- Acid p Ka Dependence in O-O Bond Heterolysis of a Nonheme FeIII-OOH Intermediate to Form a Potent FeVa? O Oxidant with Heme Compound I-Like Reactivity
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Protons play essential roles in natural systems in controlling O-O bond cleavage of peroxoiron(III) species to give rise to the high-valent iron oxidants that carry out the desired transformations. Herein, we report kinetic and mechanistic evidence that acids can control the mode of O-O bond cleavage for a nonheme S = 1/2 FeIII-OOH species [(BnTPEN)FeIII(OOH)]2+ (2, BnTPEN = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)-1,2-diaminoethane). Addition of acids having pKa values of >8.5 in CH3CN results in O-O bond homolysis, leading to the formation of hydroxyl radicals that give rise to alcohol/ketone (A/K) ratios of around 1 in the oxidation of cyclohexane. However, the introduction of acids with pKa values of III-OOH intermediate at -40 °C. These results implicate the generation of a highly reactive FeV? O species via proton-assisted O-O bond heterolysis of the FeIII-OOH intermediate, which is unprecedented for nonheme iron complexes supported by neutral pentadentate ligands and serves as a nonheme analogue for heme enzyme compounds I.
- Xu, Shuangning,Draksharapu, Apparao,Rasheed, Waqas,Que, Lawrence
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supporting information
p. 16093 - 16107
(2019/10/14)
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- High Catalytic Activity of Vanadium Complexes in Alkane Oxidations with Hydrogen Peroxide: An Effect of 8-Hydroxyquinoline Derivatives as Noninnocent Ligands
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Five monomeric oxovanadium(V) complexes [VO(OMe)(NO)2] with the nitro or halogen substituted quinolin-8-olate ligands were synthesized and characterized using Fourier transform infrared, 1H and 13C NMR, high-resolution mass spectrometry-electrospray ionization as well as X-ray diffraction and UV-vis spectroscopy. These complexes exhibit high catalytic activity toward oxidation of inert alkanes to alkyl hydroperoxides by H2O2 in aqueous acetonitrile with the yield of oxygenate products up to 39% and turnover number 1780 for 1 h. The experimental kinetic study, the C6D12 and 18O2 labeled experiments, and density functional theory (DFT) calculations allowed to propose the reaction mechanism, which includes the formation of HO· radicals as active oxidizing species. The mechanism of the HO· formation appears to be different from those usually accepted for the Fenton or Fenton-like systems. The activation of H2O2 toward homolysis occurs upon simple coordination of hydrogen peroxide to the metal center of the catalyst molecule and does not require the change of the metal oxidation state and formation of the HOO· radical. Such an activation is associated with the redox-active nature of the quinolin-8-olate ligands. The experimentally determined activation energy for the oxidation of cyclohexane with complex [VO(OCH3)(5-Cl-quin)2] (quin = quinolin-8-olate) is 23 ± 3 kcal/mol correlating well with the estimate obtained from the DFT calculations.
- Gryca, Izabela,Czerwińska, Katarzyna,MacHura, Barbara,Chrobok, Anna,Shul'Pina, Lidia S.,Kuznetsov, Maxim L.,Nesterov, Dmytro S.,Kozlov, Yuriy N.,Pombeiro, Armando J. L.,Varyan, Ivetta A.,Shul'Pin, Georgiy B.
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supporting information
p. 1824 - 1839
(2018/02/27)
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- Palladium-Catalyzed Hydrolytic Cleavage of Aromatic C?O Bonds
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Metallic palladium surfaces are highly selective in promoting the reductive hydrolysis of aromatic ethers in aqueous phase at relatively mild temperatures and pressures of H2. At quantitative conversions, the selectivity to hydrolysis products of PhOR ethers was observed to range from 50 % (R=Ph) to greater than 90 % (R=n-C4H9, cyclohexyl, and PhCH2CH2). By analysis of the evolution of products with and without incorporation of H218O, the pathway was concluded to be initiated by palladium metal catalyzed partial hydrogenation of the phenyl group to an enol ether. Water then rapidly adds to the enol ether to form a hemiacetal, which then undergoes elimination to cyclohexanone and phenol/alkanol products. A remarkable feature of the reaction is that the stronger Ph?O bond is cleaved rather than the weaker aliphatic O?R bond.
