title: Synthesis of Pd Complexes Bearing an Enantiomerically-Resolved Seven-Membered N-Heterocyclic Carbene Ligands and Initial Studies of their Use in Asymmetric Wacker-Type Oxidative Cyclization Reactions.
abstract: The development of enantiomerically-resolved, axially-chiral seven-membered N-heterocyclic carbene ((7)NHC) ligands for palladium is described. These (7)NHC ligands are derived from enatiomerically pure 2,2'-diamino-6,6'-dimethylbiphenyl, which is transformed via a synthetic sequence consisting of ortho-arylation, N-alkylation, and cyclization to afford seven-membered-ring amidinium salts. Synthesis of the 7-membered amidinium salts benefits from microwave irradiation, and in-situ metalation of the amidinium salts yields (7)NHC-Pd(II) complexes. The chiral (7)NHC-Pd complexes were examined as chiral catalysts under aerobic conditions in two intramolecular oxidative amination reactions of alkenes. In one case, enantioselectivities up to 63% e.e. were obtained, while the other substrate underwent cyclization to afford essentially racemic products. The catalytic data compare favorably to results obtained with a Pd(II) catalyst bearing a chiral five-membered-ring NHC ligand and, thereby, highlight the potential significance of this new class of chiral NHC ligands.
authors.affiliation: Department of Chemistry, University of Wisconsin-Madison, Madison, WI 53706.
authors.forename: Christopher C
authors.initials: CC
authors.lastname: Scarborough
authors.forename: Ana
authors.initials: A
authors.lastname: Bergant
authors.forename: Graham T
authors.initials: GT
authors.lastname: Sazama
authors.forename: Ilia A
authors.initials: IA
authors.lastname: Guzei
authors.forename: Lara C
authors.initials: LC
authors.lastname: Spencer
authors.forename: Shannon S
authors.initials: SS
authors.lastname: Stahl