Synthesis of Pd complexes bearing an enantiomerically resolved seven-membered N-heterocyclic carbene ligand and initial studies of their use in asymmetric Wacker-type oxidative cyclization reactions |
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Authors: | Christopher C Scarborough |
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Institution: | Department of Chemistry, University of Wisconsin-Madison, Madison, WI 53706, USA |
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Abstract: | The development of enantiomerically resolved, axially-chiral seven-membered N-heterocyclic carbene (7NHC) ligands for palladium is described. These 7NHC 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 seven-membered amidinium salts benefits from microwave irradiation, and in-situ metalation of the amidinium salts yields 7NHC-PdII complexes. The chiral 7NHC-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% ee were obtained, while the other substrate underwent cyclization to afford essentially racemic products. The catalytic data compare favorably to results obtained with a PdII catalyst bearing a chiral five-membered-ring NHC ligand and, thereby, highlight the potential significance of this new class of chiral NHC ligands. |
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Keywords: | N-Heterocyclic carbene Palladium Organometallic chemistry Wacker oxidation |
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