Sulfur Radical Cations. Kinetic and Product Study of the Photoinduced Fragmentation Reactions of (Phenylsulfanylalkyl)trimethylsilanes and Phenylsulfanylacetic Acid Radical Cations

Laser and steady-state photolysis, sensitized by NMQ⁺, of PhSCH(R)X 1−4 (R = H, Ph; X SiMe₃, CO₂H) was carried out in CH₃CN. The formation of 1^+•−4^+• was clearly shown. All radical cations undergo a fast first-order fragmentation reaction involving C−Si bond cleavage with 1^+• and 2^+• and C−C bond cleavage with 3^+• and 4^+•. The desilylation reaction of 1^+• and 2^+• was nucleophilically assisted, and the decarboxylation rates of 3^+• and 4^+• increased in the presence of H₂O. A deuterium kinetic isotope effect of 2.0 was observed when H₂O was replaced by D₂O. Pyridines too were found to accelerate the decarboxylation rate of 3^+• and 4^+•. The rate increase, however, was not a linear function of the base concentration, but a plateau was reached. A fast and reversible formation of a H-bonded complex between the radical cation and the base is suggested, which undergoes C−C bond cleavage. It is probable that the H-bond complex undergoes first a rate determining proton-coupled electron transfer forming a carboxyl radical that then loses CO₂. The steady-state photolysis study showed that PhSCH₃ was the exclusive product formed from 1 and 3 whereas [PhS(Ph)CH−]₂ was the only product with 3 and 4.