Enantioselective Diels-Alder reactions catalyzed by well-defined Arene-Ruthenium complexes. Relationship between catalyst structure and enantioselectivity

摘要

The aqua complexes [(η 6-arene)Ru(prophos)(H 2O)][SbF 6] 2 [prophos = (R)-propane-1,2- diylbis(diphenylphosphane); arene = p-MeC 6H 4iPr (p-cymene) (1), C 6Me 6 (2)] are active catalysts for the asymmetric Diels-Alder (DA) reaction of methacrolein and cyclopentadiene. Complexes 1 and 2 and the DA intermediate Lewis acid-dienophile complexes [(η 6- arene)Ru(prophos)(methacrolein)][SbF 6] 2 (arene = p-cymene (3), C 6Me 6 (4)) have been isolated and characterized by analytical and spectroscopic means, including the X-ray crystal structure determination of compounds 2, 3, and 4. Structural parameters suggest that the disposition of coordinated methacrolein is influenced by the CH/π-attractive interactions established between phenyl groups of the prophos ligand and the CHO proton of the methacrolein. Proton NMR parameters indicate that these interactions are maintained in solution. The role of the apparently innocent η 6-arene ligand in the control of the catalytic coordination site is examined from the structural data obtained. From these data, the origin of the enantioselectivity in the DA reaction is discussed. © 2011 American Chemical Society.