Istituto CNR di Metodologie Chimiche-IMC, Sezione Meccanismi di Reazione c/o Dipartimento di Chimica, Università di Roma La Sapienza, P.le A. Moro 5, 00185 Rome, Italy.
J Org Chem. 2011 Jan 21;76(2):573-82. doi: 10.1021/jo102086f. Epub 2010 Dec 16.
Steady state and laser flash photolysis (LFP) of a series of p-X-cumyl phenyl sulfides (4-X-C(6)H(4)C(CH(3))(2)SC(6)H(5): 1, X = Br; 2, X = H; 3, X = CH(3); 4, X = OCH(3)) and p-X-cumyl p-methoxyphenyl sulfides (4-X-C(6)H(4)C(CH(3))(2)SC(6)H(4)OCH(3): 5, X = H; 6, X = CH(3); 7, X = OCH(3)) has been carried out in the presence of N-methoxy phenanthridinium hexafluorophosphate (MeOP(+)PF(6)(-)) 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(+•) (λ(max) = 530 nm) and 5(+•)-7(+•) (λ(max) = 570 nm) mainly localized in the arylsulfenyl group and radical cation 4(+•) (λ(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(+•)-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.
在氮气气氛下，用 N-甲氧基菲啶六氟磷酸盐（MeOP(+)PF(6)(-)) 作为氧化剂，在 MeCN 中对一系列对-卤代（氢、甲氧基）-和对-甲氧基取代的对-（二甲氨基）苯基硫醚（4-X-C(6)H(4)C(CH(3))(2)SC(6)H(5)：1，X = Br；2，X = H；3，X = CH(3)；4，X = OCH(3)）和对-卤代（氢、甲氧基）-和对-甲氧基取代的对-（二甲氨基）苯基对甲硫基醚（4-X-C(6)H(4)C(CH(3))(2)SC(6)H(4)OCH(3)：5，X = H；6，X = CH(3)；7，X = OCH(3)）进行了稳态和激光闪光光解（LFP）研究。稳态光解表明，在自由基阳离子 1(+•)-7(+•)（2-芳基-2-丙醇和二芳基二硫醚）中 C-S 键断裂生成了产物（2-芳基-2-丙醇和二芳基二硫醚）。LFP 实验也证明了 1(+•)-7(+•)的形成，该实验证明了自由基阳离子 1(+•)-3(+•)（λ(max) = 530nm）和 5(+•)-7(+•)（λ(max) = 570nm）的吸收带主要定域在芳硫基，而自由基阳离子 4(+•)（λ(max) = 410nm，700nm）可能主要定域在芴基环。自由基阳离子以一级动力学衰减，这归因于 C-S 键的断裂。基于 DFT 计算，我们提出了 1(+•)-4(+•)和 7(+•)中最适合 C-S 键断裂的构象，其特征是 C-S 键几乎与芴基环的π 体系共线，并且芳基硫环和芴基环之间有明显的电荷和自旋离域。这种离域可能是 C-S 键断裂速率对 C-S 键离解自由能（BDFE）变化的敏感性很小的原因。在自由基阳离子中，C-S 键的断裂反应的重组能值（λ = 43.7 kcal mol(-1)）计算值相当大。这一数值远大于双环戊二烯自由基阳离子中 C-C 键断裂反应的重组能值（λ = 12 kcal mol(-1)），该反应也导致芴基碳正离子的形成。
