Effect of cationic surfactants on limonene oxidation

Kinetic regularities of free radical formation during limonene oxidation with molecular oxygen in a chlorobenzene solution at 60 °C were studied by the inhibitor method under autooxidation conditions and with additives of surfactants (cetyltrimethylammonium chloride and bromide) using dimer 2,2′-bis[2-(p-dimethylaminophenyl)indane-1,3-dione] and α-tocopherol as acceptors of radicals. In the presence of the surfactants, the rates of radical generation and oxidation increase sharply. The catalytic activity of cetyltrimethylammonium chloride is much higher than that of the corresponding bromide.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2110–2113, October, 2004.     

烷基膦酸键合的纳米氧化物上甲苯选择氧化制苯甲醛的类酶机理

在传统催化反应中,分子氧催化氧化烷烃生成目标产物的选择性最低,常产生副产物以及大量的COx和水,因需要复杂的分离过程而导致流程的投资成本高.因此,在过去几十年里,针对此类反应的研究以提高选择性为首要目标,但克服这一挑战的任务依然艰巨.在此类反应中,甲苯经O2选择性催化氧化是制备苯甲醛的最佳途径,但从未实现工业化,其中的主要难点在于苯甲醛在临氧条件下的继续氧化反应速率比甲苯氧化高4个数量级以上.因此,在高甲苯转化率下高选择性获得苯甲醛是绿色化学研究中一个极具挑战性的课题.本文发现一种甲苯/水的类皮克林乳液催化体系对甲苯转化为苯甲醛有很高的活性和选择性,其中两性纳米颗粒(十六烷基膦酸,简写为HDPA,键合的纳米铁基氧化物)作为催化剂并处在乳液相界面处.在温和条件下通过O2氧化,苯甲醛的时空产率大于2 g·g-1·h-1,整个过程没有使用H2O2、卤素或其它自由基引发剂.实验发现,用于甲苯氧化反应的相关催化剂HDPA-FeMOx/γ-Al2O3(M:V和Mo等)的作用机制类似甲烷单加氧酶.在传统固定床反应器中和温和的条件下(70～160°C,常压),催化剂能将甲苯单一氧化为苯甲醛.催化剂的活性中心与HDPA以单齿态与氧化物表面键合有关,并且它的另一个磷羟基与氧化物表面在非键合态和键合态之间循环变化,分别对应甲苯反应中催化剂的氧化态和还原态,即HDPA与氧化物表面通过单齿和双齿键合循环变化,极大地提升了该铁基复合氧化物晶格氧的移动性而使得催化剂反应活性得到明显提升.透射电子显微镜、X射线衍射和氮吸附测量结果表明,铁基复合金属氧化物在γ-Al2O3纳米棒上具有良好的分散性和较大的比表面积.X射线光电子能谱、吡啶吸附FT-IR和31P NMR结果表明,与表面键合的HDPA保留了有机膦酸属性,没有与FeMOx/γ-Al2O3生成体相磷酸盐,其中,以单齿状态与表面键合HDPA分子只占总数的2％左右.H2-TPR和CO-TPR结果表明,HDPA的键合强烈影响了晶格氧的流动性或金属氧化物的活性.反应条件下原位FT-IR结果表明,HDPA-FeVOx/γ-Al2O3在70°C下即能催化甲苯氧化成苯甲醛,且不会过度氧化,该催化剂的晶格氧物种可参与甲苯转化为苯甲醛.此外,发现HDPA-Fe2O3/γ-Al2O3对甲苯的吸附具有特殊的分子识别作用,以苯甲醇氧化作为对照反应的研究结果也表明HDPA-Fe2O3/γ-Al2O3对甲苯氧化为苯甲醛的反应存特异催化作用.宏观反应动力学测试结果表明,催化剂上苯甲醛生成的表观活化能仅为～44.5 kJ/mol.在低H2O浓度下,甲苯氧化速率对甲苯浓度为一级反应,对O2浓度为零级反应,基元反应步骤推导出来的动力学方程与实验结果良好吻合,说明了该催化剂上类酶反应基元步骤假设的合理性.     

