Pd and Ru complexes bearing axially chiral ligands for the asymmetric hydrogenation of C[dbnd]C and C[dbnd]O double bonds

Abstract

Complexes composed of either Pd or Ru as central metal and ligands with axial chirality in all cases were used as hydrogenation catalysts. The ligands were (R)- and (S)-6,6′-dimethyl-2,2′-diaminobiphenyl, (R)-(+)-1-1′-Bi(2-naphtylamine), (R)-2,2′-Bis(diphenylphosphino)-1,1′-binaphthalene and (R)-2,2′-Bis(di-p-tolylphosphino)-1,1′-binaphthyl. The Pd(II) complexes had one diamine ligand and the Ru(II) complexes had one bisphosphine and one diamine ligand, forming seven member chelate rings with the metal center. The pro-chiral substrates used were itaconic acid, α-acetamidocinnamic acid and acetophenone. The Pd complexes showed 100% chemoselectivity toward the CC bond, and toward the CO bond in the case of Ru. The yield and enantiomeric excess versus time behavior was studied using a large substrate/catalyst ratio. The addition of an organic base to the reaction with Pd complexes enhanced yield and enantiomeric excess. Use of the (S)-diamine ligand in the complex favored the (R)-products. The best results with itaconic acid were 61% yield and 56% enantiomeric excess and 55% yield and 52% enantiomeric excess with α-acetamidocinnamic acid, both catalyzed by Pd(OCOCF3)2 ((S)-6,6′-dimethyl-2,2′-diaminobiphenyl) in 2,2,2-trifluoroethanol. In the case of the Ru catalysts, (S)-1-phenylethanol formed preferentially during hydrogenation of acetophenone. Potassium tert-butoxide stabilized the enantiomeric excess. The best result was 87% yield and 41% enantiomeric excess catalyzed by ((R)-2,2′-Bis(di-p-tolylphosphino)-1,1′-binaphthyl)-RuCl2-((R)-(+)-1-1′-Bi(2-naphtylamine)).