Palladium-Catalyzed Asymmetric Allylic Amination Using Ferrocenyl Pyrazole Ligands: Steric Control of η³-Allyl Configuration and Site-Selective Nucleophilic Attack

The reaction of 1,3-diphenylallylethyl carbonate (1) with benzylamine to afford the secondary amine 2 is effectively catalyzed by Pd-complexes containing chiral ferrocenyl pyrazole ligands. The highest enantioselectivity (99% ee) was obtained using ligand 3a, 1-{(S)-1-[(R)-2-(diphenylphosphino)ferrocenyl]ethyl}-3-(1-adamantyl)-1H-pyrazole. Four different cationic Pd-allyl intermediates, 4a, 4c, 4j, and 4k (containing ligands 3a, 3c, 3j, and 3k, respectively), formed during the catalytic reaction were studied in solution by 2D-NMR spectroscopy, with the aim of clarifying configurational aspects. Depending on the size and shape of the substituent in position 3 of the pyrazole ring, it was found that the major diastereoisomeric form of these complexes either adopts an exo-syn-syn (ligands 3a and 3c, 1-{(S)-1-[(R)-2-(diphenylphosphino)ferrocenyl]-ethyl}-3-phenyl-5-methyl-1H-pyrazole) or an exo-syn-anti configuration (ligands 3j, 1-{(S)-1-[(R)-2-(diphenylphosphino)ferrocenyl]ethyl}-3-(9-anthryl)-5-methyl-1H-pyrazole and 3k, 1-{(S)-1-[(R)-2-(diphenylphosphino)ferrocenyl]ethyl}-3-(9-triptycyl)-1H-pyrazole). This analysis allows the site of nucleophilic attack on the allyl ligand to be established unequivocally, i.e., the carbon atom trans to phosphorus. The reasons responsible for this pronounced site-selectivity are discussed. The Pd-allyl complexes [Pd(η3-PhCHCHCHPh)(3c)]PF6, 4c, and [Pd(η3-PhCHCHCHPh)(3k)]PF6, 4k, were characterized by X-ray diffraction. 4c crystallizes with 1 equiv of Et2O in the orthorhombic space group P212121: a = 13.026(2) Å, b = 14.784(2) Å, c = 25.124(5) Å, Z = 4. Crystals of 4k contain 3 equiv of hexane and 1 equiv of acetone and H2O in the unit cell belonging to the trigonal system, space group P3221: a = 24.07(3) Å, c = 22.48(3) Å, Z = 6.