Chiral Polysiloxane-Fixed Metal 1,3-Diketonates (Chirasil-Metals) as Catalytic Lewis Acids for a Hetero Diels-Alder Reaction -Inversion of Enantioselectivity upon Catalyst-Polymer Binding

Article abstract of DOI:10.1002/cber.19971300711

A dimethylpolysiloxane chain is covalently bound to the monomeric Lewis acid catalyst (lR)-(+)-oxovanadium(IV) bis[3heptafluorobutanoylcamphorate] [(+)-5a] at the C-10 position of the camphor moiety yielding the novel chiral polymeric (1S)-(+)-oxovanadium(IV) bis[3-heptafluorobutanoylcamphoratej-dimethylpolysiloxane [(+)-6a]^[1]. The Lewis acid promoted hetero Diels-Alder reaction of //ans-l-methoxy-3(trimethylsilyloxy)-l,3-butadiene (1) and benzaldehyde (2) is studied in the presence of the enantiomerically pure monomeric (5a, 5b) and polymeric (6a, 6b) catalysts. The monomeric and polymeric catalysts of identical chirality'1' invoke opposite enantioselectivities toward the formation of 2-phenyl2,3-dihydro-4H-pyrone (4). This remarkable result is corroborated by numerous control experiments. Thus, it is demonstrated that the 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 monomeric 12 and polymeric 13 europium(HI) tris[3-heptafluorobutanoylcamphorates] of identical chirality invoke the same enantioselectivity toward the formation of 4. The concept of polymer attachment is utilized to recover the catalyst from the reaction mixture by precipitation and to recycle it for subsequent catalytic runs. VCH Verlagsgcscllschaft mbH,.