Enantioselective Catalytic Diels–Alder Reactions with Enones As Dienophiles

The aqua complexes (SM,RC)-[(η5-C5Me5)­M­(PROPHOS)­(H2O)]­[SbF6]2 [PROPHOS = (R)-propane-1,2-diylbis­(diphenylphosphane); M = Rh (1), Ir (2)] are active catalysts for the asymmetric Diels–Alder reaction between ketones and dienes. At low temperatures, enantioselectivities of up to 89% ee are achieved. The intermediate Lewis acid–dienophile complexes (SM,RC)-[(η5-C5Me5)­M­(PROPHOS)­(MVK)]­[SbF6]2 (MVK = methyl vinyl ketone; M = Rh (3), Ir (4)) and (SIr,RC)-[(η5-C5Me5)­Ir­(PROPHOS)­(EVK)]­[SbF6]2 (EVK = ethyl vinyl ketone (5)) have been isolated and characterized by analytical and spectroscopic means, including the determination of the crystal structure of the iridium complexes 4 and 5 by X-ray diffractometric methods. Structural parameters indicate that the dispositions of the coordinated dienophiles are controlled by the CH/π attractive interactions established between a phenyl group of the PROPHOS ligand and the α-vinyl proton of the ketones. Proton NMR parameters indicate that these interactions are maintained in solution. From these data, the stereoselectivity of the catalytic reaction is discussed.