Electron and hydrogen atom donor photocatalysts in situ generated from benzimidazolium salts and hydride reagents

Photocatalytic systems consisting of 2-substituted benzimidazoliums (BI⁺–R, R = polycyclic aryl, triarylamine or phenyl-sulfate) and stoichiometric hydride donor reagents were developed. Light emitting diode irradiation of these photocatalysts in the presence of NaBH₄ or picoline borane promotes desulfonylation reactions of an N-sulfonyl indole, N-sulfonyl amide and α-sulfonyl ketone. Absorption spectroscopic and redox potential measurements as well as density functional theory calculations were carried out to gain mechanistic information. Benzimidazolines (BIH–R), generated in situ by hydride reduction of BI⁺–R, serve as both an electron and hydrogen atom donor photocatalysts in these reductive desulfonylation reactions, which contrasts to ordinary reducing photocatalysts that simply donate electrons.