CATALYTIC ENANTIOSELECTIVE HYDROGENATION OF 1,1-DISUBSTITUTED ALKENES WITH OPTICALLY-ACTIVE TITANOCENE AND ZIRCONOCENE COMPLEXES CONTAINING EITHER IDENTICAL OR DIFFERENT LIGANDS

The synthesis of a variety of nonbridged optically active C-2- and C-1-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 2-(alpha-naphthyl)-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 bicycle-fused ligand involved. For a series of verbenone-related metallocenes, an increase in the size of a substituent positioned alpha 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.