Asymmetric N-oxidation catalyzed by bisguanidinium dinuclear oxodiperoxomolybdosulfate

Abstract

N-oxides play a pivotal role in natural products and emerging drug design, while also serving as valuable ligand scaffolds in organometallic chemistry. Among heteroatom oxidations, the conversion of amines to N-oxides is a critical and challenging facet. We present here a highly enantioselective N-oxidation methodology for both cyclic and acyclic amines. The method employs an ion-pair catalyst comprising a chiral bisguanidinium [BG]2+ cation and an achiral oxodiperoxomolybdosulfate anion [(µ-SO4)2Mo2O2(µ-O2)2(O2)2]2-. Notably, the bisguanidinium cation undergoes modification through silyl group incorporation and is elucidated by X-ray crystallography. Our findings underscore the crucial role of the side chain in the determination of the chiral pocket size, allowing for the oxidation of diverse tertiary amines with enantioselectivities. Comprehensive mechanistic investigations are conducted to explain the catalytic system’s efficacy in achieving dynamic kinetic resolution (DKR) with high efficiency.