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-pi-arene)-(phosphine)chromium complexes 3 were capable of coordinating to a rhodium(I) cation in a bidentate fashion at the (pi-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 (eta(2)-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 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)-phosplaino group on the eta(6)-arene ring, was found to be a far better chiral ligand in the rhodium-catalyzed asymmetric reactions showing excellent enantioselectivity and high yields.