Enantioselective Diels–Alder Reactions Catalyzed by Well-Defined Arene–Ruthenium Complexes. Relationship between Catalyst Structure and Enantioselectivity

The aqua complexes [(η6-arene)­Ru­(prophos)­(H2O)]­[SbF6]2 [prophos = (R)-propane-1,2-diylbis­(diphenylphosphane); arene = p-MeC6H4iPr (p-cymene) (1), C6Me6 (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 [(η6-arene)­Ru­(prophos)­(methacrolein)]­[SbF6]2 (arene = p-cymene (3), C6Me6 (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 η6-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.