No abstract

Relative rate constants have been measured for the oxidation of aryl methyl sulfides and sulfoxides by dimethyldioxirane in acetone, in mixtures of acetone with aprotic co-solvents of both higher and lower relative permittivity, and in aqueous acetone mixtures. Correlation analyses of the effects of substituents in the different solvents show that, with one exception, reactions take place via a single step mechanism in which the formation of the new SO bond and the elimination of acetone occur concertedly. The exception was oxidation of the sulfides in aqueous acetone containing the highest proportion of water of those studied (20% v/v). Here, the behaviour of the reaction is consistent with a two-step mechanism in which the oxidant reversibly attacks the sulfide to form an open-chain sulfonium betaine that subsequently fragments to sulfoxide and acetone. There is no evidence for the participation of an intermediate dioxathietane as has been found in the case of sulfide oxidations by (trifluoromethyl)methyldioxirane in CH(2)Cl(2) and similar aprotic solvents. It is not justified to generalise a mechanism involving a betaine, with or without a derived dioxathietane, to the reactions of dimethyldioxirane in acetone.

The photochemical reactions of styrene, (E)-stilbene, phenylethyne, and diphenylethyne with pyrrole and N-methylpyrrole have been investigated. The pyrroles quench the fluorescence of the phenylethenes and phenylethynes efficiently and broad structureless emissions attributed to exciplexes are observed from the N-methylpyrrole systems. Substituted pyrroles (1:l and 2:l adducts) are formed from irradiation of the phenylethenes with pyrrole, and 2-styrylpyrroles result from the phenylethynes. Styrene, but none of the other arenes, undergoes photoaddition to N-methylpyrrole in the presence of 10 % acetic acid.

The oxidations of methyl 4-nitrophenyl sulfide and sulfoxide by dimethyldioxirane, in acetone and mixtures of acetone with water, methanol, acetonitrile and hexane, have been followed by UV-Vis spectroscopy to monitor the decay of the substrates. The data show that, under all the conditions studied, both oxidations obey second-order kinetics. Grunwald-Winstein and Kamlet-Taft analyses of the influence of solvents on the second-order rate constants have been used to obtain mechanistic information on the two reactions. Activation parameters for the two oxidations in acetone and aqueous acetone have been calculated from rate constants for reactions in the temperature range 283-313 K and compared with those from sulfide and sulfoxide oxidations with other oxidants. For sulfoxide oxidations in acetone and 1-20% v/v water in acetone, the results support a concerted nucleophilic displacement by sulfur of oxygen from dimethyldioxirane with the rate being dependent on the solvent's polarity. Sulfide oxidations in acetone and 1-5% v/v water in acetone also proceed by a concerted mechanism. However, in the most polar solvent system studied, 20% v/v water in acetone, the mechanism changes in favour of a two-step reaction involving a betaine intermediate. Importantly, the sulfide oxidation shows a different solvent dependence to that of the sulfoxide, with the rate of oxidation being determined by the hydrogen bond donor capacity and electron-pair donicity of the solvent.

The Photoaddition of Pyrroles to Arylethenes and Arylethynes. -Pyrrole and N-methylpyrrole efficiently quenched the fluorescence of the arylethenes and arylethynes. Substituted pyrroles (1:1 and 2:1 adducts) are formed from irradiation of the phenylethenes with pyrrole, and 2-styrylpyrroles result from the phenylethynes. Data for the quenching of the fluorescence are presented. -(AUSTIN, M.; COVELL, C.; GILBERT, A.; HENDRICKX, R.; Liebigs Ann./Recl. (1997) 5, 943-946; Dep. Chem., Univ. Reading, Whiteknights, Reading, Berkshire RG6 2AD, UK; EN)