Kinetic Resolution of Planar-Chiral Ferrocenylphosphine Derivatives by Molybdenum-Catalyzed Asymmetric Ring-Closing Metathesis and Their Application in Asymmetric Catalysis

Highly enantioselective kinetic resolution
of racemic planar-chiral
metallocenylphosphine sulfides was realized by the molybdenum-catalyzed
asymmetric ring-closing metathesis reaction with the krel values of up to 147. The enantiomerically enriched
1,4-but-2-enylene-bridged ferrocenylphosphine sulfides thus obtained
could be purified to enantiomerically pure forms by simple recrystallization
from hot methanol, and subsequent reduction of the phosphine sulfides
provided the corresponding planar-chiral phosphines with retention
of the enantiomeric homogeneity. This is a rare example of preparing
planar-chiral ferrocenylphosphines by catalytic asymmetric reactions.
The single-enantiomer planar-chiral ferrocenylphosphines were applied
as chiral ligands in the rhodium-catalyzed asymmetric 1,4-addition
reaction (the Hayashi–Miyaura conjugate addition reaction)
of phenylboronic acid to 2-cyclohexenone to show excellent enantioselectivity
and high yields. The NMR studies clarified that the butenylene-bridged
ferrocenylphosphine coordinated to a rhodium­(I) cation in a monodentate
fashion and an interaction of the bridging olefin moiety to the rhodium
atom was not detected.