CHIRAL POLYSILOXANE-FIXED METAL 1,3-DIKETONATES (CHIRASIL-METALS) AS CATALYTIC LEWIS-ACIDS FOR A HETERO-DIELS-ALDER REACTION - INVERSION OFENANTIOSELECTIVITY UPON CATALYST-POLYMER BINDING

A dimethylpolysiloxane chain is covalently bound to the monomeric Lewi s acid catalyst (1R)-(+)-oxovanadium(IV) bis[3-heptafluorobutanoylcamp horate] [(+)-5a] at the C-10 position of the camphor moiety yielding t he novel chiral polymeric (1S)-(+)-oxovanadium(IV) bis[3-heptanuorobut anoylcamphorate] -dimethylpolysiloxane [(+)-6a]([1]). The Lewis acid p romoted hetero Diels-Alder reaction of rans-1-methoxy-3-(trimethylsily loxy)-1,3-butadiene (1) and benzaldehyde (2) is studied in the presenc e of the enantiomerically pure monomeric (5a, 5b) and polymeric (6a, 6 b) catalysts. The monomeric and polymeric catalysts of identical chira lity([1]) invoke opposite enantioselectivities toward the formation of 2-phenyl-2,3-dihydro-4H-pyrone (4). This remarkable result is corrobo rated by numerous control experiments. Thus, it is demonstrated that t he environment of the active site of the catalyst is changed when the complex is fixed to a polymeric chain. In contrast to 5 and 6, the mon omeric 12 and polymeric 13 europium(III) tris[3-heptafluorobutanoylcam phorates] of identical chirality invoke the same enantioselectivity to ward the formation of 4. The concept of polymer attachment is utilized to recover the catalyst from the reaction mixture by precipitation an d to recycle it for subsequent catalytic runs.