Catalyst-free reductive desulfurization of thioamides with ammonia borane through a concerted double-hydrogen transfer mechanism

Abstract

The green reductive transformation of thioamides is highly desired yet faces challenges in broad substrate scope and selectivity for CS and C–N cleavage. Existing catalytic hydrogenation methods are still limited and require harsh reaction conditions. Here, 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 to play an important self-catalytic role in the reaction. This practical and selective protocol provides an important example for designing catalyst-free reductive systems.