Asymmetric synthesis of (S,S)-(+)-1,1'-bis-(methyl-phenyl-phosphino) ferrocene

The asymmetric synthesis of 1,1'-bis-(methyl-phenpl-phosphino) ferrocene 1 is described using the oxazaphospholidine borane complex 5 as synthon. Two strategies were investigated, based either on P-C bond formation by ring opening of complex 5 with the 1,1'-dilithio ferrocene 10, or on homocoupling of the cyclopentadienyl-methyl-phenyl-phosphine borane 2 anion with FeCl2. The first leads to a diastereomeric mixture of the diphosphine 1 in a 55:45 ratio, and the low stereoselectivity is explained by the steric hindrance of the ferrocene dianion. In the second strategy, the (R)-cyclopentadienyl phosphine borane 2 (85% eel was prepared by the reaction of CpNa with the optically active chlorophosphine borane 14, derived from the aminophosphine borane 6a. The coupling of 2 leads to the 1,1'-diphosphino ferrocene borane 13, which is obtained diastereomerically pure by recrystallization, then decomplexed to the corresponding (S,S)-diphosphine 1.