Studies on the photooxidation mechanism of polymers. V. Oxidation of polybutadienes by singlet oxygen from microwave discharge and in dye-photosensitized reactions

The singlet oxygen oxidation of cis- and trans- 1,4-polybutadienes was studied by using singlet oxygen generated in a microwave generator and in dye-photosensitized reactions of these polymers in the solid state and in solution. It was shown that the reactions of singlet oxygen result in formation of hydroperoxide groups, whereas ultraviolet oxidation by molecular oxygen in addition leads to formation of carbonyl groups. During dye-photosensitized oxidation of polydienes in benzene solution, a very rapid decrease in the molecular weight was observed.     

The heterogeneous catalytic oxidation of propylene in the presence of singlet oxygen in the reaction mixture

Three methods for the introduction of singlet oxygen into the reaction mixture were tested, including thermal generation of singlet oxygen on the catalyst itself, the introduction of singlet oxygen from an external source, and photogeneration of singlet oxygen on the catalyst. Zeolites with admixtures of Mo, Bi, V, and Ni and SiO₂ with deposited Mo, V, and Bi were used. Common to all reactions was an increase in the yield of deep oxidation products in the presence of singlet oxygen. A sharp increase in the yield of mild oxidation products was observed in the oxidation of propylene on a Bi/SiO₂ catalyst. The generation of singlet oxygen under irradiation at 240–260 nm was found to cause deep oxidation only. Mild oxidation products could only form under the action of total mercury lamp light.     

Studies on the photooxidation mechanism of polymers. I. Photolysis and photooxidation of polystyrene

A new mechanism has been proposed for the photooxidation of polystyrene as film and in benzene. The initial stage of the photooxidative degradation may involve reactions of singlet oxygen with polystyrene molecules. Singlet oxygen may be formed in the reaction between excited benzene ring in polystyrene molecule and molecular oxygen. The addition of singlet oxygen quenchers such as 1,3-cyclohexadiene or β-carotene reduces the rate of polymer degradation in benzene solution. The mechanisms of the photolysis of polystyrene as film and in benzene solution, in vacuo and in the presence of oxygen, are discussed and interpretations proposed. The pronounced yellowing of polystyrene during the photooxidation process is interpreted as a reaction involving benzene ring-opening photooxidation in polystyrene molecule. These results were obtained by comparing ultraviolet and infrared spectra in experiments of photooxidation of pure liquid benzene and polystyrene film.     

CuO+氧化苯直接形成苯酚反应机理的理论研究

在UB3LYP/6-31G(d,p) 水平下研究了CuO+氧化苯形成苯酚反应的详细机理,同时计算了单重态和三重态势能面。计算结果表明,苯与CuO+间相互作用主要为?配键,反馈?键较弱. CuO+氧化苯形成苯酚反应通过非自由基氢摘取机理完成,主要包括C-H键活化和苯基与羟基耦合两步反应. C-H键活化为整个反应的决速步骤. C-H键活化步骤涉及势能面交叉,且自旋交叉与动力学相关。CuO+氧化苯形成苯酚反应在气相中很容易进行.     

在Pt/Al2O3催化剂上苯的内扩散对负0.9级动力学的影响

在Pt/Al₂O₃催化剂上用外循环反应器研究了内扩散对苯完全氧化动力学的影响,当用30～40目(即0.45～0.60mm)催化剂时反应在动力学区域进行.若O₂过量时则动力学区域苯的完全氧化可用-0.9级速率方程描述.当催化剂粒径增至φ6×5mm时,反应在内扩散区域进行并变为一0.1级反应.催化剂有效因子η在0.24～0.12之间.在同一温度下,η_实验随苯分压p_苯的增加而增大;而p_苯相近时,η_实验则随温度的升高而减小.动力学区域的反应活化能为55.5kJ/mol,内扩散区域的反应活化能为34.9kJ/mol,其值约为动力学区域的活化能与苯分子扩散活化能的算术平均值.     

