A Bimetallic Palladium Catalyst for Asymmetric Allylic Substitution Reactions

Abstract

With 1,1‘-diacetylruthenocene ( 9 ) as starting material, the preparation of 1,1‘-bis[1-(1-{( R )-1-[( S )-2-(diphenylphosphino)ferrocenyl]ethyl})-1 H -pyrazol-3-yl]ruthenocene (( R, R )-( S, S )- 12 ) is attained in three steps. 12 coordinates to two independent Pd(II)−π-allyl units to form the dicationic complex 15, isolated as a bis(hexafluorophosphate) salt. This has been characterized by X-ray and 2D NMR methods in solution, along with the analogous derivative 14, which contains 1-{( R )-1-[( S )-2-(diphenylphosphino)ferrocenyl]ethyl}-3-ruthenocenyl-1 H -pyrazole (( R )-( S )- 8b ). The three Pd(II)−allyl fragments in these two complexes show very similar conformational features. In the Pd-catalyzed substitution reaction of ethyl (1,3-diphenylallyl)carbonate ( 1 ) with benzylamine, the bimetallic catalyst containing ligand 12 and the mononuclear catalyst formed by 8b afford the same high enantioselectivity (99.3%).