Immobilized catalysts for enantioselective hydrogenation: The effect of site-isolation

Functionalized PPM (PPM: (2S,4S)-4-diphenylphosphino-2-(diphenylphosphino-methyl) pyrrolidine), a chiral diphosphine ligand, was bound to silica gel with different loadings [mmol ligand/g support] and used as model to detect possible effects of site isolation in the enantioselective hydrogenation of methyl-acetamidecinnamate (MAC) with cationic and with neutral Rh-catalysts and of N-(2-methyl-6-ethylphen-1-yl)methoxymethylmethylketimin (MEI) with an Ir-catalyst. To investigate the influence of high local catalyst concentration in absence of a support, two new bis-PPM ligands with spacers of different length were prepared and tested. The results indicate that site isolation effects occur with two of the catalytic systems used: Whereas immobilized cationic Rh-catalysts are practically not affected, the neutral Ph-catalysts lose most of their activity with increasing catalyst loading. Probably, this is due to the formation of chloride-bridged Rh-dimers which is favored by high catalyst concentration as indicated by experiments with the bis-PPM-ligands. Immobilization has a spectacular effect with the Lr-system. It is known that Ir-complexes in presence of hydrogen can form stable hydrogen-bridged dimers that are catalytically inactive. The fact that the activity (and also productivity) of the immobilized catalysts increases with decreasing catalyst loading indicates, that the formation of these dimers can be prevented by immobilization.