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 CO, The steady-state photolysis study showed that PhSCH3 Was the exclusive product formed from I and 3 whereas [PhS(Ph)CH-](2) was the only product with 3 and 4.