Enantioselective-Arylation of Ketones with Aryl Triflates Catalyzed by Difluorphos Complexes of Palladium and Nickel
The asymmetric α-arylation of ketones with aryl triflates is described, and the use of this electrophile with nickel and palladium catalysts containing a segphos derivative increases substantially the scope of highly enantioselective arylations of ketone enolates. The combination of aryl triflates as reactant, difluorphos as ligand, palladium catalysts for reactions of electron-neutral or electron-rich aryl triflates, and nickel catalysts for reactions of electron-poor aryl triflates led to a series of α-arylations of tetralone, indanone, cyclopentanone, and cyclohexanone derivatives. Enantioselectivities ranged from 70% to 98% with 10 examples over 90%. Systematic studies on these α-arylations have revealed a number of factors that affect enantioselectivity. Ligands containing biaryl backbones with smaller dihedral angles generate catalysts that react with higher enantioselectivity than related ligands with larger dihedral angles. In addition, faster rates for reactions of aryl triflates versus those for reactions of aryl bromides allow the α-arylations of aryl triflates to be conducted at lower temperatures, and this lower temperature improves enantioselectivity. Finally, studies that compare the enantioselectivities of catalytic reactions to those of stoichiometric reactions of isolated [(segphos)Pd(Ar)(Br)], [(segphos)Pd(Ar)(I)], and [(segphos)Ni(C6H4-4-CN)Br] suggest that catalyst decomposition affects enantioselectivity.
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