Sulfur Radical Cations. Kinetic and Product Study of the
Photoinduced Fragmentation Reactions of
(Phenylsulfanylalkyl)trimethylsilanes and Phenylsulfanylacetic Acid
Radical Cations
posted on 2006-02-03, 00:00authored byEnrico Baciocchi, Tiziana Del Giacco, Fausto Elisei, Andrea Lapi
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.