Dependence of enantioselectivity on the distribution of a chiral hydrogenation catalyst between an aqueous and a micellar phase: Investigations usingpulsed field gradient spin echo NMR spectroscopy

The enantioselectivity obtained from rhodium complex catalyzed hydrogenatio ns conducted in water can often be increased considerably by the addition o f amphiphiles. At present the reasons for this increase in selectivity are not fully understood. The application of pulsed field gradient spin echo NM R (PGSE-NMR) spectroscopy to determine the average diffusion coefficients o f the catalysts in both known and novel examples of asymmetric hydrogenatio n shows definitively that the increase in enantioselectivity is coupled wit h an aggregation of the catalyst to the micelles. This aggregation or solub ilization of the catalyst in the micelles leads to the formation of a new c olloidal phase in the aqueous solution. This phase has stronger hydrophobic properties, and thus the hydrogenation is more comparable to those conduct ed in a hydrophobic or less polar organic solvent. In the case of anionic a mphiphiles. which form amphiphilic salts with the cationic catalyst, the em bedment of the catalyst complex into the micelle is generally complete. The whole hydrogenation then takes place exclusively inside the micelles. lead ing to high enantioselectivity. If the catalyst is not completely embedded into the micelle, for example in the cases of nonionic or cationic surfacta nt solutions. the solubility of the substrate plays an important role. For soluble substrates the hydrogenation of the substrate occurs predominately in the aqueous phase itself, leading to very poor enantioselectivities. In these cases, only the use of a large excess of amphiphile, far above the cr itical micelle concentration (cmc), will lead to higher enantioselectivitie s due to a shift of the equilibrium towards the micellar bonded forms of ca talyst and substrate. In contrast. poorly soluble substrates exhibit a high tendency to be incorporated into micelles. which leads to much higher enan tioselectivities if the cmc of the surfactant is small enough. Changes in t he cmc of amphiphiles caused by their aggregation with catalysts could also be estimated. The variation in selectivity observed for the catalysts cont aining seven-membered, flexible chelate rings is apparently due to changes in their conformation in the less polar micellar medium, and this effect is also seen in organic solvents. As expected, catalysts containing smaller c helate rings show this effect to a considerably lower extent since they are conformationally more rigid.