THE EFFECTS OF PHOTOOXIDATION BY PROFLAVINE ON HeLa CELLS—1. THE MOLECULAR MECHANISMS*

Abstract— DNA and RNA syntheses were inhibited immediately after proflavine treated HeLa cells were irradiated with visible light (400–500 nm). The molecular mechanism for this photooxidation may be either a free radical-mediated (Type I) or singlet oxygen-mediated (Type II) reaction. Non-toxic free radical and singlet oxygen quenchers were added to cells and sensitizer before irradiation to quench the appropriate excited state intermediate. Photooxidative damage (the inhibition of incorporation of [¹⁴C]-thymidine) in this system was greatly reduced in the presence of free radical quenchers (glutathione, penicillamine) and not significantly affected by the presence of singlet oxygen quenchers (α-tocopherol, β-carotene, DABCO). This suggests that at least part of the photodynamic damage in HeLa cells is via a Type I mechanism.     

Effects of oxygen and antioxidants on the <Emphasis Type="Italic">cis-trans</Emphasis>-isomerization of unsaturated fatty acids caused by thiyl radicals

The interaction of unsaturated compounds methyllinoleate and β-carotene with thiyl radicals occurred in exchange radical reactions with mercaptoethanol. β-Carotene was shown to be an effective thiyl radical acceptor. In the case of methyllinoleate, thiyl radicals catalyze cis/trans isomerization, which can be reduced by the natural antioxidant α-tocopherol; the cis/trans isomerization rate decreases in an oxygen atmosphere.     

Dehydrogenation of nitro derivatives of bibenzyl to corresponding nitro stilbene with dioxygen catalyzed by 2,2,6,6-tetramethylpiperidine-1-oxyl

A simple and effective oxidative dehydrogenation of nitro derivatives of bibenzyl was developed using a mixture of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and metal salts as catalyst under an atmospheric pressure of oxygen. The oxidation of several nitro derivatives of bibenzyl led to corresponding nitro stilbene in good yields, and the yields increased with the increase in the substituted nitro group on the bibenzyl. For example, the aerobic dehydrogenation of 2,2??,4,4??,6,6??-hexanitrobibenzyl in the presence of a catalytic amount of TEMPO with metal salts gave 2,2??,4,4??,6,6??-hexanitrostilbene in 81?% yield and 4,4??-dinitrostilbene-2,2??-disulfonic acid (75?%) was obtained from 4,4??-dinitrobibenzyl-2,2??-disulfonic acid.     

Unexpected catalyzed C=C bond cleavage by molecular oxygen promoted by a thiyl radical.

Olefin oxidation with molecular oxygen, promoted by a transition metal catalyst and a thiophenol, involved C=C bond cleavage into the corresponding carbonyl derivatives. This new reaction proceeds under one atmosphere of oxygen, at room temperature, in the presence of an excess of thiophenol and a catalyst such as MnL(2) 3a or VClL(2) 3c. It was applied to aromatic and aliphatic olefins, as well as to functionalized or unfunctionalized acyclic compounds, providing the corresponding ketones and aldehydes in up to 98% yield. The synthetic interest of this catalytic oxidation was illustrated by a one-step preparation of the fragrance (-)-4-acetyl-1-methylcyclohexene 7e in 73% isolated yield. The C=C bond cleavage probably results from a catalyzed decomposition of the beta-hydroperoxysulfide intermediate 12 that is formed by the radical addition of thiophenol to the olefin in the presence of oxygen. Although an excess of the thiophenol was used, it was transformed into the disulfide which could then be reduced without purification in 83% overall yield, thereby allowing for recycling. In addition, the C=C bond cleavage under oxygen could be promoted by catalytic quantities of the thiyl radical, generated by photolysis of the disulfide; thus, in the presence of 0.1 equiv of bis(4-chlorophenyl) disulfide 4b and 5% of the manganese complex 3a, trans-methylstilbene 1b gave, under radiation, benzaldehyde 6a and acetophenone 7a in up to 95% yield. This new reaction offers an alternative to the classical C=C bond cleavage procedures, and further developments in the fields of bioinorganic and environmental chemistry are likely.     

A comparison between artificial and natural water oxidation

Two artificial water oxidation catalysts, the blue dimer and the Llobet catalyst, have been studied using hybrid DFT methods. The results are compared to those for water oxidation in the natural photosystem II enzyme. Studies on the latter system have now reached a high level of understanding, at present much higher than the one for the artificial systems. A recent high resolution X-ray structural investigation of PSII has confirmed the main features of the structure of the oxygen evolving complex (OEC) suggested by previous DFT cluster studies. The O-O bond formation mechanism suggested is of direct coupling (DC) type between an oxygen radical and a bridging oxo ligand. A similar DC mechanism is found for the Llobet catalyst, while an acid-base (AB) mechanism is preferred for the blue dimer. All of them require at least one oxygen radical. Full energy diagrams, including both redox and chemical steps, have been constructed illustrating similarities and differences to the natural system. Unlike previous DFT studies, the results of the present study suggest that the blue dimer is rate-limited by the initial redox steps, and the Llobet catalyst by O(2) release. The results could be useful for further improvement of the artificial systems.     

