Catalyst-Free Reductive Desulfurization of Thioamides with Ammonia Borane through Concerted Double-hydrogen Transfer Mechanism

The green reductive transformation of thioamides is highly desired yet faces challenges in broad substrate scope and selectivity for C=S and C-N cleavage. Existing catalytic hydrogenation methods are still limited and require harsh reaction conditions. Here, a computation-aided design discovered a catalyst-free protocol for thioamides’ reductive desulfurization with ammonia borane (AB). The system uses dimethylamine borane (DMAB) to directly reduce thioamides to amines under catalyst-free, practical, economical, green, and easy-to-handle conditions. It covers a broad scope for primary, secondary, and tertiary thioamides. The experimental and theoretical studies revealed a concerted double-hydrogen transfer mechanism for this catalyst-free system, in which DMAB was found an important self-catalysis role in the reaction. This practical and selective protocol could provide an important example for designing catalyst-free reductive systems.