Enantioselective catalysts based on the chiral fragment (η5-C5Me5)Ir(Prophos) for Diels–Alder reactions

• (SIr,RC)–[(η5-C5Me5)Ir(Prophos)(H2O)][SbF6]2 is an enantioselective catalyst precursor for Diels–Alder reactions between enals and diverse dienes.
• A catalyst-enal intermediate can be diastereoselectivity isolated and characterized.
• CH/π intramolecular interactions fix the enal conformation.
• The stereochemistry of the coordinated enal accounts for the catalytic outcome.

The aqua complex (SIr,RC)–[(η5-C5Me5)Ir(Prophos)(H2O)][SbF6]2 [Prophos = (R)-propane-1,2-diyl-bis(diphenylphosphane)] is an active precursor for the asymmetric Diels–Alder reaction of acyclic enals with cyclopentadiene, 2,3-dimethylbutadiene and isoprene. Enantioselectivities up to 78% ee are achieved. The intermediate Lewis acid-dienophile complex (SIr,RC)–[(η5-C5Me5)Ir(Prophos)(ethyl acrolein)][SbF6]2 has been isolated and completely characterized, including the X-ray crystal structure determination. Structural parameters indicate that the disposition of the coordinated dienophile is controlled by CH/π attractive interactions established between a phenyl group of the Prophos ligand and the aldehyde proton of the coordinated enal. Proton NMR data indicate that these interactions are maintained in solution. From diffractometric and spectroscopic data, the origin of the enantioselectivity is discussed.

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