Stereochemically Dynamic 2,2‘-Biphosphole Ligands for Asymmetric Catalysis

The stereochemically dynamic 2,2‘-biphospholes 8, 9, and 10 have been synthesized by
asymmetric alkylation of 2,2‘-biphospholyl anion with enantiomerically pure diol ditosylates.
A good carbon-to-phosphorus induction has been obtained in particular in the case of
compound 8. The introduction of a chiral linker between the two phosphorus atoms resulted
in partial chirality control of the central and axial chiralities of the 2,2‘-biphosphole
framework, as three diastereoisomers a, b, and c are obtained among the six expected. In
solution at room temperature a, b, and c diastereoisomers exist as an equilibrium mixture;
the interconversion occurred by phosphorus-inversion inducing atropoinversion. Reaction
of the equilibrium mixture of 2,2‘-biphosphole with [MCl2(CH3CN)2] resulted in dynamic
resolution leading to diastereo- and enantiopure Pd- and Pt-complexes. Palladium-catalyzed
asymmetric allylic substitution can be achieved with the ligands 8 and 10 with high activities
but still moderate enantioselectivities.