ASYMMETRIC-SYNTHESIS OF (S,S)-(-1,1'-BIS-(METHYL-PHENYL-PHOSPHINO) FERROCENE())

The asymmetric synthesis of 1,1'-bis-(methyl-phenpl-phosphino) ferroce ne 1 is described using the oxazaphospholidine borane complex 5 as syn thon. Two strategies were investigated, based either on P-C bond forma tion by ring opening of complex 5 with the 1,1'-dilithio ferrocene 10, or on homocoupling of the cyclopentadienyl-methyl-phenyl-phosphine bo rane 2 anion with FeCl2. The first leads to a diastereomeric mixture o f the diphosphine 1 in a 55:45 ratio, and the low stereoselectivity is explained by the steric hindrance of the ferrocene dianion. In the se cond strategy, the (R)-cyclopentadienyl phosphine borane 2 (85% eel wa s prepared by the reaction of CpNa with the optically active chloropho sphine borane 14, derived from the aminophosphine borane 6a. The coupl ing of 2 leads to the 1,1'-diphosphino ferrocene borane 13, which is o btained diastereomerically pure by recrystallization, then decomplexed to the corresponding (S,S)-diphosphine 1.