Oxidation of alkylarenes by nitrate catalyzed by polyoxophosphomolybdates: synthetic applications and mechanistic insights

Alkylarenes were catalytically and selectively oxidized to the corresponding benzylic acetates and carbonyl products by nitrate salts in acetic acid in the presence of Keggin type molybdenum-based heteropolyacids, H(3+)(x)()PV(x)()Mo(12)(-)(x)()O(40) (x = 0-2). H(5)PV(2)Mo(10)O(40) was especially effective. For methylarenes there was no over-oxidation to the carboxylic acid contrary to what was observed for nitric acid as oxidant. The conversion to the aldehyde/ketone could be increased by the addition of water to the reaction mixture. As evidenced by IR and (15)N NMR spectroscopy, initially the nitrate salt reacted with H(5)PV(2)Mo(10)O(40) to yield a N(V)O(2)(+)[H(4)PV(2)Mo(10)O(40)] intermediate. In an electron-transfer reaction, the proposed N(V)O(2)(+)[H(4)PV(2)Mo(10)O(40)] complex reacts with the alkylarene substrate to yield a radical-cation-based donor-acceptor intermediate, N(IV)O(2)[H(4)PV(2)Mo(10)O(40)]-ArCH(2)R(+)(*). Concurrent proton transfer yields an alkylarene radical, ArCHR(*), and NO(2). Alternatively, it is possible that the N(V)O(2)(+)[H(4)PV(2)Mo(10)O(40)] complex abstracts a hydrogen atom from alkylarene substrate to directly yield ArCHR(*) and NO(2). The electron transfer-proton transfer and hydrogen abstraction scenarios are supported by the correlation of the reaction rate with the ionization potential and the bond dissociation energy at the benzylic positions of the alkylarene, respectively, the high kinetic isotope effect determined for substrates deuterated at the benzylic position, and the reaction order in the catalyst. Product selectivity in the oxidation of phenylcyclopropane tends to support the electron transfer-proton transfer pathway. The ArCHR(*) and NO(2) radical species undergo heterocoupling to yield a benzylic nitrite, which undergoes hydrolysis or acetolysis and subsequent reactions to yield benzylic acetates and corresponding aldehydes or ketones as final products.     

Oxygen transfer from sulfoxides: selective oxidation of alcohols catalyzed by polyoxomolybdates

Benzylic, allylic, and aliphatic alcohols are oxidized to aldehydes and ketones in a reaction catalyzed by Keggin-type polyoxomolybdates, PV(x)Mo(12-x)O(40)(-(3+x)) (x = 0, 2), with DMSO as a solvent. The oxidation of benzylic alcohols is quantitative within hours and selective, whereas that of allylic alcohols is less selective. Oxidation of aliphatic alcohols is slower but selective. Further mechanistic studies revealed that, for H(3)PMo(12)O(40) as a catalyst and benzylic alcohols as substrates, the sulfoxide is in fact an oxygen donor in the reaction. Postulated reaction steps as determined from isotope-labeling experiments, kinetic isotope effects, and Hammett plots include (a) sulfoxide activation by complexation to the polyoxometalate and (b) oxygen transfer from the activated sulfoxide and elimination of water from the alcohol. The mechanism is supported by the reaction kinetics.     

Kinetics and mechanism of oxygen atom transfer from methyl phenyl sulfoxide to triarylphosphines catalyzed by an oxorhenium(V) dimer

An oxorhenium(V) dimer, [PMeReO(mtp)](2), D, where mtpH(2) is 2-(mercaptomethyl)thiophenol, catalyzes oxygen atom transfer reaction from methyl phenyl sulfoxide to triarylphosphines. Kinetic studies in benzene-d(6) at 23 degrees C indicate that the reaction takes place through the formation of an adduct between D and sulfoxide. The equilibrium constants, K(DL), for adduct formation were determined by spectrophotometric titration, and the values of K(DL) for MeS(O)C(6)H(4)-4-R were obtained as 14.1(2), 5.7(1), and 2.1(1) for R = Me, H, and Br, respectively. Following sulfoxide binding, oxygen atom transfer occurs with either internal or external nucleophilic assistance. Because [MeReO(mtp)](2) is a much more reactive catalyst than its monomerized form, MeReO(mtp)PPh(3), loss of the active catalyst during the time course of the reaction must be taken into account as a part of the kinetic analysis. As it happens, sulfoxide catalyzes monomerization. Monomerization by triarylphosphines was also studied in the presence of sulfoxide, and a mechanism for that reaction was also proposed. Both the phosphine-assisted monomerization and the phosphine-assisted pathway for oxygen atom transfer involve transition states with ternary components, D, sulfoxide, and phosphine, which we suggest are structural isomers of one another.     

