C–S bond cleavage in the sensitized photooxygenation of tert-alkyl phenyl sulfides. The role of superoxide anion

Abstract

The N-methylquinolinium tetrafluoroborate (NMQ+)-photosensitized oxidation of tert-alkyl phenyl sulfides 1a–c (1a, tert-alkyl=tert-butyl; 1b, tert-alkyl=2-phenyl-2-propyl; 1c, tert-alkyl=1,1-diphenylethyl) and benzyl phenyl sulfide (2) were investigated in CH3CN by nanosecond laser flash photolysis (LFP) and steady-state irradiation either under nitrogen or in the presence of O2. By laser irradiation, the formation of sulfide radical cations 1a+–c+ in the monomeric form (λmax=520 nm) and of 2+ in both the monomeric (λmax=520 nm) and dimeric form (λmax=780 nm) were observed within the laser pulse. In both cases, the radical cations decayed by second-order kinetics without any apparent formation of transients attributable to C–S bond rupture. In line with these results, very small amounts of photoproducts were obtained under nitrogen thus suggesting that the sulfide radical cations mainly undergo a back electron transfer process with the reduced N-methylquinolinium (NMQ). A different situation was found in the presence of O2 since steady-state photolysis produced substantial amounts of C–S bond cleavage products (alcohols, alkenes, and ketones from 1a–c and benzaldehyde from 2), in contrast with LFP experiments. Formation of products was, however, significantly reduced in the presence of benzoquinone, a trap for O2− generated by NMQ and O2. For the tert-alkyl phenyl sulfides, 1a–c, these results have been interpreted by suggesting that C–S bond cleavage products in the presence of oxygen mostly derive from the decomposition of a thiadioxirane 6 formed by the reaction of the sulfide radical cation with O2−. In this cleavage a sulfinate and a carbocation formed. The former is oxidized to sulfonate, whereas the carbocation can react with adventitious water to form the alcohol (and the alkene therefrom) and with O2− to produce the ketone. For 2 (a sulfide with α-CH bonds) probably a different mechanism holds, benzaldehyde coming from the α-phenylthio carbon radical formed from deprotonation by O2− of 2n+.