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

• Published in: Journal of organic chemistry 2011-01, Vol.76 (2), p.573-582
• Main Authors: Baciocchi, Enrico, Bettoni, Marta, Del Giacco, Tiziana, Lanzalunga, Osvaldo, Mazzonna, Marco, Mencarelli, Paolo
• Format: Article
• Language: English
• Subjects: Cations - chemistry; Chemistry; Exact sciences and technology; Free Radicals - chemistry; Kinetics and mechanisms; Lasers; Light; Magnetic Resonance Spectroscopy; Molecular Structure; Noncondensed benzenic compounds; Organic chemistry; Photolysis; Preparations and properties; Reactivity and mechanisms; Sulfhydryl Compounds - chemistry; Sulfides - chemistry
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo102086f

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+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 +•−7 +• (2-aryl-2-propanols and diaryl disulfides). Formation of 1 +•−7 +• was also demonstrated by LFP experiments evidencing the absorption bands of the radical cations 1 +•−3 +• (λmax = 530 nm) and 5 +•−7 +• (λmax = 570 nm) mainly localized in the arylsulfenyl group and radical cation 4 +• (λmax = 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 +•−4 +• and 7 +• are characterized by having the C−S bond almost collinear with the π 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 (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 = 12 kcal mol−1) found for the C−C bond cleavage in bicumyl radical cations, a reaction that also leads to cumyl carbocations.