CONTRIBUTION OF DIFFERENT MECHANISMS AND DIFFERENT ACTIVE-SITES TO THE CLAY-CATALYZED DIELS-ALDER REACTIONS

Cation-exchanged K10 montmorillonites display a high Lewis/Bronsted ac idity ratio, and efficiently promote the reaction of (-)-menthyl acryl ate with cyclopentadiene. The clays silylated with trimethylchlorosila ne are worse catalysts, due to the decrease in the number of acid site s. This effect is particularly important when the silylation is carrie d out under argon atmosphere. On the other hand, rehydration of the si lylated clay gives rise to the appearance of Bronsted acid sites, able to promote the reaction. The elimination of Lewis acidity by silylati on shows the role of the hydroxyl groups of the clay. The use of aniso le as a solvent improves the catalytic activity of the silylated clays , but the endo/exo and the diastereofacial selectivities are lower, wh ich is due to a radical cation mechanism. Given that the Zn(II)-exchan ged clay also shows this effect, the structural Fe(III) of the clay mu st contribute to this radical cation mechanism. The results obtained s how how different treatments modify the properties of the clays.