Catalytic Direct‐Type Addition Reactions of Alkylarenes with Imines and Alkenes

Catalytic addition reactions of very weakly acidic nonactivated alkylarenes such as toluene and its derivatives were developed by using a strongly basic mixed catalyst system under mild reaction conditions. The addition reactions with imines and alkenes proceeded smoothly under proton‐transfer conditions to afford the desired products in good to high yields, and high levels of regio‐ and stereoselectivity were achieved. It was also revealed that the asymmetric addition reaction of an alkylarene was possible.     

Oxidative C-C bond cleavage of primary alcohols and vicinal diols catalyzed by H5PV2Mo10O40 by an electron transfer and oxygen transfer reaction mechanism

Primary alcohols such as 1-butanol were oxidized by the H5PV2Mo10O40 polyoxometalate in an atypical manner. Instead of C-H bond activation leading to the formation of butanal and butanoic acid, C-C bond cleavage took place leading to the formation of propanal and formaldehyde as initial products. The latter reacted with the excess 1-butanol present to yield butylformate and butylpropanate in additional oxidative transformations. Kinetic studies including measurement of kinetic isotope effects, labeling studies with 18O labeled H5PV2Mo10O40, and observation of a prerate determining step intermediate by 13C NMR leads to the formulation of a reaction mechanism based on electron transfer from the substrate to the polyoxometalate and oxygen transfer from the reduced polyoxometalate to the organic substrate. It was also shown that vicinal diols such as 1,2-ethanediol apparently react by a similar reaction mechanism.     

应用超微电极和粉末微电极技术研究有机溶剂和正极材料对氧气还原反应和氧气生成反应的影响

采用碳纤维超微电极分别研究了O_2在二甲基亚砜、乙腈和四甘醇二甲醚3种有机溶剂中的电化学反应,结果表明,当阳离子只含四丁胺离子时,反应呈可逆的一电子转移;而阳离子只含锂离子时,O_2的还原和氧化均经历了多电子转移过程.利用超导炭黑和乙炔黑制作粉末微电极进行电化学测试,结果表明,在这2种正极材料上,氧气还原反应(ORR)过程相似,氧气生成反应(OER)过程区别明显.此外,Tafel分析结果表明,对于不同有机溶剂和正极材料,O_2还原均经历了初始的一电子转移步骤.     

Decomposition of Aryldiazonium Ions in Homogenous Solutions Part II: Arylations with benzenediazonium ions and p-nitrobenzenediazonium ions in dimethyl sulfoxide

When p-nitrobenzenediazonium tetrafluoroborate is decomposed in the presence of dimethyl sulfoxide/benzene or dimethyl sulfoxide/nitrobenzene systems under nitrogen, the respective biphenyl derivatives are obtained in good yield. With benzenediazonium tetrafluoroborate in the same systems the yields are low. The influence of oxygen, iodine, iodobenzene and N, N-diphenylhydroxylamine on the products of the reactions were determined. The by-products, isomer distribution with nitrobenzene as a substrate, and the partial and total rate factors were also determined. From the above data it can be shown that the reaction with p-nitrobenzenediazonium salt is a homolytic process and that with the unsubstituted benzenediazonium salt is essentially a heterolytic reaction. The problem of the mechanism of the radical forming steps (electron transfer or adduct formation) and the mechanistic relationship to other modes of arylation (aroylperoxide, classical two-phase Gomberg-Bachmann reaction, catalysis by transition metal ions, nitrite ions, etc. electron transfer properties of DMSO) are discussed.     

