Chiral-at-Ruthenium Catalysts with Mixed Normal and Abnormal N-Heterocyclic Carbene Ligands

Abstract

We recently reported a new class of chiral ruthenium catalysts in which two achiral bidentate N-(2-pyridyl)-substituted N-heterocyclic carbene ligands in addition to two labile acetonitriles are coordinated to a central ruthenium and generate a stereogenic metal center which is responsible for the overall chirality (Zheng et al. J. Am. Chem. Soc. 2017, 139, 4322). Here we now report our discovery of related chiral-at-ruthenium catalysts in which normal and abnormal N-heterocyclic carbene (NHC) ligands are present at the same time. The synthesis of racemic complexes, their resolution into individual enantiomers by a chiral auxiliary approach, and a catalytic application are reported. The mixed normal/abnormal NHC complexes display significantly increased turnover numbers and turnover frequencies for a nitrene-mediated enantioselective C(sp3)–H amination.