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.