Solid-phase synthesis of homogeneous ruthenium catalysts on silica for the continuous asymmetric transfer hydrogenation reaction

• Published in: Chemistry : a European journal Vol. 7; no. 6; pp. 1202 - 1208
• Main Authors: Sandee, AJ, Petra, DGI, Reek, JNH, Kamer, PCJ, van Leeuwen, PWNM
• Format: Journal Article
• Language: English
• Published: WEINHEIM Wiley 16.03.2001
• Subjects: Chemistry; Chemistry, Multidisciplinary; Physical Sciences; Science & Technology; KETONES; immobilization ruthenium; COMPLEXES; COMBINATORIAL; asymmetric catalysis; LIGAND; hydrogen transfer; solid-phase synthesis; DISCOVERY
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/1521-3765(20010316)7:6<1202::AID-CHEM1202>3.3.CO;2-I

The solid-phase synthesis of new asymmetric transfer hydrogenation catalysts as well as the use of these silica supported systems in batch and flow reactors is reported. The ruthenium complex of NH-benzyl-(1R,2S)-(-)-norephedrine covalently tethered to silica showed a high activity and enantioselectivity in the reduction of acetophenone. In three consecutive batchwise catalytic runs, we obtained ee values 88%. In a continuous flow reactor, a very constant catalytic activity was observed; no catalyst deactivation occurred over a period of one week, This has been ascribed to successful site isolation. Using optimized conditions in this flow reactor, the ee was as high as 90% at 95% conversion, The supported catalysts generally show the same trend in catalyst performance as in solution. The viability of our approach was further shown in one example, the ruthenium(II) complex of (1S,2R)-(+)-2-amino-1,2-diphenylethanol, for which an enantiomeric excess of 58% was observed, which is nearly three times higher than its homogeneous analogue.