A New Class of Tunable Dendritic Diphosphine Ligands: Synthesis and Applications in the Ru-Catalyzed Asymmetric Hydrogenationof Functionalized Ketones

摘要

A series of tunable G_0-G_3 dendritic 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP) ligands was prepared by attaching polyaryl ether dendrons onto the four phenyl rings on the P atoms. Their ruthenium complexes were employed in the asymmetric hydrogenation of b-ketoesters, aketoesters, and a-ketoamides to reveal the effects of dendron size on the catalytic properties. The second-and thirdgeneration catalysts exhibited excellent enantioselectivities, which are remarkably higher than those obtained from the small molecular catalysts and the first-generation catalyst. Molecular modeling indicates that the incorporation of bulky dendritic wedges can influence the steric environments around the metal center. In addition, the ruthenium catalyst bearing a second-generation dendritic ligand could be recycled and reused seven times without any obvious decrease in enantioselectivity.