Comparison of Lewis acids of different hardness supported on silica gel as catalysts of Diels-Alder reactions of (E)-2-cyanocinnamates

The behaviour of several Lewis acids, derived from zinc, aluminium and titanium, supported on silica gel, as catalysts of the Diels-Alder reactions of cyclopentadiene with methyl, (1R,2S,5R)-menthyl, and (R)-pantolactone (E)-2-cyanocinnamates has been compared. The best catalytic activities were observed with the zinc catalysts. The extent and even the direction of the asymmetric induction changes for the same chiral auxiliary depending on the catalyst used. Ab initio theoretical calculations, carried out on model dienophile-catalyst intermediates, show that the coordination of the softer zinc derivatives at the nitrogen atom is thermodynamically favoured, whereas coordination to the carbonyl oxygen atom is preferred for the harder aluminium catalysts. However, in both cases the most reactive intermediate comes from the coordination of the Lewis acid to the carbonyl group of the dienophile in the s-trans conformation.This work was made possible by the generous financial support of the Comision Interministerial de Ciencia y Technologia (project MAT96-1053-C02-01). EP thanks the Diputacion General de Aragon for a grant, and TT thanks the Ministerio de Educacion y Cultura for a post-doctoral fellowship.Peer reviewe