Gauging the significance of atomic oxygen [O(3P)] in sulfoxide photochemistry. A method for hydrocarbon oxidation

A liquid-phase photolysis of 1,2-benzodiphenylene sulfoxide, 1, and dibenzothiophene sulfoxide, 2, was used to generate atomic oxygen [O(3P)] or an equivalent active oxygen species. The reaction for sulfoxide photodeoxygenation was similar to a microwave discharge method for generating O(3P) atoms in the condensed phase (Zadok, E.; Rubinraut, S.; Mazur, Y. J. Org. Chem. 1987, 52, 385-90). Sulfoxide photodeoxygenation is a potentially clean method for O(3P) production compared to the microwave discharge method. With Argon purging of the sulfoxide sample before photolysis, the method can preclude a secondary oxidation process involving molecular oxygen. Our study focused on the results of oxidation products in the reaction of styrene, 3, and on the dependence of substrates that provided an opportunity to vary the electronic and steric effects. The sulfoxide photochemistry is rationalized with the primary formation of O(3P) in which a charge-transfer interaction between O(3P) and substrate precedes oxidation. Functionalization of hydrocarbons takes place under mild photolysis conditions of 1 and 2, which leads to an interesting possibility for the synthetic use of atomic oxygen, O(3P). Alkanes give principally alcohols. Alkenes give principally epoxides and ketones. For comparison, hydroxyl radicals are more reactive and less selective toward hydrocarbons compared to O(3P) atoms. On the other hand, O(3P) atoms balance reactivity and selectivity and involve the oxidation of inert alkanes typically inaccessible to peracid, dioxirane, ozone, and singlet molecular oxygen chemistry. The findings from this study may be useful to those interested in generating high-value oxygenated compounds from readily available petroleum components.     

Selenocysteine as a Substrate,an Inhibitor and a Mechanistic Probe for Bacterial and Fungal Iron-Dependent Sulfoxide Synthases

Sulfoxide synthases are non-heme iron enzymes that participate in the biosynthesis of thiohistidines, such as ergothioneine and ovothiol A. The sulfoxide synthase EgtB from Chloracidobacterium thermophilum (CthEgtB) catalyzes oxidative coupling between the side chains of N-α-trimethyl histidine (TMH) and cysteine (Cys) in a reaction that entails complete reduction of molecular oxygen, carbon–sulfur (C−S) and sulfur–oxygen (S−O) bond formation as well as carbon–hydrogen (C−H) bond cleavage. In this report, we show that CthEgtB and other bacterial sulfoxide synthases cannot efficiently accept selenocysteine (SeCys) as a substrate in place of cysteine. In contrast, the sulfoxide synthase from the filamentous fungus Chaetomium thermophilum (CthEgt1) catalyzes C−S and C−Se bond formation at almost equal efficiency. We discuss evidence suggesting that this functional difference between bacterial and fungal sulfoxide synthases emerges from different modes of oxygen activation.     

Direct photooxidation and xanthene-sensitized oxidation of naphthols: quantum yields and mechanism

The photoinduced oxidation of 1-naphthol to 1,4-naphthoquinone and of 5-hydroxy-1-naphthol to 5-hydroxy-1,4-naphthoquinone was studied by steady-state and time-resolved techniques. The direct photooxidation of naphthols in methanol or water takes place by reaction of the naphoxyl radical ((?)ONaph) with the superoxide ion radical (O(2)(?-)), the latter of which results from the reaction of the solvated electron with oxygen after photoionization. The sensitized oxidation takes place by energy transfer from the xanthene triplet state to oxygen. From the two oxygen atoms, which are consumed, one is incorporated into the naphthol molecule giving naphthoquinone and the second gives rise to water. The effects of eosin, erythrosin, and rose bengal in aqueous solution, pH, and the oxygen and naphthol concentrations were studied. The quantum yield of the photosensitized transformation was determined, which increases with the naphthol concentration and is largest at pH > 10. The quantum yield of oxygen uptake is similar. The pathway involving singlet molecular oxygen is suggested to operate for the three sensitizers. The alternative pathway via electron transfer from the naphthol to the xanthene triplet state and subsequent reaction of (?)ONaph with O(2)(?-), the latter of which is formed by scavenging of the xanthene radical anion by oxygen, does also contribute.     

