The β-sulfur substituent effect in radical reactions. Chlorine-atom abstraction from alkyl 2-chloroethyl sulfides, hydrogen-atom abstraction from dialkyl sulfides and the question of possible 1,4-hydrogen-atom transfer in β-alkylthioethyl radicals

Abstract

The reactions of MeSCH₂CH₂Cl with the nucleophilic metalloidal radicals Me₃N→ḂHBu and Et₃Si˙ have been studied in solution between 180 and 240 K using EPR spectroscopy. With the amine-boryl radical the dominant reaction is abstraction of chlorine, while with the silyl radical S_H2 dealkylation at sulfur takes place more rapidly than the abstraction of halogen. The relative rates of abstraction of chlorine from YCH₂CH₂Cl by Me₃N→ḂHBu or Et₃Si˙ increase along the series Y = MeCH₂ < MeO < MeS and the high reactivity of the 2-chloroethyl sulfide is attributed to a combination of favourable polar and enthalpic factors. In contrast, the relative reactivities of MeSCH₂CH₃ and MeSCH₂CH₂SMe towards hydrogen-atom abstraction by the tert-butoxyl radical indicate that a β-MeS group slightly retards hydrogen transfer from the SCH₂ groups to this electrophilic radical. No evidence was found to support the proposed rearrangement of the β-alkylthioalkyl radicals MeSCH₂ĊH₂ or EtSCH₂ĊH₂ to the more stable isomeric α-alkylthioalkyl radicals H₂ĊSCH₂Me or MeĊHSCH₂Me, respectively, by an intramolecular 1,4-H-atom shift. UV photolysis of dimethyl disulfide in the presence of ethene at low temperatures affords the EPR spectra of MeSCH₂ĊH₂ and of an α-alkylthioalkyl radical identified as MeSCH₂CH₂SĊH₂. The latter radical is thought to arise by addition of MeSCH₂ĊH₂ to thioformaldehyde, itself formed as a product of disproportionation of methanethiyl radicals. A series of ab initio molecular orbital calculations has been carried out in support of the experimental work and the computed activation energy for the rearrangement of MeSCH₂ĊH₂ to H₂ĊSCH₂Me by a 1,4-H-atom shift is very large (89.9 kJ mol^–1), such that this process would not be detectable in solution by EPR spectroscopy at moderate temperatures.