- Wang, Meng,Shi, Hui,Camaioni, Donald M.,Lercher, Johannes A.
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supporting information
p. 2110 - 2114
(2017/02/15)
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- Hydroxylation versus Halogenation of Aliphatic C?H Bonds by a Dioxygen-Derived Iron–Oxygen Oxidant: Functional Mimicking of Iron Halogenases
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An iron–oxygen intermediate species generated in situ in the reductive activation of dioxygen by an iron(II)–benzilate complex of a monoanionic facial N3ligand, promoted the halogenation of aliphatic C?H bonds in the presence of a protic acid and a halide anion. An electrophilic iron(IV)–oxo oxidant with a coordinated halide is proposed as the active oxidant. The halogenation reaction with dioxygen and the iron complex mimics the activity of non-heme iron halogenases.
- Chatterjee, Sayanti,Paine, Tapan Kanti
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supporting information
p. 7717 - 7722
(2016/07/07)
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- Alkane oxidation with peroxides catalyzed by cage-like copper(II) silsesquioxanes
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Isomeric cage-like tetracopper(II) silsesquioxane complexes [(PhSiO1.5)12(CuO)4(NaO0.5)4] (1a), [(PhSiO1.5)6(CuO)4(NaO0.5)4(PhSiO1.5)6] (1b) and binuclear complex [(PhSiO1.5)10(CuO)2(NaO0.5)2] (2) have been studied by various methods. These compounds can be considered as models of some multinuclear copper-containing enzymes. Compounds 1a and 2 are good pre-catalysts for the alkane oxygenation with hydrogen peroxide in air in an acetonitrile solution. Thus, the 1a-catalyzed reaction with cyclohexane at 60°C gave mainly cyclohexyl hydroperoxide in 17% yield (turnover number, TON, was 190 after 230 min and initial turnover frequency, TOF, was 100 h-1). The alkyl hydroperoxide partly decomposes in the course of the reaction to afford the corresponding ketone and alcohol. The effective activation energy for the cyclohexane oxygenation catalyzed by compounds 1a and 2 is 16 ± 2 and 17 ± 2 kcal mol-1, respectively. Selectivity parameters measured in the oxidation of linear and branched alkanes and the kinetic analysis revealed that the oxidizing species in the reaction is the hydroxyl radical. The analysis of the dependence of the initial reaction rate on the initial concentration of cyclohexane led to a conclusion that hydroxyl radicals attack the cyclohexane molecules in proximity to the copper reaction centers. The oxidations of saturated hydrocarbons with tert-butylhydroperoxide (TBHP) catalyzed by complexes 1a and 2 exhibit unusual selectivity parameters which are due to the steric hindrance created by bulky silsesquioxane ligands surrounding copper reactive centers. Thus, the methylene groups in n-octane have different reactivities: the regioselectivity parameter for the oxidation with TBHP catalyzed by 1a is 1:10.5:8:7. Furthermore, in the oxidation of methylcyclohexane the position 2 relative to the methyl group of this substrate is noticeably less reactive than the corresponding positions 3 and 4. Finally, the oxidation of trans-1,2-dimethylcyclohexane with TBHP catalyzed by complexes 1a and 2 proceeds stereoselectively with the inversion of configuration. The 1a-catalyzed reaction of cyclohexane with H216O2 in an atmosphere of 18O2 gives cyclohexyl hydroperoxide containing up to 50% of 18O. The small amount of cyclohexanone, produced along with cyclohexyl hydroperoxide, is 18O-free and is generated apparently via a mechanism which does not include hydroxyl radicals and incorporation of molecular oxygen from the atmosphere.
