Phosphorus-chiral analogues of 1,1'-bis(diphenylphosphino)ferrocene: Asymmetric synthesis and application in highly enantioselective rhodium- catalyzed hydrogenation reactions

Abstract

Five new ferrocene ligands 1a-e (1 = 1,1'-bis(aryl- phenylphosphino)ferrocene with aryl residues a = 1-naphthyl, b = 2-naphthyl, c = 2-anisyl, d = 2-biphenylyl, and e = 9-phenanthryl) bearing stereogenic phosphorus atoms have been prepared in enantiomerically pure form via an asymmetric synthesis protocol. The use of (+)- or (-)-ephedrine as the optically active auxiliary during subsequent nucleophilic displacement reactions at borane-protected phosphorus centers gave rise to the title compounds in 41-65% overall yield. The absolute configuration of the ferrocenyldiphosphines was confirmed by crystal structure analysis of 1a, 1b, and 1d. Their efficiency as chiral ligands was demonstrated in rhodium- catalyzed asymmetric hydrogenation reactions of α-(acylamino)cinnamic acid derivatives. While low reactivity or enantiomeric discrimination were observed when employing ligands 1b and 1d, the in situ formed catalysts prepared from diphosphines 1a, 1c, or 1e effected complete conversion and enantioselectivities of up to 98.7% ee.