Six-Membered, Chiral NHCs Derived from Camphor: Structure–Reactivity Relationship in Asymmetric Oxindole Synthesis

Abstract

A series of three chiral, expanded six-membered NHC–palladium­(II) complexes was prepared with successively increased sterical demand, while retaining natural <i>d</i>-(+)-camphor as a chiral motif. The catalysts showed different reaction profiles in the asymmetric, intramolecular α-arylation of amides. The molecular structure of two N<i>-</i>heterocyclic and one nitrogen acyclic carbene palladium isonitrile complex was unequivocally determined by X-ray crystallographic analysis. The results reported herein account for a correlation of catalytic activity and enantiodiscrimination in relation to the degree of chiral substitution and steric congestion at the metal center. The modular and convergent synthetic route of these air- and moisture-stable palladium isonitrile complexes underlines the usefulness of this approach