Sulfur Radical Cations. Kinetic and Product Study of the Photoinduced Fragmentation Reactions of (Phenylsulfanylalkyl)trimethylsilanes and Phenylsulfanylacetic Acid Radical Cations
journal contributionposted on 03.02.2006, 00:00 by Enrico 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.
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base concentrationdecarboxylation ratesfragmentation reactioncleavagePhSCH 3bondPhotoinduced Fragmentation ReactionsformationCO 2Radical Cations LaserD 2 OCH 3 CNrate increasedecarboxylation ratedesilylation reactionphotolysis studyH 2 OProduct StudycationNMQSulfur Radical Cationsisotope effectCO 2 H