Asymmetric homogeneous hydrogenation of olefins catalyzed by alkylphosphine complexes of rhodium(I)

Abstract

The cationic rhodium(I) complexes of the optically active ferrocenes, C5H5FeC5H3[CH(CH3)N(CH3)2][ER2]-1,2, catalyze the asymmetric hydrogenation of acetamidoacrylic acid derivatives, itaconic acid and styrene, when ER2 is P(C6H5)2 or P(C(CH3)3)2. The configuration of the product is reversed on substituting C6H5 for C(CH3)3 groups. The catalyst with the tert-butyl groups can afford higher optical yields and is faster overall. The rhodium(I) complexes of the arsenic derivatives ER2 = As(C6H5)2 or As(CH3)2 do not catalyze the hydrogenation reactions. The nmr spectrum of the arsenic complex (ER2 = As(C6H5)2) is temperature dependent which seems to be due to a process involving the making and breaking of the Rh—N bond. The unsuccessful use of the MEM group (β-methoxyethoxymethyl) to protect an alcohol function α to a ferrocene ring is described. The hydrogenation results are discussed in the light of models which are currently used to predict the results of rhodium(I) catalyzed reactions but which are inapplicable to this work.