The role of oxygen in the reduction of tetrazolium salts with nadh mediated by 5-methyl phenazonium methyl sulfate. An EPR and voltammetric study

The reduction of tetrazolium salts (namely INT, MTT and BNT) with NADH in the presence of catalytic amounts of PMS (5-methylphenazonium methylsulfate) was studied, and the influence of oxygen on the system was considered. The redox potentials of all the investigated compounds, the kinetic measurements, as well as the application of the Marcus theory confirmed that PMS and tetrazolium ions are reduced by NADH through an inner-sphere mechanism. The EPR investigations led to the detection of all possible radical species coming from PMS and tetrazolium ions and are in agreement with a mechanism which excludes any role of oxygen in the system NADH/PMS/tetrazolium for the formation of formazan.     

Methyl chloride production from methane over lanthanum-based catalysts

The mechanism of selective production of methyl chloride by a reaction of methane, hydrogen chloride, and oxygen over lanthanum-based catalysts was studied. The results suggest that methane activation proceeds through oxidation-reduction reactions on the surface of catalysts with an irreducible metal-lanthanum, which is significantly different from known mechanisms for oxidative chlorination. Activity and spectroscopic measurements show that lanthanum oxychloride (LaOCl), lanthanum trichloride (LaCl3), and lanthanum phases with an intermediate extent of chlorination are all active for this reaction. The catalyst is stable with no noticeable deactivation after three weeks of testing. Kinetic measurements suggest that methane activation proceeds on the surface of the catalyst. Flow and pulse experiments indicate that the presence of hydrogen chloride is not required for activity, and its role appears to be limited to maintaining the extent of catalyst chlorination. In contrast, the presence of gas-phase oxygen is essential for catalytic activity. Density-functional theory calculations suggest that oxygen can activate surface chlorine species by adsorbing dissociatively and forming OCl surface species, which can serve as an active site for methane activation. The proposed mechanism, thus, involves changing of the formal oxidation state of surface chlorine from -1 to +1 without any changes in the oxidation state of the underlying metal.     

Dirhodium-catalyzed phenol and aniline oxidations with T-HYDRO. Substrate scope and mechanism of oxidation

Dirhodium caprolactamate, Rh(2)(cap)(4), is a very efficient catalyst for the generation of the tert-butylperoxy radical from tert-butyl hydroperoxide, and the tert-butylperoxy radical is a highly effective oxidant for phenols and anilines. These reactions are performed with 70% aqueous tert-butyl hydroperoxide using dirhodium caprolactamate in amounts as low as 0.01 mol % to oxidize para-substituted phenols to 4-(tert-butyldioxy)cyclohexadienones. Although these transformations have normally been performed in halocarbon solvents, there is a significant rate enhancement when Rh(2)(cap)(4)-catalyzed phenol oxidations are performed in toluene or chlorobenzene. Electron-rich and electron-poor phenolic substrates undergo selective oxidation in good to excellent yields, but steric influences from bulky para substituents force oxidation onto the ortho position resulting in ortho-quinones. Comparative results with RuCl(2)(PPh(3))(3) and CuI are provided, and mechanistic comparisons are made between these catalysts that are based on diastereoselectivity (reactions with estrone), regioselectivity (reactions with p-tert-butylphenol), and chemoselectivity in the formation of 4-(tert-butyldioxy)cyclohexadienones. The data obtained are consistent with hydrogen atom abstraction by the tert-butylperoxy radical followed by radical combination between the phenoxy radical and the tert-butylperoxy radical. Under similar reaction conditions, para-substituted anilines are oxidized to nitroarenes in good yield, presumably through the corresponding nitrosoarene, and primary amines are oxidized to carbonyl compounds by TBHP in the presence of catalytic amounts of Rh(2)(cap)(4).     

