH (TEMPOH = 2,2′-6,6′-tetramethylpiperidine-1-ol), which has a bond dissociation free energy (BDFE) of 66.5 kcal/mol, in MeCN at 25 °C. The hydrogen/deuterium kinetic isotope effect of 1.8 observed for this reaction implies a concerted proton-electron transfer pathway. The [Mn III(OH)(dpaq)]+ complex also oxidizes xanthene (C-H BDFE of 73.3 kcal/mol in dimethylsulfoxide) and phenols, such as 2,4,6-tri-t- butylphenol, with BDFEs of less than 79 kcal/mol. Saturation kinetics were observed for phenol oxidation, implying an initial equilibrium prior to the rate-determining step. On the basis of a collective body of evidence, the equilibrium step is attributed to the formation of a hydrogen-bonding complex between [MnIII(OH)(dpaq)]+ and the phenol substrates.
- Wijeratne, Gayan B.,Corzine, Briana,Day, Victor W.,Jackson, Timothy A.
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supporting information
p. 7622 - 7634
(2014/08/05)
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- Photo-induced proton-coupled electron transfer reactions of acridine orange: Comprehensive spectral and kinetics analysis
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The triplet excited state of acridine orange (3*AO) undergoes a proton-coupled electron transfer (PCET) reaction with tri-tert-butylphenol (ttbPhOH) in acetonitrile. Each of the reaction components possesses a spectroscopic signature, providing a rare opportunity to monitor the individual proton transfer, electron transfer, and H ?-transfer components in parallel via transient absorption spectroscopy. This enhanced optical tracking, along with excited-state thermochemical analysis, facilitates assignment of the mechanism of excited-state PCET reactivity. 3*AO is quenched via concerted proton-electron transfer (CPET) from ttbPhOH to form acridine radical (AOH?) and ttbPhO? (kCPET = 3.7 × 108 M-1 s-1, KIE = 1.3). Subsequently, AOH? reduces the phenoxyl radical (kET = 5.5 × 109 M-1 s-1), forming AOH + and ttbPhO-, followed by proton transfer (kPT = 1.0 × 109 M-1 s-1) to regenerate the starting reactants.
- Eisenhart, Thomas T.,Dempsey, Jillian L.
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p. 12221 - 12224
(2014/11/08)
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- Copper-catalyzed oxyazidation of unactivated alkenes: A facile synthesis of isoxazolines featuring an azido substituent
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A novel and efficient Cu(OAc)2-catalyzed oxyazidation of unactivated alkenes was developed. The reactions are easy to conduct, occur under mild conditions, and form azido-substituted isoxazolines in good yields.
- Zhu, Liping,Yu, Hongmei,Xu, Zhaoqing,Jiang, Xianxing,Lin, Li,Wang, Rui
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supporting information
p. 1562 - 1565
(2014/04/17)
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- Aerobic dehydrogenative α-diarylation of benzyl ketones with aromatics through carbon-carbon bond cleavage
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Substituted benzyl ketones reacted with aromatics in the presence of K 2S2O8 in CF3COOH at room temperature, yielding α-diaryl benzyl ketones through a carbon-carbon bond cleavage. In the reaction, two new carbon-carbon bonds were formed and one carbon-carbon bond was cleaved. It is very interesting that two different nucleophiles such as benzyl ketones and aromatics were coupled together without metal, which is unusual in organic synthesis.
- More, Nagnath Yadav,Jeganmohan, Masilamani
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supporting information
p. 804 - 807
(2014/03/21)
-