- Vinogradov, Mikhail M.,Kozlov, Yuriy N.,Bilyachenko, Alexey N.,Nesterov, Dmytro S.,Shul'pina, Lidia S.,Zubavichus, Yan V.,Pombeiro, Armando J. L.,Levitsky, Mikhail M.,Yalymov, Alexey I.,Shul'pin, Georgiy B.
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p. 187 - 199
(2015/02/19)
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- Photocatalytic oxidation of benzyl alcohol by homogeneous CuCl2/solvent: A model system to explore the role of molecular oxygen
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The oxidation of alcohols to the corresponding carbonyl compounds is a pivotal reaction in organic synthesis. Under visible light irradiation, the homogeneous CuCl2 and cheap solvent oxidized benzyl alcohol into benzaldehyde with a selectivity higher than 95% using molecular oxygen as an oxidant. The formation of a visible light responsive complex between Cu(II) and solvent is responsible for the occurrence of the oxidation of benzyl alcohol. During the photocatalytic process, molecular oxygen was not incorporated into the final benzaldehyde and only involved in the oxidation of Cu(I) into Cu(II) in which it served as a terminal hydrogen acceptor to form H2O. A similar role of molecular oxygen has also been observed in the heterogeneous TiO2 photocatalytic system. The understanding of the role of molecular oxygen helps us to further design new classes of synthetic organic reactions by photocatalytic processes.
- Meng, Chao,Yang, Kai,Fu, Xianzhi,Yuan, Rusheng
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p. 3760 - 3766
(2015/06/16)
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- Catalytic oxidation of alkanes by a (salen)osmium(VI) nitrido complex using H2O2 as the terminal oxidant
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The osmium(vi) nitrido complex, [OsVI(N)(L)(CH3OH)]+ (1, L = N,N′-bis(salicylidene)-o-cyclohexyldiamine dianion) is an efficient catalyst for the oxidation of alkanes under ambient conditions using H2O2 as the oxidant. Alkanes are oxidized to the corresponding alcohols and ketones, with yields up to 75% and turnover numbers up to 2230. Experimental and computational studies are consistent with a mechanism that involves O-atom transfer from H2O2 to [OsVI(N)(L)]+ to generate an [OsVIII(N)(O)(L)]+ active intermediate.
- Chen, Man,Pan, Yi,Kwong, Hoi-Ki,Zeng, Raymond J.,Lau, Kai-Chung,Lau, Tai-Chu
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supporting information
p. 13686 - 13689
(2015/09/02)
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- Mechanistic elucidation of C-H oxidation by electron rich non-heme iron(IV)-oxo at room temperature
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Non-heme iron(iv)-oxo species form iron(iii) intermediates during hydrogen atom abstraction (HAA) from the C-H bond. While synthesizing a room temperature stable, electron rich, non-heme iron(iv)-oxo compound, we obtained iron(iii)-hydroxide, iron(iii)-alkoxide and hydroxylated-substrate-bound iron(ii) as the detectable intermediates. The present study revealed that a radical rebound pathway was operative for benzylic C-H oxidation of ethylbenzene and cumene. A dissociative pathway for cyclohexane oxidation was established based on UV-vis and radical trap experiments. Interestingly, experimental evidence including O-18 labeling and mechanistic study suggested an electron transfer mechanism to be operative during C-H oxidation of alcohols (e.g. benzyl alcohol and cyclobutanol). The present report, therefore, unveils non-heme iron(iv)-oxo promoted substrate-dependent C-H oxidation pathways which are of synthetic as well as biological significance.
