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(l,5-COD)(2)]BF4, [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 nu(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)fer ethyl}-3, 5-dimethyl- 1H-pyrazole}Rh(l, 5-COD)]BF4 (3a), [{1-{(S)-1-[(R)-2-(diphenylphosphino)ferrocenyl]ethyl}-3, 5-bis(trifluoromethyl) 1-[(R)-2-(diphenylphosphino)ferrocenyl]e (5k), and [(1-{(S)-1-[(R)-2-(diphenylphosphino)fer Cl] (5n) have been characterized by X-ray diffraction and were found to display very similar conformational features.