title: Solid-phase synthesis of homogeneous ruthenium catalysts on silica for the continuous asymmetric transfer hydrogenation reaction.
abstract: 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 of 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.
authors.affiliation: Institute of Molecular Chemistry, Amsterdam, The Netherlands.
authors.forename: A J
authors.initials: AJ
authors.lastname: Sandee
authors.forename: D G
authors.initials: DG
authors.lastname: Petra
authors.forename: J N
authors.initials: JN
authors.lastname: Reek
authors.forename: P C
authors.initials: PC
authors.lastname: Kamer
authors.forename: P W
authors.initials: PW
authors.lastname: van Leeuwen