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

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.