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(+) ClO(4)(-))-photosensitized oxidation of phenyl alkyl sulfoxides (PhSOCR(1)R(2)R(3), 1, R(1) = R(2) = H, R(3) = Ph; 2, R(1) = H, R(2) = Me, R(3) = Ph; 3, R(1) = R(2) = Ph, R(3) = H; 4, R(1) = R(2) = Me, R(3) = Ph; 5, R(1) = R(2) = R(3) = 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, R(1)R(2)R(3)COH, and acetamides, R(1)R(2)R(3)CNHCOCH(3)) accompanied by sulfur-containing products (phenyl benzenethiosulfinate, diphenyl disulfide, and phenyl benzenethiosulfonate). By laser irradiation, the formation of 3-CN-NMQ(center dot) (lambda(max) = 390 nm) and sulfoxide radical cations 1(center dot+), 2(center dot 1), and 5(center dot+) (lambda(max) = 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(center dot+) and 4(center dot+) fragment too rapidly, decaying within the laser pulse. The absorption band of the cation Ph(2)CH(+) (lambda(max) = 440 nm) was observed with 3 while the absorption bands of 3-CN-NMQ(center dot) and PhSO(center dot) (lambda(max)= 460 nm) were observed just after the laser pulse in the UP experiment with 4. No competitive beta-C-H bond cleavage has been observed in the radical cations from 1-3. The C-S bond cleavage rates were measured for 1(center dot+), 2(center dot+), and 5(center dot+). For 3(center dot+) and 4(center dot+), only a lower limit (ca. > 3 x 10(7) s(-1)) could be given. Quantum yields (Phi) 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, Phi 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(center dot+) (1.4 x 10(6) s(-1)) with the upper limit (101 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 cot-responding sulfide radical cations