A chromium(III)-superoxo complex in oxygen atom transfer reactions as a chemical model of cysteine dioxygenase

Metal-superoxo species are believed to play key roles in oxygenation reactions by metalloenzymes. One example is cysteine dioxygenase (CDO) that catalyzes the oxidation of cysteine with O(2), and an iron(III)-superoxo species is proposed as an intermediate that effects the sulfoxidation reaction. We now report the first biomimetic example showing that a chromium(III)-superoxo complex bearing a macrocyclic TMC ligand, [Cr(III)(O(2))(TMC)(Cl)](+), is an active oxidant in oxygen atom transfer (OAT) reactions, such as the oxidation of phosphine and sulfides. The electrophilic character of the Cr(III)-superoxo complex is demonstrated unambiguously in the sulfoxidation of para-substituted thioanisoles. A Cr(IV)-oxo complex, [Cr(IV)(O)(TMC)(Cl)](+), formed in the OAT reactions by the chromium(III)-superoxo complex, is characterized by X-ray crystallography and various spectroscopic methods. The present results support the proposed oxidant and mechanism in CDO, such as an iron(III)-superoxo species is an active oxidant that attacks the sulfur atom of the cysteine ligand by the terminal oxygen atom of the superoxo group, followed by the formation of a sulfoxide and an iron(IV)-oxo species via an O-O bond cleavage.     

Kinetics and mechanisms of catalytic oxygen atom transfer with oxorhenium(V) oxazoline complexes

The rhenium(V) monooxo complexes (hoz)2Re(O)Cl (1) and [(hoz)2Re(O)(OH2)][OTf] (2) have been synthesized and fully characterized (hoz = 2-(2'-hydroxyphenyl)-2-oxazoline). A single-crystal X-ray structure of 2 has been solved: space group = P1, a = 13.61(2) A, b = 14.76(2) A, c = 11.871(14) A, alpha = 93.69(4) degrees, beta = 99.43(4) degrees, gamma = 108.44(4) degrees, Z = 4; the structure was refined to final residuals R = 0.0455 and Rw = 0.1055. 1 and 2 catalyze oxygen atom transfer from aryl sulfoxides to alkyl sulfides and oxygen-scrambling between sulfoxides to yield sulfone and sulfide. Superior catalytic activity has been observed for 2 due to the availability of a coordination site on the rhenium. The active form of the catalyst is a dioxo rhenium(VII) intermediate, [Re(O)2(hoz)2]+ (3). In the presence of sulfide, 3 is rapidly reduced to [Re(O)(hoz)2]+ with sulfoxide as the sole organic product. The transition state is very sensitive to electronic influences. A Hammett correlation plot with para-substituted thioanisole derivatives gave a reaction constant rho of -4.6 +/- 0.4, in agreement with an electrophilic oxygen transfer from rhenium. The catalytic reaction features inhibition by sulfides at high concentrations. The equilibrium constants for sulfide binding to complex 2 (cause of inhibition), K2 (L x mol(-1)), were determined for a few sulfides: Me2S (22 +/- 3), Et2S (14 +/- 2), and tBu2S (8 +/- 2). Thermodynamic data, obtained from equilibrium measurements in solution, show that the S=O bond in alkyl sulfoxides is stronger than in aryl sulfoxides. The Re=O bond strength in 3 was estimated to be about 20 kcal x mol(-1). The high activity and oxygen electrophilicity of complex 3 are discussed and related to analogous molybdenum systems.     

Mechanism of the oxidation of aromatic sulfides catalysed by a water soluble iron porphyrin

