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 SH2 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.