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.