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)2SC6H5: 1, X=Br; 2, X = H; 3, X = CH3; 4, X = OCH3) and p-X-cumyl p-methoxyphenyl sulfides (4-X-C6H4C(CH3)2SC6H4OCH3: 5,X=H; 6,X=CH3; 7,X=OCH3) has been carried out in the presence of N-methoxy phenanthridinium hexafluorophosphate (MeOP þ PF6 -) under nitrogen in MeCN. Steady state photolysis showed the formation of products deriving from theC-S bond cleavage in the radical cations 1 þ•-7 þ• (2-aryl-2-propanols and diaryl disulfides). Formation of 1 þ•-7 þ• was also demonstrated by LFP experiments evidencing the absorption bands of the radical cations 1 þ•-3 þ• (λmax = 530 nm) and 5 þ•-7 þ• (λmax=570 nm)mainly localized in the arylsulfenyl group and radical cation 4 þ• (λmax=410, 700 nm) probably mainly localized in the cumyl ring.The radical cations decayed by first-order kinetics with a process attributable to theC-S bond cleavage.On thebasis ofDFTcalculations ithasbeen suggested that the conformationsmost suitable forC-S bond cleavage in 1 þ•-4 þ• and 7 þ• are characterized by having theC-S bond almost collinear with the π system of the cumyl ring and by a significant charge and spin delocalization from theArS ring to the cumyl ring. Such a delocalization is probably at the origin of the observation that the rates ofC-S bond cleavage result in very little sensitivity to changes in theC-S bond dissociation free energy (BDFE). A quite large reorganization energy value (λ=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 (λ=12kcalmol-1) found for the C-C bond cleavage in bicumyl radical cations, a reaction that also leads to cumyl carbocations.