Photosensitized Oxidation of Alkyl Phenyl Sulfoxides. C-S Bond Cleavage in Alkyl Phenyl Sulfoxide Radical Cations

The 3-cyano-N-methylquinolinium perchlorate (3-CN-NMQ+ ClO4 -)-photosensitized oxidation of phenyl alkyl sulfoxides (PhSOCR1R2R3, 1, R1 ) R2 ) H, R3 ) Ph; 2, R1 ) H, R2 ) Me, R3 ) Ph; 3, R1 ) R2 ) Ph, R3 ) H; 4, R1 ) R2 ) Me, R3 ) Ph; 5, R1 ) R2 ) R3 ) Me) has been investigated by steady-state irradiation and nanosecond laser flash photolysis (LFP) under nitrogen in MeCN. Steady-state photolysis showed the formation of products deriving from the heterolytic C-S bond cleavage in the sulfoxide radical cations (alcohols, R1R2R3COH, and acetamides, R1R2R3CNHCOCH3) accompanied by sulfur-containing products (phenyl benzenethiosulfinate, diphenyl disulfide, and phenyl benzenethiosulfonate). By laser irradiation, the formation of 3-CN-NMQ• (λmax ) 390 nm) and sulfoxide radical cations 1•+, 2•+, and 5•+ (λmax ) 550 nm) was observed within the laser pulse. The radical cations decayed by first-order kinetics with a process attributable to the heterolytic C-S bond cleavage leading to the sulfinyl radical and an alkyl carbocation. The radical cations 3•+ and 4•+ fragment too rapidly, decaying within the laser pulse. The absorption band of the cation Ph2CH+ (λmax ) 440 nm) was observed with 3 while the absorption bands of 3-CN-NMQ• and PhSO• (λmax ) 460 nm) were observed just after the laser pulse in the LFP experiment with 4. No competitive -C-H bond cleavage has been observed in the radical cations from 1-3. The C-S bond cleavage rates were measured for 1•+, 2•+, and 5•+. For 3•+ and 4•+, only a lower limit (ca. >3 × 107 s-1) could be given. Quantum yields (Φ) and fragmentation first-order rate constants (k) appear to depend on the structure of the alkyl group and on the bond dissociation free energy (BDFE) of the C-S bond of the radical cations determined by a thermochemical cycle using the C-S BDEs for the neutral sulfoxides 1-5 obtained by DFT calculations. Namely, Φ and k increase as the C-S BDFE becomes more negative, that is in the order 1 < 5 < 2 < 3, 4, which is also the stability order of the alkyl carbocations formed in the cleavage. An estimate of the difference in the C-S bond cleavage rate between sulfoxide and sulfide radical cations was possible by comparing the fragmentation rate of 5•+ (1.4 × 106 s-1) with the upper limit (104 s-1) given for tert-butyl phenyl sulfide radical cation (Baciocchi, E.; Del Giacco, T.; Gerini, M. F.; Lanzalunga, O. Org. Lett. 2006, 8, 641-644). It turns out that sulfoxide radical cations undergo C-S bond breaking at a rate at least 2 orders of magnitude faster than that of corresponding sulfide radical cations.