PALLADIUM-CATALYZED ASYMMETRIC ALLYLIC AMINATION USING FERROCENYL PYRAZOLE LIGANDS - STERIC CONTROL OF ETA(3)-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-complex es containing chiral ferrocenyl pyrazole ligands. The highest enantios electivity (99% ee) was obtained using ligand 3a, ino)ferrocenyl]ethyl }-3-(1-adamantyl)-1H-pyrazole. Four different cationic Pd-allyl interm ediates, 4a, 4c, 4j, and 4k (containing ligands 3a, 3c, 3j, and 3k, re spectively), formed during the catalytic reaction were studied in solu tion by 2D-NMR spectroscopy, with the aim of clarifying configurationa l aspects. Depending on the size and shape of the substituent in posit ion 3 of the pyrazole ring, it was found that the major diastereoisome ric form of these complexes either adopts an exo-syn-syn (ligands 3a a nd 3c, )ferrocenyl]-ethyl}-3-phenyl-5-methyl-1H-pyrazole) or an exo-sy n-anti configuration (ligands 3j, rrocenyl]ethyl}-3-(9-anthryl)-5-meth yl-1H-pyrazole and 3k, ino)ferrocenyl]ethyl}-3-(9-triptycyl)-1H-pyrazo le. 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-selectiv ity are discussed. The Pd-allyl complexes [Pd(eta(3)-PhCHCHCHPh)(3c)]P F6, 4c, and [Pd(eta(3)-PhCHCHCHPh)(3k)]PF6, 4k, were characterized by X-ray diffraction. 4c crystallizes with 1 equiv of Et(2)O in the ortho rhombic space group P2(1)2(1)2(1): a = 13.026(2) Angstrom, b = 14.784( 2) Angstrom, c = 25.124(5) Angstrom, Z = 4. Crystals of 4k contain 3 e quiv of hexane and 1 equiv of acetone and H2O in the unit cell belongi ng to the trigonal system, space group P3(2)21: a = 24.07(3) Angstrom, c = 22.48(3) Angstrom, Z = 6.