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.
Baciocchi
Enrico
E
Dipartimento di Chimica, Università La Sapienza, Piazzale A. Moro 5, 00185 Rome, Italy. enrico.baciocchi@uniroma1.it
Del Giacco
Tiziana
T
Elisei
Fausto
F
Lapi
Andrea
A
D000085:Acetates / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D005609:Free Radicals / Q000737:chemistry; D007700:Kinetics; D008745:Methylation; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010777:Photochemistry; D012821:Silanes / Q000737:chemistry*; D013057:Spectrum Analysis; D013440:Sulfides / Q000737:chemistry*; D013455:Sulfur / Q000737:chemistry*
D000085:Acetates; D002412:Cations; D005609:Free Radicals; D012821:Silanes; D013440:Sulfides; C512014:phenylsulfanylacetic acid; D013455:Sulfur