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

Pulse radiolysis with UV−vis/ESR detection and steady-state γ-radiolysis, combined with chromatographic techniques, were used to investigate the detailed mechanism of the •OH-induced oxidation of α-(methylthio)acetamide (α-MTA) in aqueous solution. The main pathway involves the formation of hydroxysulfuranyl radicals α-MTA-(>S•OH) and α-(alkylthio)alkyl radicals H3CS•CHC(O)NH2 (λmax ≤ 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, α-MTA-(>S•OH) radicals undergo efficient conversion to intermolecularly three-electron-bonded dimeric radical cations of α-MTA-(>S∴SS•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 H3CSCH2C(O)•NH, which subsequently decays in three independent channels. The first decay channel begins with a β-scission followed by hydrolysis and a subsequent Hofmann rearrangement. One of the end products of this first decay channel is CO2, which was detected. The second decay channel involves an intramolecular hydrogen transfer from the δC carbon atom to the radical imine site producing the α-(alkylthio)alkyl radical H2C•SCH2C(O)NH2. In the third decay channel there is a 1,3-hydrogen shift in the imine radical which forms the radical H3CS•CHC(O)NH2. The presence of the amide group induces more complex radical chemistry that leads unexpectedly to the degradation of the CH3SCH2CONH2 molecule into gaseous products, CO2 and NH3. These features of the mechanism of the •OH-induced oxidation of α-MTA are quite different from those seen in other organic sulfides in neutral solutions.