Phenol synthesis by liquid-phase oxidation of benzene with molecular oxygen over iron-heteropoly acid

The phenol synthesis by liquid-phase oxidation of benzene with molecular oxygen over iron-heteropoly acid (HPA) system was studied. When iron salts were used with H₃PW₁₂O₄₀, the highest activity was obtained. Spectroscopic studies showed that the state of the iron ion was changed after interaction with heteropoly acid (HPA) while the Keggin structure of HPA remained. The acidic nature of HPA activated benzene to form cationic species. Insoluble iron ion-exchanged heteropoly acid was obtained by partial ion exchange method, which also showed high activity on oxidation of benzene with molecular oxygen. The effects of reaction conditions were studied and the mechanism of deactivation was discussed. For the regeneration of catalytic activity, the addition of -ascorbic acid as a reducing agent was suggested.     

SINGLET OXYGEN GENERATION BY FUROCOUMARIN TRIPLET STATES—I. LINEAR FUROCOUMARINS (PSORALENS)

Abstract— Triplet extinction coefficients and hence singlet → triplet intersystem crossing quantum yields have been measured in benzene for a number of linear furocoumarins including pseudopsoralen, 5, 8-dimethoxypsoralen, 4, 5', 8-trimethylpsoralen and 3-carbethoxypseudopsoralen. These triplet yields were then used in conjunction with the corresponding quantum yields of singlet oxygen formation, measured in oxygenated solution, to estimate the fractions of furocoumarin triplets which when quenched by ground state oxygen produce singlet excited oxygen, similar data being obtained for psoralen, 5-methoxypsoralen, 8-methoxypsoralen and 3-carbethoxypsoralen. The superoxide anion radical was not detected from these oxygen quenching reactions, nor was a contribution to the singlet oxygen yield found from furocoumarin excited singlet state quenching by oxygen. The fraction of furocoumarin-oxygen quenching interactions leading to singlet oxygen varied between 0.13 (for 5, 8-dimethoxypsoralen) and unity (for 3-carbethoxypsoralen), and thus needs to be taken into account, as well as the triplet quantum yields, in assessing photobiological processes involving singlet oxygen.     

Photo-induced catalytic partial oxidation of citronellol with assistance of phthalocyanines

Attention was paid to the role of sulfonated phthalocyanines as efficient photocatalysts in the photo-induced partial oxidation of citronellol. Performance of the singlet oxygen species inducing the formation of citronellol hydroperoxides was studied in detail. The involved reactions may be of great importance in the catalytic production of rose oxide — a highly appreciated component of many synthetic fragrances.     

Ergosterol (provitamin D2) triplet state: an efficient sensitiser of singlet oxygen, O2(1 delta g), formation

The triplet state of ergosterol (provitamin D2) has been produced in benzene by pulse radiolysis and characterised in terms of absorption spectrum, lifetime, self-quenching properties and relaxed triplet energy. The amount of singlet oxygen, O₂(¹Δ_g), produced as a consequence of the oxygen quenching of this species has been determined by kinetic infrared emission spectroscopy. Ergosterol is significantly more efficient as a singlet oxygen sensitiser in benzene than is naphthalene, the absolute standard employed in this work.     

Comparative studies of reactions of commercial polymers with molecular oxygen,singlet oxygen,atomic oxygen and ozone—I. Reactions with cis-1,4-polybutadiene

The oxidation of cis-1,4-polybutadiene by molecular oxygen, singlet oxygen, atomic oxygen and ozone has been studied using u.v. and i.r. spectroscopic methods. Some possible implications of the results of oxidation in the presence of singlet oxygen (parallel free radical oxidation) and atomic oxygen (formation of NO₂ and its reaction with polymer) are discussed. Chain scission was observed during all types of oxidation. A new mechanism involving opening of double bonds and formation of biradicals has been considered in detail.     

Importance of single electron-transfer in singlet oxygen reaction in aqueous solution : Oxidation of electron-rich thioanisoles

Oxidation of substituted thioanisoles by chemically generated singlet oxygen was investigated in polar aqueous media. The formation of the superoxide ion was observed during sulphoxidation of 4-hydroxythioanisole (4) in phosphate buffer at pH 7.5. Control experiments indicated that the superoxide ion was formed by a direct reaction between singlet oxygen and 4. The kinetics of the trapping reaction by diphenylsulphoxide indicated the involvement of a single intermediate. The overall rate constants of the reaction of thioanisoles with singlet oxygen in methanol-water (1:1) are one order of magnitude larger than those in benzene. On the basis of these results, a mechanism involving a charge-transfer complex has been proposed for the reaction of electron-rich thioanisoles with singlet oxygen, whereby the charge-transfer complex would produce persulphoxide directly or dissociate to the cation radical and superoxide ion in polar aqueous media.     

