Comparison of AlEt2Cl and ZnCl2 supported on silica gel as catalysts of Diels-Alder reactions. Influence of the nature of the dienophile

Non-modified silica gel, silica gel treated with AlEt2Cl, and ZnCl2 supported on silica gel are tested as catalysts in the Diels-Alder reactions of cyclopentadiene (1) with methyl acrylate (2a), (-) -menthyl acrylate (2b) and acrylonitrile (2c). In general, supported catalysts are more efficient than non-modified silica gel. Two different kinds of silica gel are tested and the amount of aluminium incorporated when they are treated with AlEt2Cl changes from one to another, which influences the chemical yield but not the selectivity of the reactions. In general, aluminium catalysts are more efficient than ZnCl2-supported ones in the reactions of acrylates, but the latter solid is a better catalyst in the reactions of acrylonitrile (2c). Differences in acid strength and hardness of the catalytic sites are invoked to account for these results.