New rhodium pyridylphosphine complexes and their application in hydrogenation reactions

New phosphine ligands containing substituted pyridyl rings, namely tris[3-(2,6-dimethoxypyridyl)]phosphine (1), bis[3-(2,6-dimethoxypyridyl)]phenylphosphine (2), and 3-(2,6-dimethoxypyridyl)phenylphosphine(3), have been prepared. The purpose was to overcome previous problems associated with the use of pyridylphosphine ligands in catalytic hydrogenation reactions and to develop a new class of catalysts that can be separated from reaction mediums via phase separation. To prevent the formation of polymeric transition-metal complexes of pyridylphosphines (which took place via the coordination of the P and N atoms of the ligands to separate metal centers), the ortho-hydrogen atoms of the pyridyl rings were replaced with more bulky substituents. The use of methoxy substituents at the ortho positions of the pyridyl ring in this new class of pyridylphosphine ligands successfully prevented the coordination of the pyridyl group to the metal center and allowed the formation of discrete rhodium pyridylphosphine complexes, which were found to be active in the catalytic hydrogenation of olefins, aldehydes, and imines. The solubility of these rhodium pyridylphosphine complexes in organic solvent or in aqueous solution can be altered by adjusting the acidity of the solution. The separation of the catalyst from the reaction product in a water-immiscible organic solvent was achieved by extracting the catalyst with hydrochloric acid. Upon neutralization of the aqueous extract with sodium carbonate, the catalyst could be re-extracted into an organic layer and could be reused in a new batch of hydrogenation reaction with similar reactivity as the freshly prepared complexes.