Characterization of the triplet state of tris(8-hydroxyquinoline)aluminium(III) in benzene solution

The lowest triplet state of tris(8-hydroxyquinoline)aluminium(III) (Alq3) has been prepared by pulse radiolysis/energy transfer from appropriate donors in benzene solutions and has an absorption maximum around 510 nm with a lifetime of about 50 mus. It is quenched by molecular oxygen, leading to singlet oxygen formation. From flash photolysis and singlet oxygen formation measurements, a quantum yield of triplet formation of 0.24 was determined for direct photolysis of the complex. A value of 2.10 +/- 0.10 eV was determined for the energy of the lowest triplet state by energy transfer studies and was confirmed by phosphorescence measurements on Alq3, either in the heavy atom solvent ethyl iodide or photosensitized by benzophenone in benzene. Dexter (exchange) energy transfer was observed from triplet Alq3 to platinum(II) octaethylporphyrin.     

Variable Markovnikov orientation and "cis effect" in ene reactions of nitrosocarbonyl intermediates

Nitrosocarbonyl intermediates, generated at room temperature by the mild oxidation of nitrile oxides, undergo clean ene reactions with trisubstituted olefins. Allylic hydrogens on the more congested side of the alkene are exclusively abstracted (the "cis effect"), thus resembling singlet oxygen behavior. Nitrosocarbonyl benzene follows a Markovnikov orientation and abstracts preferentially the twix hydrogens over the lone ones. With the more sterically demanding nitrosocarbonyl mesitylene, the Markovnikov directing effect is relieved, and comparable twix and lone abstraction are observed.     

SOME PREVALENT BIOMOLECULES AS DEFENSES AGAINST SINGLET OXYGEN DAMAGE

Abstract— We have compared the relative abilities of some putative biological protectors to block oxidation of 2,5-bis(hydroxymethyl)furan (BHMF)† in illuminated solutions containing the photosensitizer rose bengal and in the separated-surface-sensitizer (S-S-S) system involving pure singlet oxygen (¹ΔA_gO₂). While L-histidine is a well-known quencher of singlet oxygen, free L-histidine is not commonly found in high concentrations in nature. L-Carnosine (β-alanyl-L-histidine), however, is present in the striated muscles of many organisms, most notably mammals, in concentrations up to 40 m M . At neutral pH, carnosine quenched singlet oxygen more effectively than did equimolar histidine, both in solubilized sensitizer studies and in the S-S-S system. In the pure singlet oxygen system, 1 m M carnosine reduced the rate of BHMF oxidation as effectively as 3 m M histidine alone, or a mixture of 3 mM histidine and 3 m M β-alanine. The fungal product L-ergothioneine (2-thiol-L.-histidine betaine) and its synthetic analogue, 2-thiolhistidine, at 1 m M blocked photosensitized BHMF oxidation using solubilized rose bengal, as did urate at 0.5 m M . All three compounds failed to reduce the rate of BHMF oxidation by singlet oxygen in the S-S-S system, however. Homocarnosine (-γ-aminobutyryl-L-histidine) gave levels of protection against BHMF oxidation identical to histidine, but is present in the central nervous system only at micromolar concentrations. Neither 1 m M imidazole nor 5 m M urea reduced BHMF oxidation in either system. We conclude that some prevalent biomolecules may afford protection either by preventing singlet oxygen production (urate, L-ergothioneine) or by intercepting singlet oxygen once formed (L-carnosine). Such protective devices may be of importance in natural systems.     

Comparative studies of reactions of commercial polymers with molecular oxygen,singlet oxygen,atomic oxygen and ozone—II. Reactions with 1,2-polybutadiene

The oxidations of 1,2-polybutadiene by molecular oxygen, singlet oxygen, atomic oxygen and ozone have been studied using u.v. and i.r. spectroscopic methods. Some possible implications of the results of oxidation in the presence of singlet oxygen (parallel free radical oxidation) and atomic oxygen (formation of NO₂ and its reaction with polymer) are discussed. Crosslinking was observed during all types of oxidation. A new mechanism involving formation of free radicals has been considered in detail. During ozonization of 1,2-polybutadiene, formation of formaldehyde and formic acid were detected. An ozonization mechanism has been proposed.     

