First examples of structurally imposing eight-membered-ring (diazocanylidene) N-heterocyclic carbenes: salts, free carbenes, and metal complexes

A series of three chiral, expanded six-membered NHC–palladium(II) complexes was prepared with successively increased sterical demand, while retaining natural d-(+)-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-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