Structure and C-S Bond Cleavage in Aryl 1-Methyl-1-arylethyl Sulfide Radical Cations

Steady state and laser flash photolysis (LFP) of a series of p-X-cumyl phenyl sulfides (4-X-C6H4C-(CH3)(2)SC6H5: 1, X = Br; 2, X = H; 3, X = CH3; 4, X = OCH3) and p-X-cumyl p-methoxyphenyl sulfides (4-X-C6H4C(CH3)(2)SC6H4OCH3: 5, X = H; 6, X = CH3; 7, X = OCH3) has been carried out in the presence of N-methoxy phenanthridinium hexafluorophosphate (MeOP+PF6-) under nitrogen in MeCN. Steady state photolysis showed the formation of products deriving from the C-S bond cleavage in the radical cations 1(+center dot)-7(+center dot) (2-aryl-2-propanols and diaryl disulfides). Formation of 1(+center dot)-7(+center dot) was also demonstrated by LFP experiments evidencing the absorption bands of the radical cations 1(+center dot)-3(+center dot) (lambda(max) = 530 nm) and 5(+center dot)-7(+center dot) (lambda(max) = 570 nm) mainly localized in the arylsulfenyl group and radical cation 4(+center dot) (lambda(max) = 410, 700 nm) probably mainly localized in the cumyl ring. The radical cations decayed by first-order kinetics with a process attributable to the C-S bond cleavage. On the basis of DFT calculations it has been suggested that the conformations most suitable for C-S bond cleavage in 1(+center dot)-4(+center dot) and 7(+center dot) are characterized by having the C-S bond almost collinear with the pi system of the cumyl ring and by a significant charge and spin delocalization from the ArS ring to the cumyl ring. Such a delocalization is probably at the origin of the observation that the rates of C-S bond cleavage result in very little sensitivity to changes in the C-S bond dissociation free energy (BDFE). A quite large reorganization energy value (lambda = 43.7 kcal mol(-1))has been calculated for the C-S bond scission reaction in the radical cation. This value is much larger than that (A = 12 kcal mol(-1)) found for the C-C bond cleavage in bicumyl radical cations, a reaction that also leads to cumyl carbocations.