Interactions between Vitamin E Homologues and Ascorbate Free Radicals in Murine Skin Homogenates Irradiated with Ultraviolet Light

The mechanism of oxidation of ascorbic acid in mouse skin homogenates by UV light was investigated by measuring ascorbate free radical formation using electron spin resonance signal formation. Addition of vitamin E (α-tocopherol or α-tocotrienol) had no effect, whereas short-chain homologues (2,5,7,8-tetramethyl-6-hydroxy-chroman-2-carboxylic acid [Trolox] and 2,2,5,7,8-penta-methyl-6-hydroxychromane [PMC]) accelerated ascorbate oxidation. The similar hydrophilicity of ascorbate, Trolox and PMC increased their interaction, thus rapidly depleting ascorbate. When dihydrolipoic acid was added simultaneously with the vitamin E homologues, the accelerated ascorbate oxidation was prevented. This was due to the regeneration of ascorbate and PMC from their free radicals by a recycling mechanism between ascorbate, vitamin E homologues and dihydrolipoic acid. Potentiation of antioxidant recycling may be protective against UV irradiation-induced damage. The rate of ascorbate oxidation in the presence of vitamin E homologues was enhanced by a photosensitizer (riboflavin) but was not influenced by reactive oxygen radical quenchers, superoxide dismutase or 5,5-dimethyl-l-pyrroline-iV-ox-ide. These experimental results suggest that the UV irradiation-induced ascorbate oxidation in murine skin homogenates is caused by photoactivated reactions rather than reactive oxygen radical reactions.     

Mechanism of Pd(OAc)2/DMSO-catalyzed aerobic alcohol oxidation: mass-transfer-limitation effects and catalyst decomposition pathways

Pd(OAc)(2) in DMSO is an effective catalyst for the aerobic oxidation of alcohols and numerous other organic substrates. Kinetic studies of the catalytic oxidation of primary and secondary benzylic alcohol substrates provide fundamental insights into the catalytic mechanism. In contrast to the conclusion reached in our earlier study (J. Am. Chem. Soc. 2002, 124, 766-767), we find that Pd(II)-mediated alcohol oxidation is the turnover-limiting step of the catalytic reaction. At elevated catalyst loading, however, the rate of catalytic turnover is limited by the dissolution of oxygen gas into solution. This mass-transfer rate is measured directly by using gas-uptake methods, and it correlates with the maximum rate observed during catalysis. Initial-rate studies were complemented by kinetic analysis of the full-reaction timecourses at different catalyst concentrations. Kinetic fits of these traces reveal the presence of unimolecular and bimolecular catalyst decomposition pathways that compete with productive catalytic turnover.     

六(甲氧基甲基)三聚氰胺/多元醇/二甲基丙烯酸酯的混合体系的聚合

六(甲氧基甲基)三聚氰胺(HMMM)-二缩三乙二醇(T_3EG)和二缩三乙二醇二甲基丙烯酸酯(T_3EGMA)的混合体系加入潜酸催化剂后,在较高温度下可同时进行缩聚和自由基聚合并表现出协同效应.在这一混合聚合体系中HM-MM不仅是交联剂,而且是体系中活泼亚甲基氧化为过氧化氢物的催化剂,由潜酸催化剂分解出的酸是缩聚的催化剂,也是过氧化氢物分解为自由基的催化剂,生成的自由基可引发T_3EGMA聚合.有关凝胶时间实验,吸氧实验和活性氧测定的结果支持上述论断.     

First asymmetric synthesis of both enantiomers of andirolactone

We have achieved the first asymmetric synthesis of (+)- and (−)-andirolactone. The key steps were separation of limonene oxide diastereomers, asymmetric oxidation induced by the chiral intermediate and ring-closing metathesis in the presence of catalytic amounts of Lewis acid to form the spirocyclic butenolides.     

Kinetics of the liquid-phase oxidation of cyclooctene in the presence of MnO2

The kinetics of the liquid-phase oxidation of cyclooctene by molecular oxygen in the presence of a MnC₂ catalyst is studied. It is found that MnC₂ is an initiator of this reaction and has no effect on the steps of chain propagation and termination. The oxidation occurs by a radical chain mechanism with the escape of radicals to the bulk of the reaction mixture. Radicals are formed by the interaction of the olefin with the solid catalyst surface. The kinetic parameters of the reaction are calculated.     

