Batch versus flow stereoselective hydrogenation of α-acetamido-cinnamic acid catalyzed by an Au(I) complex

Abstract

A chiral gold (I) (2S,4S)-1-tert-butoxycarbonyl-4-diphenylphosphino-2-(diphenylphosphino- methyl) pyrrolidine (BPPM) complex has been prepared using [Au(SMe2)Cl] as precursor. The heterogenization of the Au-BPPM catalyst onto the CNT support followed two routes, ie (i) the non-covalent immobilization of the gold(I)complex by dry-impregnation, and (b) covalent immobilization of the gold(I)complex on a pre-functionalized CNT. These catalysts afford the stereoselective hydrogenation of α-acetamidocinnamic acid to the (R)-N-acetyl-phenylalanine enantiomer. The nature of the solvent affected both the enantioselectivity and TOFs. Among MeOH, EtOH, and TFE, methanol appeared to be the most efficient one (at 80 °C a TOF of 0.37 h−1 for a total enantioselectivity to the R-isomer). Transferring the reaction in the flow reactor, under similar conditions (methanol, room temperature) led to a 10 time increase of the TOF with no change in the stereoselectivity. The decrease of the TOF in time for both the reference Rh and the Au catalysts was assigned to their partial modification under the reaction conditions. The heterogenization of the Au-BPPM catalyst onto the CNT support, for the same content of Au-complex, led to a very important increase of the conversion with no change in the selectivity. However, the covalent bonding was more efficient affording a very high increase of the conversion even at room temperature (95% after 24 h), thus demonstrating that the anchoring a support increases the dispersion, and in consequence the efficiency. These CNT-Au-BPPM catalysts preserved the catalytic performances during recycling as also confirmed by the characterization results.