Mechanistic and Preparative Studies of Radical Chain Homolytic Substitution Reactions of N‑Heterocyclic Carbene Boranes and Disulfides

Reactions of 1,3-dimethylimidazol-2-ylidene–borane
(diMe-Imd-BH3) and related NHC–boranes with diaryl
and diheteroaryl
disulfides provide diverse NHC–boryl monosulfides (diMe-Imd-BH2SAr) and NHC–boryl disulfides (diMe-Imd-BH­(SAr)2). Heating in the dark with 1 equiv of disulfide favors monosulfide
formation, while irradiation with 2 equiv disulfide favors disulfide
formation. With heteroaryl disulfides, the NHC–borane in the
primary NHC–boryl sulfide product migrates from sulfur to nitrogen
to give new products with a thioamide substructure. Most substitution
reactions are thought to proceed through radical chains in which homolytic
substitution of a disulfide by an NHC–boryl radical is a key
step. However, with electrophilic disulfides under dark conditions,
a competing ionic path may also be possible.