Mechanism of SH2 Reactions of Disulfides: Frontside vs Backside, Stepwise vs Concerted

Density functional theory calculations indicate that the S(H)2 reactions of disulfides with alkyl or aryl radicals take place via concerted backside displacement. The activation energies for reactions of Me-center dot with RSSR (R = Me, Et, Pr-i, Bu-t) increase with the size of R, since larger R groups prevent the formation of an ideal geometry for SOMO-LUMO overlap. Frontside transition states can also be located, but these lie at least 11 kcal mol(-1) above the corresponding backside transition states.