- Rana, Sujoy,Dey, Aniruddha,Maiti, Debabrata
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supporting information
p. 14469 - 14472
(2015/09/28)
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- How small amounts of impurities are sufficient to catalyze the interconversion of carbonyl compounds and iminium ions, or is there a metathesis through 1,3-oxazetidinium ions? Experiments, speculations, and calculations
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Under the 'best anhydrous' conditions, we were able to achieve, the bicyclic oxazolidinones derived from proline and pivalaldehyde (or cyclohexanone) equilibrate with added carbonyl compounds in (D6)DMSO and in (D6)benzene. With (su
- Seebach, Dieter,Yoshinari, Tomohiro,Beck, Albert K.,Ebert, Marc-Olivier,Castro-Alvarez, Alejandro,Vilarrasa, Jaume,Reiher, Markus
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p. 1177 - 1203
(2015/04/14)
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- Highly efficient alkane oxidation catalyzed by [MnV(N)(CN) 4]2-. Evidence for [MnVII(N)(O)(CN) 4]2- as an active intermediate
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The oxidation of various alkanes catalyzed by [MnV(N)(CN) 4]2- using various terminal oxidants at room temperature has been investigated. Excellent yields of alcohols and ketones (>95%) are obtained using H2O2 as oxidant and CF3CH 2OH as solvent. Good yields (>80%) are also obtained using (NH4)2[Ce(NO3)6] in CF 3CH2OH/H2O. Kinetic isotope effects (KIEs) are determined by using an equimolar mixture of cyclohexane (c-C6H 12) and cyclohexane-d12 (c-C6D12) as substrate. The KIEs are 3.1 ± 0.3 and 3.6 ± 0.2 for oxidation by H2O2 and Ce(IV), respectively. On the other hand, the rate constants for the formation of products using c-C6H12 or c-C6D12 as single substrate are the same. These results are consistent with initial rate-limiting formation of an active intermediate between [Mn(N)(CN)4]2- and H2O2 or CeIV, followed by H-atom abstraction from cyclohexane by the active intermediate. When PhCH2C(CH3)2OOH (MPPH) is used as oxidant for the oxidation of c-C6H12, the major products are c-C6H11OH, c-C6H10O, and PhCH2C(CH3)2OH (MPPOH), suggesting heterolytic cleavage of MPPH to generate a Mn=O intermediate. In the reaction of H2O2 with [Mn(N)(CN)4]2- in CF 3CH2OH, a peak at m/z 628.1 was observed in the electrospray ionization mass spectrometry, which is assigned to the solvated manganese nitrido oxo species, (PPh4)[Mn(N)(O)(CN)4] -·CF3CH2OH. On the basis of the experimental results the proposed mechanism for catalytic alkane oxidation by [MnV(N)(CN)4]2-/ROOH involves initial rate-limiting O-atom transfer from ROOH to [Mn(N)(CN)4]2- to generate a manganese(VII) nitrido oxo active species, [MnVII(N)(O) (CN)4]2-, which then oxidizes alkanes (R'H) via a H-atom abstraction/O-rebound mechanism. The proposed mechanism is also supported by density functional theory calculations.
- Ma, Li,Pan, Yi,Man, Wai-Lun,Kwong, Hoi-Ki,Lam, William W.Y.,Chen, Gui,Lau, Kai-Chung,Lau, Tai-Chu
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p. 7680 - 7687
(2014/06/10)
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- Evidence for light-induced oxygen exchange in the oxidation of liquid hydrocarbons on oxygen 18-labelled titanium dioxide
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Titanium dioxide (TiO2) nanoparticles labelled with 18O isotopes were prepared via a sol-gel method with titanium alkoxide and 18O water precursors. The presence of 18O in the labelled TiO2 sample was confirmed using Raman spectroscopy and secondary-ion mass spectrometry. The photocatalysis of cyclohexane under UV light was investigated using 18O-labelled TiO2 as the catalyst; this reaction yielded 18O-containing cyclohexanone as the major product. The 18O content in the cyclohexanone product dramatically decreased with increasing reaction time. This isotopic experiment demonstrates that the oxygen atom in cyclohexanone originates from surface oxygen in 18O-labelled TiO2. Similar photoreactions performed with toluene and para-xylene as the hydrocarbon substrates afforded oxidation products containing 18O isotopes. The migration of oxygen atoms in 18O-labelled TiO2 into liquid hydrocarbons provides direct evidence for the involvement of the solid oxide in the interfacial reaction between a liquid substrate and surface oxygen in a solid catalyst.
