Electronic Effects in Asymmetric Catalysis. Synthesis and Structure of Model Rhodium Complexes Containing Ferrocenyl Ligands for Use in the Hydroboration Reaction

Abstract

A series of new rhodium complexes with chiral ferrocenyl chelating ligands containing a tertiary phosphine and a pyrazole moiety have been obtained in good yields from the reaction of the corresponding P,N ligand (1a−p) with [Rh(1,5-COD)2]BF4, [Rh(CO)2Cl]2, and [Rh(COE)2Cl]2. The electronic influence of the P,N ligands has been evaluated by the measurement of the carbonyl stretching vibration in the compounds 5a−p. Qualitative trend correlations between ν(CO) values for such complexes and the enantioselectivities obtained in the Rh-catalyzed hydroboration of styrene utilizing the corresponding ligands have been found. Thus, both electron-donating groups on the pyrazole and electron-withdrawing substituents on the phosphine lead to both higher enantioselectivities and higher CO stretching frequencies. The compounds [{1-{(S)-1-[(R)-2-(diphenylphosphino)ferrocenyl]ethyl}-3,5-dimethyl-1H-pyrazole}Rh(1,5-COD)]BF4 (3a), [{1-{(S)-1-[(R)-2-(diphenylphosphino)ferrocenyl]ethyl}-3,5-bis(trifluoromethyl)-1H-pyrazole}Rh(1,5-COD)]BF4 (3h), [{1-{(S)-1-[(R)-2-(diphenylphosphino)ferrocenyl]ethyl}-3,5-dimethyl-1H-pyrazole}Rh(CO)Cl] (5k), and [{1-{(S)-1-[(R)-2-(diphenylphosphino)ferrocenyl]ethyl}-3,5-dimethyl-1H-pyrazole}Rh(CO)Cl] (5n) have been characterized by X-ray diffraction and were found to display very similar conformational features.