Laser and steady-state photolysis, sensitized by NMQ+, of PhSCH(R)X 1-4 (R ) H, Ph; X dSiMe3, 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.
Sulfur Radical Cations. Kinetic and Product Study of the Photoinduced Fragmentation Reactions of (Phenylsulfanylalkyl) trimethylsilanes and Phenylsulfanylacetic Acid Radical Cations.
DEL GIACCO, Tiziana;ELISEI, Fausto;
2006
Abstract
Laser and steady-state photolysis, sensitized by NMQ+, of PhSCH(R)X 1-4 (R ) H, Ph; X dSiMe3, 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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
