Asymmetric Induction at Remote Quaternary Centers of Cyclohexadienones by Rhodium-Catalyzed Conjugate Hydrosilylation.

The enantioselective desymmetrizing conjugate hydrosilylation of prochiral differently γ,γ-disubstituted cyclohexadienone derivatives 2 to furnish the corresponding cyclohexenones 4 with a remote chiral all-carbon quaternary center at the γ position is described. Chiral rhodium-bis(oxazolinyl)phenyl complexes 1 were effective catalysts for this transformation. This catalytic system was extended to the asymmetric transformation of spirocarbocyclic cyclohexadienones 5 to give the corresponding products 6 with high enantiomeric ratios.

Construction of a Chiral Silicon Center by Rhodium-Catalyzed Enantioselective Intramolecular Hydrosilylation.

Rhodium-catalyzed enantioselective desymmetrizing intramolecular hydrosilylation of symmetrically disubstituted hydrosilanes is described. The original axially chiral phenanthroline ligand (S)-BinThro (Binol-derived phenanthroline) was found to work as an effective chiral catalyst for this transformation. A chiral silicon stereogenic center is one of the chiral motifs gaining much attention in asymmetric syntheses and the present protocol provides cyclic five-membered organosilanes incorporating chiral silicon centers with high enantioselectivities (up to 91 % ee). The putative active Rh(I) catalyst takes the form of an N,N,O-tridentate coordination complex, as determined by several complementary experiments.