Specific features of the reaction of vanadyl acetylacetonate with <Emphasis Type="Italic">tert</Emphasis>-butyl hydroperoxide

Reaction of vanadyl acetylacetonate with tert-butyl hydroperoxide (benzene, 20°C) at any molar ratio leads to the elimination of ligand and its oxidation mainly to CO₂ and acetic acid. At the (acac)₂VO: t-BuOOH ratio above 1:10 liberation of oxygen partially in the singlet state takes place.     

Kinetics and Mechanism of the Photosensitized Oxidation of Furosemide*

Detection of O2(1δg) phosphorescence emission, γmax = 1270 nm, following laser excitation and steady-state competitive methods was employed to measure total rate constants, kT, for the reactions of the diuretic furosemide, 2-methylfurane and furfurylamine with singlet oxygen in several solvents. Correlation of kT values with solvent parameters and product identification shows that the reaction mechanism is strongly solvent dependent. In aliphatic alcohols, the dependence of kT on solvent parameters is similar to the one observed for triethylamine, suggesting a reaction mechanism involving partial charge transfer from the amino group to the singlet oxygen. In nonprotic solvents, the dependence of kT on solvent parameters resembles the behavior found for 2-methylfur-ane and furfurylamine, implying that mostly a 2 + 4 cy-cloaddition mechanism of singlet oxygen to furane ring of furosemide occurs in these solvents. These mechanistic differences are explained in terms of hydrogen-bonding interactions between the carboxylic group in the aromatic ring and the amino group of furosemide. Furthermore, direct generation of C2(1δg) by furosemide was detected. Quantum yields of 0.047 ± 0.003 and 0.078 ± 0.004 were determined in acetonitrile and benzene, respectively. This last result may be related, at least partially, to the photodynamic effects of this diuretic drug.     

COMPARATIVE STUDY OF OXIDATION OF NUCLEIC ACID COMPONENTS BY HYDROXYL RADICALS, SINGLET OXYGEN AND SUPEROXIDE ANION RADICALS

Abstract— Superoxide radicals, singlet oxygen and hydroxyl radicals are individually or in combination involved in radiation or photochemical processes and in various enzymatic reactions. The reactivity and the mechanism of reaction of these oxygen species with some biologically significant DNA components were investigated through the characterization of the final oxidation products.
Superoxide radicals appear to be unreactive with purine and pyrimidine 2'-deoxyribonucleosides. However, the autoxidation reaction of 6-hydroxydopamine leads to extensive degradation of thymine through the intermediary of hydroxyl radicals. Chemically and microwave-discharge generated singlet oxygen oxidation is specific to 2'-deoxyguanosine. The main oxidized products of these reactions were also characterized as well as an as yet unidentified nucleoside in the methylene-blue photooxydation of 2-deoxyguanosine. These results, in addition to specific deuterium effect experiments, lend support to the involvment of singlet oxygen (type II mechanism) in the methylene-blue photosentization. No singlet oxygen effect was observed in aqueous irradiated system.     

Stereoselective photooxidation of enecarbamates: reactivity of ozone vs singlet oxygen

[reaction: see text]. Oxazolidinone-functionalized enecarbamates show contrasting behavior upon oxidation by singlet oxygen and by ozone. The observed stereoselectivity difference indicates that the oxidation with ozone is subject to classic steric effects, whereas the very high selectivity in the photooxidation with singlet oxygen is derived from vibrational deactivation.     

Improving Singlet Oxygen Resistance during Photochemical Water Oxidation by Cobalt Porphyrin Catalysts

Enabling the production of solar fuels on a global scale through artificial photosynthesis requires the development of water oxidation catalysts with significantly improved stability. The stability of photosystems is often reduced owing to attack by singlet oxygen, which is produced during light harvesting. Here, we report photochemical water oxidation by CoFPS , a fluorinated Co‐porphyrin designed to resist attack by singlet oxygen. CoFPS exhibits significantly improved stability relative to its non‐fluorinated analogue, as shown by a large increase in turnover numbers. This increased stability results from resistance of CoFPS to attack by singlet oxygen, the formation of which was monitored in situ by using 9,10‐diphenylanthracene as a chemical probe. Dynamic light scattering (DLS) confirms that CoFPS remains homogeneous, proving its stability during water oxidation catalysis.