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; <bold>2</bold>, 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 CS bond cleavage in the radical cations 1<bold>(+center dot)-</bold>4<bold>(+center dot)</bold> have been observed in the steady-state photolysis experiments. By laser irradiation, the formation of 3-CN-NMQ(center dot<bold>) (</bold>lambda(max<bold>) = </bold>390<bold> </bold>nm<bold>) </bold>and<bold> </bold>1<bold>(+center dot)-</bold>4<bold>(+center dot)</bold> (lambda(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 (k(f)) that decrease by increasing the electron donating power of the arylsulfinyl substituent from 1.8 x 10(6) s(-1) (<bold>4(</bold>+center dot<bold>)) </bold>to<bold> </bold>2.3<bold> x </bold>10(5<bold>) </bold>s(-1) (1(+center dot<bold>)). </bold>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 (lambda = 62 kcal mol(-1)) has been calculated for the C-S bond scission reaction in 1(+center dot)-4(+center dot).