Mechanism of S_H2 Reactions of Disulfides: Frontside vs Backside, Stepwise vs Concerted

Density functional theory calculations indicate that the S_H2 reactions of disulfides with alkyl or aryl radicals take place via concerted backside displacement. The activation energies for reactions of Me^• with RSSR (R = Me, Et, ⁱPr, ^tBu) 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.