The synthesis of a variety of nonbridged optically active C2- and C1-symmetric titanocenes and zirconocenes is reported together with a detailed investigation of the ability of these complexes to effect the catalytic asymmetric hydrogenation of 2-phenyl-1-butene and 24anaphthyl)- 1-butene. The respective dihydro products are shown to be produced with enantiomeric excesses ranging from 4% to 69%. The absolute configuration of these 2-arylbutanes was noted to be sensitive to the three-dimensional characteristics of the particular bicyclo-fused ligand involved. For a series of verbenone-related metallocenes, an increase in the size of a substituent positioned a and syn to the metal center was found to be unfavorable for stereoinduction. This phenomenon has been rationalized in transition state terms, with the alkene approaching laterally to engage in hydrogen atom transfer. The three-dimensional stereochemical model advanced in explanation of the observed catalytic selectivity underscores the development of steric biases during olefin coordination to the reactive metal hydride intermediate.
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