- Choi, Jongmyung,Kang, Donghyeon,Lee, Kyu Hyung,Lee, Byeongno,Kim, Kyung Joong,Hur, Nam Hwi
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p. 9402 - 9407
(2013/07/27)
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- Evidence for an alternative to the oxygen rebound mechanism in C-H bond activation by non-heme FeIVO complexes
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The hydroxylation of alkanes by heme FeIVO species occurs via the hydrogen abstraction/oxygen rebound mechanism. It has been assumed that non-heme FeIVO species follow the heme FeIVO paradigm in C-H bond activation reactions. Herein we report theoretical and experimental evidence that C-H bond activation of alkanes by synthetic non-heme Fe IVO complexes follows an alternative mechanism. Theoretical calculations predicted that dissociation of the substrate radical formed via hydrogen abstraction from the alkane is more favorable than the oxygen rebound and desaturation processes. This theoretical prediction was verified by experimental results obtained by analyzing iron and organic products formed in the C-H bond activation of substrates by non-heme FeIVO complexes. The difference in the behaviors of heme and non-heme FeIVO species is ascribed to differences in structural preference and exchange-enhanced reactivity. Thus, the general consensus that C-H bond activation by high-valent metal-oxo species, including non-heme FeIVO, occurs via the conventional hydrogen abstraction/oxygen rebound mechanism should be viewed with caution.
- Cho, Kyung-Bin,Wu, Xiujuan,Lee, Yong-Min,Kwon, Yoon Hye,Shaik, Sason,Nam, Wonwoo
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supporting information
p. 20222 - 20225
(2013/02/23)
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- Reactions of Ozone with 1-Methylcyclohexene and Methylenecyclohexane in Air
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Reactions of ozone with 1-methylcyclohexene and methylenecyclohexane were studied for the purpose of obtaining the reaction mechanism as a prototype of the reaction of ozone with pinenes.The similarity of the yields of corresponding gaseous products between 1-methylcyclohexene and α-pinene and between methylenecyclohexane and β-pinene indicates the similarity of the reaction mechanisms.
- Hatakeyama, Shiro,Akimoto, Hajime
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p. 2701 - 2703
(2007/10/02)
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- On the Mechanism of the Baeyer-Villiger Oxidation of Ketones by Bis(trimethylsilyl) Peroxomonosulfate. Intermediacy of Dioxiranes
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The Baeyer-Villiger oxidation of cyclohexanone (2a) and of acetophenone (2b) by bis(trimethylsilyl)peroxomonosulfate (1) has been reinvestigated using (18)O-labeling techniques.Starting with doubly labeled Me3Si(18)O(18)OSO3SiMe3, mass spectrometric analyses allowed determination of the amount of label appearing in the carbonyl and the OR moiety of the ester (or of the lactone).It has been observed that 2a also promotes the decomposition of 1 to yield oxygen gas, which was analyzed for its (18)O content.Furthermore, ketones 2a, 4-heptanone, and acetone were foundto enhance significantly the rate of oxidation of 1-methylcyclohexene (10) and of trans-β-methylstyrene (13) by 1, yielding 2-methylcyclohexanone (12) and 1-phenylpropanone (15) derived from the isomerization of the initially formed epoxides.These observations, most notably the (18)O-tracer results, point to a mechanism involving the intermediacy of dioxiranes as the prevailing pathway.
- Camporeale, Michele,Fiorani, Tiziana,Troisi, Luigino,Adam, Waldemar,Curci, Ruggero,Edwards, John O.
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