Preparation of Al2O3-supported nano-Cu2O catalysts for the oxidative treatment of industrial wastewater

Cuprous oxide (Cu₂O) nanoparticles supported on Al₂O₃ prepared using two different methods (hereafter referred to as catalyst I and II, respectively) were characterized by XRD and TEM. The catalytic activities of catalyst I and II during the treatment of industrial wastewater were then investigated. Specifically, the progress of the catalytic oxidation of industrial wastewater was observed by monitoring the time-dependent change in the chemical oxygen demand (COD) of industrial wastewater when the catalysts were applied. The results indicated that the catalytic activity of catalyst II was greater than that of catalyst I. Furthermore, under optimal conditions the COD removal efficiency was 94.59%. Finally, the mechanism by which the oxidative degradation of the industrial wastewater occurred could be explained based on a hydroxyl radical mechanism.     

Kinetics and Mechanism of 4-Nitrotoluene Oxidation with Ozone in an Acetic Acid Solution in the Presence of a Metal–Bromide Catalyst

The kinetics and mechanism of a reaction between ozone and 4-nitrotoluene in an acetic acid solution in the presence of a cobalt–bromide catalyst were studied at 100°. Under these conditions, 4-nitrobenzoic acid was the main reaction product (96.6%). The rate of oxidation exhibited a first order with respect to the substrate, ozone, and the catalyst and a zero order with respect to oxygen. The main stages of the catalytic cycle were considered. It is believed that the selective oxidation of 4-nitrotoluene at the methyl group in the presence of a catalyst occurs as a nonchain ion–radical process, in which the role of ozone is reduced to the generation of an active catalyst species (Co³⁺Br^– Co²⁺Br^·). This species involves 4-nitrotoluene in the oxidation process at a high rate.     

Oxidation of a mustard gas analogue using an aldehyde/O2 system catalyzed by V-doped mesoporous silica

Vanadium-doped mesoporous silica was shown to be an effective heterogeneous catalyst for the oxidation of a mustard gas analogue, 2-chloroethyl ethyl sulfide (CEES), in the presence of an aldehyde and molecular oxygen. The oxidation was shown to involve a radical mechanism, which was indicated by the appearance of an induction period when the reaction occurred in the presence of a free radical scavenger. The reaction was initially selective for the oxidation of CEES to the sulfoxide, CEESO, although oxidation of the sulfoxide to the sulfone occurred once all the CEES had been oxidized. Chemical analysis indicated that V species did not leach from the silica support when the reaction was performed in the fluorinated solvent HFE-7100.     

The β-carotene dilemma: ESR study with coordinated peroxyl radicals

The apparently unpredictable behaviour of β-carotene in the supplementation of the diet of smokers is discussed in the light of the reactions of peroxyl radicals with β-carotene in the absence of oxygen. The decay of tert-butylperoxyl radicals in the presence of β-carotene was studied at ambient temperature in non-polar solvents by ESR spectroscopy. The primary reaction in the absence of oxygen is interpreted as a spin-trapping effect of a peroxyl radical by β-carotene producing an intermediate labile free radical, which disappears after recombination with a second tert-butylperoxyl radical. The result is the transformation of β-carotene to a diamagnetic compound with two peroxy bonds. In the presence of chelating transition metals with unpaired d-electrons as electron donors the peroxy group of the oxidized β-carotene can be split to alkoxyl free radicals. The primary attack of tert-butylperoxyl radicals is completely inhibited in the presence of vitamin E followed by production of free aryloxy radicals and the presence of oxygen has no significant effect on this reaction. Spin-trapping of peroxyl radicals by the double bond of vitamin A leads to its oxidation in the absence of vitamin E. Transition metal ions such as Co, Cr, Fe, and Mn, known to be present in the aerosol of cigarette smoke, homolyse the peroxyl bonds of peroxidised β-carotene, which results in cell damage.     

Catalytic Oxidation of Alcohols to Corresponding Aldehydes or Ketones with TEMPO-Mediated Iodosobenzene in Water in the Presence of a Surfactant

An efficient, facile, and rapid oxidation of alcohols to the corresponding aldehydes or ketones with a stoichiometric amount of iodosobenzene (PhIO) in the presence of catalytic amounts of 2,2,6,6-tetramethyl-1-piperidinyloxyl free radical (TEMPO), KBr, and a surfactant, such as SDS (sodium dodecylsulfate), was reported. The oxidation proceeded in water at room temperature to afford aldehydes or ketones in excellent yields and high selectivity without overoxidation to carboxylic acids. Selective oxidation of primary alcohols in the presence of secondary alcohols was also achieved with the catalytic system of PhIO/TEMPO/KBr/SDS. A possible mechanism for the oxidation was supposed.