The oxygen atom transfer-electron transfer (ET) mechanistic dichotomy has been investigated in the oxidation of a number of aryl sulfides by H2O2 in acidic (pH 3) aqueous medium catalysed by the water soluble iron(III) porphyrin 5,10,15,20-tetraphenyl-21H,23H-porphine-p,p',p",p"'-tetrasulfonic acid iron(III) chloride (FeTPPSCl). Under these reaction conditions, the iron-oxo complex porphyrin radical cation, P+. Fe(IV)=O, should be the active oxidant. When the oxidation of a series of para-X substituted phenyl alkyl sulfides (X = OCH3, CH3, H, Br, CN) was studied the corresponding sulfoxides were the only observed product and the reaction yields as well as the reactivity were little influenced by the nature of X as well as by the bulkiness of the alkyl group. Labelling experiments using H(2)18O or H(2)18O2 clearly indicated that the oxygen atom in the sulfoxides comes exclusively from the oxidant. Moreover, no fragmentation products were observed in the oxidation of a benzyl phenyl sulfide whose radical cation is expected to undergo cleavage of the beta C-H and C-S bonds. These results would seem to suggest a direct oxygen atom transfer from the iron-oxo complex to the sulfide. However, competitive experiments between thioanisole (E degree = 1.49 V vs. NHE in H2O) and N,N-dimethylaniline (E degree = 0.97 V vs. NHE in H2O) resulted in exclusive N-demethylation, whereas the oxidation of N-methylphenothiazine (10, E degree = 0.95 V vs. NHE in CH3CN) and N,N-dimethyl-4-methylthioaniline (11, E degree = 0.65 V vs. NHE in H2O) produced the corresponding sulfoxide with complete oxygen incorporation from the oxidant. Since an ET mechanism must certainly hold in the reactions of 10 and 11, the oxygen incorporation experiments indicate that the intermediate radical cation, once formed, has to react with PFe(IV)=O (the reduced form of the iron-oxo complex which is formed by the ET step) in a fast oxygen rebound. Thus, an ET step followed by a fast oxygen rebound is also suggested for the other sulfides investigated in this work.     

A Visible‐Light‐Induced α‐H Chlorination of Alkylarenes with Inorganic Chloride under NanoAg@AgCl

An efficient, photocatalytic chlorination of alkylarene α‐H groups using NaCl/HCl as a chlorine source has been developed, which involves a radical mechanism under visible‐light (including sunlight) conditions. A chlorine radical is proposed to be formed by an electron transfer from chloride ion to O₂ in air through the bandgap hole of the semiconductor AgCl. The chlorination protocol is characterized by its use of natural sunlight or other visible light, mild conditions, cheap source of chlorine, green solvent, and high selectivity. The yield of benzylchloride is 95 % with a toluene conversion as high as 40 %, which rivals traditional chlorination methods.     

碳纳米管粉末微电极技术研究超氧自由基的电产生及化学性质

将多壁碳纳米管填充在粉末微电极尖端的小孔里制成碳纳米管粉末微电极,研究氧单电子还原产生超氧自由基的电化学行为.在二甲亚砜(DMSO)介质中,该电极反应是一个近乎可逆的还原/氧化过程,峰电位差(ΔEp)120mV,并显示出良好的稳态伏安曲线.根据极化曲线算得该电极反应的异相电荷传递速率常数ks=9.8×10-3cm/s.此外,还研究了超氧自由基的氧化性和碱性,并对相关反应过程作了讨论.     

THE PEROXIDASE/OXIDASE ACTIVITY OF SOYBEAN LIPOXYGENASE – II. TRIPLET CARBONYLS AND RED PHOTOEMISSION DURING POLYUNSATURATED FATTY ACID AND GLUTATHIONE OXIDATION

During the aerobic reaction of soybean lipoxygenase with polyunsaturated fatty acids (linoleic, linolenic, and arachidonic acid) oxygen uptake is followed by excited carbonyl photoemission. The chemiluminescence yield of phi cl = 10(-10) photons/O2 molecule consumed is enhanced 2-3 orders of magnitude by the carbonyl sensitizers 9,10-dibromo-anthracene-2-sulfonate (kET tau 0 = 10(4) M-1; phi cl = 10(-8) photons/O2) and chlorophyll-a (kET tau 0 = 10(6) M-1; phi cl = 10(-7) photons/O2), respectively. alpha,beta-Saturated triplet excited carbonyls as from 1,2-dioxetane cleavage are discussed to arise from a secondary peroxidase/oxidase reaction with aldehydes formed in the course of enzymic lipid peroxidation. When 1 mM glutathione is added to the aerobic lipoxygenase/arachidonate reaction, carbonyl emission (375-455 nm) is replaced by intense red bands (630-645 nm and 695-715 nm) resembling the characteristic spectrum of (1 delta g)O2-singlet oxygen dimol-emission. The quantum yield (phi cl = 10(-8) photons/O2) remains unaffected by chlorophyll indicating that the red emission is independent of excited carbonyls. The effect of GSH is attributed to dioxetane interception and subsequent glutathione peroxidation generating 1O2 by electron transfer from the superoxide anion radical to a peroxysulfenyl radical.     

