Efficient α-(Alkylthio)alkyl-Type Radical Formation in •OH-Induced Oxidation of α-(Methylthio)acetamide

Abstract

Pulse radiolysis with UV-vis/ESR detection and steady-state gamma-radiolysis, combined with chromatographic techniques, were used to investigate the detailed mechanism of the (*)OH-induced oxidation of alpha-(methylthio)acetamide (alpha-MTA) in aqueous solution. The main pathway involves the formation of hydroxysulfuranyl radicals alpha-MTA-(>S(*)-OH) and alpha-(alkylthio)alkyl radicals H(3)C-S-(*)CH-C( horizontal lineO)-NH(2) (lambda(max) </= 260 and 340 nm). The latter radicals are highly stabilized through the combined effect of both substituents in terms of the captodative effect. At low pH, alpha-MTA-(>S(*)-OH) radicals undergo efficient conversion to intermolecularly three-electron-bonded dimeric radical cations of alpha-MTA-(>S thereforeS<)(+) (lambda(max) = 480 nm), especially for high alpha-MTA concentrations. In contrast, at low proton concentrations, alpha-MTA-(>S(*)-OH) radicals decompose via the elimination of water, formed through intramolecular hydrogen (attached to the nitrogen atom) transfer to the hydroxysulfuranyl moiety within a six-membered structure. This process leads to the formation of the imine radical H(3)C-S-CH(2)-C( horizontal lineO)(*)NH, which subsequently decays in three independent channels. The first decay channel begins with a beta-scission followed by hydrolysis and a subsequent Hofmann rearrangement. One of the end products of this first decay channel is CO(2), which was detected. The second decay channel involves an intramolecular hydrogen transfer from the deltaC carbon atom to the radical imine site producing the alpha-(alkylthio)alkyl radical H(2)C(*)-S-CH(2)-C( horizontal lineO)-NH(2). In the third decay channel there is a 1,3-hydrogen shift in the imine radical which forms the radical H(3)C-S-(*)CH-C( horizontal lineO)-NH(2). The presence of the amide group induces more complex radical chemistry that leads unexpectedly to the degradation of the CH(3)SCH(2)CONH(2) molecule into gaseous products, CO(2) and NH(3). These features of the mechanism of the (*)OH-induced oxidation of alpha-MTA are quite different from those seen in other organic sulfides in neutral solutions.