Photochemical electron-transfer reactions between sulfides and tetranitromethane.: Oxidation vs fragmentation of the sulfide radical-cation intermediate

Oxidation and/or fragmentation products are observed in the photochemical reaction of the alkyl phenyl sulfides 1a-d with tetranitromethane (TNM). The product distribution depends markedly on the substrate structure. Thus, methyl phenyl sulfide (la) and benzyl phenyl sulfide (Ib) give only the corresponding sulfoxides (oxidation). However, when the radical cation 1b(.+) is generated by chemical oxidation with triarylaminium salts (Ar3N.+) in acetonitrile, in addition to oxidation fragmentation is also observed, and with an excess of Ar3N.+ oxidation is facilitated and no fragmentation is produced. For the photoreaction of diphenylmethyl phenyl sulfide (Ic) with TNM, fragmentation is the main reaction, while for triphenylmethyl phenyl sulfide (1d only this process is observed. The ease of C-S bond scission in these sulfur-centered radical cations 1(.+) follows the ease of alkyl cation formation, i.e., Ph3C > Ph2CH > PhCH2 > CH3.