Efficient photocatalytic oxygenation of aromatic alkene to 1,2-dioxetane with oxygen via electron transfer

[reaction: see text] Photocatalytic oxygenation of tetraphenylethylene (TPE) with oxygen occurs efficiently via electron-transfer reactions of TPE and oxygen with a photogenerated electron transfer state of 9-mesityl-10-methylacridniium ion, followed by the radical-coupling reaction between TPE radical cation and O2*- to produce 1,2-dioxetane selectively. The further photocatalytic cleavage of the O-O bond of dioxetane affords benzophenone as the final oxygenated product.     

Synthesis of Vitamin E Succinate from Candida rugosa Lipase in Organic Medium

A screening of commercially available lipases for the synthesis of vitamin E succinate showed that lipase from Candida rugosa presented the highest yield. The synthesis of vitamin E succinate in organic solvents with different lgP values ranging from -1.3 to 3.5 was investigated. Of particular interest was that dimethyl sulfoxide (DMSO) with the lowest lgP exhibited the highest yield among all the organic solvents used. It suggests that lgP is incapable of satisfactorily predicting the biocompatibility of organic solvents due to the complexity of enzymatic reaction with hydrophilic and hydrophobic substrates in organic solvent. Effects of different operating conditions, such as molar ratio of substrate, enzyme concentration, reaction temperature, mass transfer, and reaction time were also studied. Under the optimum conditions of 10 g/L enzyme, a stirring rate of 100 r/min, a substrate molar ratio of 5:1 at 55℃ for 18 h, a satisfactory yield(46.95%) was obtained. The developed method has a potential to be used for efficient enzymatic production of vitamin E succinate.     

On the mechanisms of oxidation of organic sulfides by H2O2 in aqueous solutions

The mechanism of oxidation of organic sulfides in aqueous solutions by hydrogen peroxide was investigated via ab initio calculations. Specifically, two reactions, hydrogen transfer of hydrogen peroxide to form water oxide and the oxidation of dimethyl sulfide (DMS) by hydrogen peroxide to form dimethyl sulfoxide, were studied as models of these processes in general. Solvent effects are included both via including explicitly water molecules and via the polarizable continuum model. The former was found to have a much more significant effect than the latter. When explicit water molecules are included, a mechanism different from those proposed in the literature was found. Specific interactions including hydrogen bonding with 2-3 water molecules can provide enough stabilization for the charge separation of the activation complex. The energy barrier of the oxidation of DMS by hydrogen peroxide was estimated to be 12.7 kcal/mol, within the experimental range of the oxidation of analogous compounds (10-20 kcal/mol). The major reaction coordinates of the reaction are the breaking of the O-O bond of H2O2 and the formation of the S-O bond, the transfer of hydrogen to the distal oxygen of hydrogen peroxide occurring after the system has passed the transition state. Reaction barriers of the hydrogen transfer of H2O2 are an average of 10 kcal/mol or higher than the reaction barriers of the oxidation of DMS. Therefore, a two-step oxidation mechanism in which, first, the transfer of a hydrogen atom occurs to form water oxide and, second, the transfer of oxygen to the substrate occurs is unlikely to be correct. Our proposed oxidation mechanism does not suggest a pH dependence of oxidation rate within a moderate range around neutral pH (i.e., under conditions in which hydronium and hydroxide ions do not participate directly in the reaction), and it agrees with experimental observations over moderate pH values. Also, without including a protonated solvent molecule, it has activation energies that correspond to measured activation energies.     

多相/均相TiO2/N-羟酰亚胺混合体系中可见光诱导分子氧可控光催化氧化苄基反应

均相催化和多相催化通常被认为是独立甚至相互对立的学科.本文提出了一种新型的用于分子氧选择性氧化烷基苯的杂多酸/均相混合催化体系.该催化体系由N-羟基邻苯二甲酰亚胺(NHPI,用于自由基链式反应的均相有机催化剂)和纳米TiO2(多相紫外光活性光氧化催化剂)两种组分组成.NHPI与TiO2的协同作用使光氧化活性从紫外光转移到可见光,并产生邻苯二甲酰亚胺-N-氧基(PINO)自由基.NHPI/PINO催化的自由基链式反应能够在没有额外光输入的情况下进行,从而从根本上提高能源效率.通过控制NHPI/TiO2比率优化产物选择性,进而使烷基芳烃优先形成过氧化氢或酮.