Structural effects on the C-S bond cleavage in aryl tert-butyl sulfoxide radical cations

The oxidation of a series of aryl tert-butyl sulfoxides (4-X-C6H4SOC(CH3)3: 1, X = OCH3; 2, X = CH3; 3, X = H; 4, X = Br) photosensitized by 3-cyano-N-methylquinolinium perchlorate (3-CN-NMQ+) has been investigated by steady-state irradiation and nanosecond laser flash photolysis (LFP) under nitrogen in MeCN. Products deriving from the C-S bond cleavage in the radical cations 1+o-4+o have been observed in the steady-state photolysis experiments. By laser irradiation, the formation of 3-CN-NMQo (?max = 390 nm) and 1+o-4+o (?max = 500-620 nm) was observed. A first-order decay of the sulfoxide radical cations, attributable to C-S bond cleavage, was observed with fragmentation rate constants (kf) that decrease by increasing the electron donating power of the arylsulfinyl substituent from 1.8 × 106 s-1 (4+o) to 2.3 × 105 s-1 (1+o). DFT calculations showed that a significant fraction of the charge is delocalized in the tert-butyl group of the radical cations, thus explaining the small substituent effect on the C-S bond cleavage rate constants. Via application of the Marcus equation to the kinetic data, a very large value for the reorganization energy (? = 62 kcal mol-1) has been calculated for the C-S bond scission reaction in 1+o-4+o.