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

The solid-phase synthesis of new asymmetric transfer hydrogenation catalyst s as well as the use of these silica supported systems in batch and flow re actors is reported. The ruthenium complex of NH-benzyl-(1R,2S)-(-)-norephed rine covalently tethered to silica showed a high activity and enantioselect ivity in the reduction of acetophenone. In three consecutive batchwise cata lytic runs, we obtained ee values 88%. In a continuous flow reactor, a very constant catalytic activity was observed; no catalyst deactivation occurre d over a period of one week, This has been ascribed to successful site isol ation. Using optimized conditions in this flow reactor, the ee was as high as 90% at 95% conversion, The supported catalysts generally show the same t rend 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 ob served, which is nearly three times higher than its homogeneous analogue.