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Catalysis Letters, Vol.91, No.3-4, 145-148, 2003
Heterogeneous enantioselective synthesis of a dihydropyran using Cu-exchanged microporous and mesoporous materials modified by bis(oxazoline)
The formation of dihydropyran from the Diels - Alder reaction between E-ethyl-2-oxo-3-pentenonate and vinyl ethyl ether is investigated using copper (II) bis(oxazoline) as catalyst. The homogeneously and heterogeneously catalyzed reactions are contrasted. Immobilization using mesoporous materials (Cu-MCM-41, Cu-AlSBA-15, Cu-MSU-2) and zeolite Y is found to produce an effective heterogeneous catalyst. Although the level of enantioselection is not high in this initial study, the CuH-zeolite Y/bis(oxazoline) catalyst gives the highest ee (41% ee), which is significantly higher than that observed for the Cu(OTf)(2) homogeneous catalyst (20% ee) under comparable conditions. In addition, with the heterogeneously catalyzed reaction, the enantioselection changes from the initial 2R, 4S product to the 2S, 4R diastereoisomer. This behavior is not observed with the homogeneously catalyzed reaction, which always yields the 2R, 4S product. These results are discussed in terms of the confinement of the catalyst complex within the pores of the heterogeneous catalyst.