Aqueous micellar and non-micellar effects during the asymmetric hydrogenation of dehydroamino acid derivatives: influence of amphiphiles on enantioselectivity and alpha-CH/CD exchange

The effect of different amphiphiles on the CH/CD exchange in the homogeneou sly catalysed asymmetric hydrogenation/deuteration of methyl (Z)-alpha -ace tamidocinnamate or methyl alpha -acetamidoacrylate in an aqueous micellar m edium has been investigated in connection with the effect of amphiphiles on the enhancement of enantioselectivity. In comparison with the exchange of alpha -CH/CD in water, the amphiphiles inhibit the reaction in the order: c ationic < zwitterionic <much less than> anionic. In mixtures of cationic (c etyltrimethylammonium hydrogen sulfate, C16H33N(CH3)(3)+HSO4) and anionic a mphiphiles (sodium dodecyl sulfate, SDS) the H/D-exchange amount is low in the presence of an excess of SDS, but it increases rapidly near a CTA(+)HSO (4)(-) mole fraction of 0.5 to give a high level of exchange. The enantiose lectivity drops to a minimum in the 1:1 mixture because of the low solubili ty of the cationic-anionic aggregates and the absence of micelles. The resu lts obtained with mixed micelles of Brij 35 (polyethyle-neoxide(23) monodod ecylether, C12E23) and SDS are quite different. This mixture is dispersible and able to form micelles over the entire range of mole fractions (0 to 1) . As a consequence, the isotope exchange is almost constant from a mole fra ction of 0.3-0.9 of SDS. The enantioselectivity is nearly constant over the whole range. The inhibition of HID exchange in the presence of long-chain alkyl sulfates seems to be caused by a specific interaction with the cataly tic rhodium complex. (C) 2001 Elsevier Science B.V. All rights reserved.