Electron Transfer and Singlet Oxygen Mechanisms in the Photooxygenation of Dibutyl Sulfide and Thioanisole in MeCN Sensitized by N-Methylquinolinium Tetrafluoborate and 9,10-Dicyanoanthracene. The Probable Involvement of a Thiadioxirane Intermediate in Electron Transfer Photooxygenations

TLDR: It was conclusively shown that with both sensitizers, the photooxygenations of PhSMe occur by an electron transfer (ET) mechanism, as no sulfoxidation was observed in the presence of benzoquinone (BQ), which is a trap for O2-*, NMQ*, and DCA-*.

Abstract: Photooxygenations of PhSMe and Bu2S sensitized by N-methylquinolinium (NMQ+) and 9,10-dicyanoanthracene (DCA) in O2-saturated MeCN have been investigated by laser and steady-state photolysis. Laser photolysis experiments showed that excited NMQ+ promotes the efficient formation of sulfide radical cations with both substrates either in the presence or in absence of a cosensitizer (toluene). In contrast, excited DCA promotes the formation of radical ions with PhSMe, but not with Bu2S. To observe radical ions with the latter substrate, the presence of a cosensitizer (biphenyl) was necessary. With Bu2S, only the dimeric form of the radical cation, (Bu2S)2+•, was observed, while the absorptions of both PhSMe+• and (PhSMe)2+• were present in the PhSMe time-resolved spectra. The decay of the radical cations followed second-order kinetics, which in the presence of O2, was attributed to the reaction of the radical cation (presumably in the monomeric form) with O2-• generated in the reaction between NMQ• or DCA-• a...