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

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 ]BF 4, [Rh(CO) 2 Cl] 2, and [Rh(COE) 2 Cl] 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-1 H -pyrazole}Rh(1,5-COD)]BF 4 ( 3a ), [{1-{( S )-1-[( R )-2-(diphenylphosphino)ferrocenyl]ethyl}-3,5-bis(trifluoromethyl)-1 H -pyrazole}Rh(1,5-COD)]BF 4 ( 3h ), [{1-{( S )-1-[( R )-2-(diphenylphosphino)ferrocenyl]ethyl}-3,5-dimethyl-1 H -pyrazole}Rh(CO)Cl] ( 5k ), and [{1-{( S )-1-[( R )-2-(diphenylphosphino)ferrocenyl]ethyl}-3,5-dimethyl-1 H -pyrazole}Rh(CO)Cl] ( 5n ) have been characterized by X-ray diffraction and were found to display very similar conformational features.