Phosphine–Olefin Ligands Based on a Planar-Chiral (π-Arene)chromium Scaffold: Design, Synthesis, and Application in Asymmetric Catalysis

The NMR and X-ray crystallographic studies clarified that planar-chiral alkenylene-bridged (phosphino-π-arene)­(phosphine)chromium complexes 3 were capable of coordinating to a rhodium­(I) cation in a bidentate fashion at the (π-arene)-bound phosphorus atom and at the olefin moiety. The P-olefin chelate coordination of 3 constructs the effective chiral environment at the metal center, and thus, these rhodium complexes display high performances in various rhodium-catalyzed asymmetric 1,4- and 1,2-addition reactions with arylboron nucleophiles. The control experiments demonstrated that the (η2-olefin)–Rh interaction as well as the bridging structure in 3 played the pivotal roles in the high enantioselectivity of the Rh-catalyzed asymmetric reactions. To enhance the synthetic utilities of these phosphine–olefin ligands, an enantiospecific and scalable synthetic method was developed. The novel synthetic method is flexible in terms of the substituent variation, and a library of the planar-chiral (arene)­chromium-based phosphine–olefin ligands was established by the combinatorial approach. Among the newly prepared ligand library, compound 3g, which is with a bis­(3,5-dimethylphenyl)­phosphino group on the η6-arene ring, was found to be a far better chiral ligand in the rhodium-catalyzed asymmetric reactions showing excellent enantioselectivity and high yields.