Contribution of different mechanisms and different active sites to the clay-catalyzed Diels–Alder reactions
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文摘
Cation-exchanged K10 montmorillonites display a high Lewis/Brønsted acidity ratio, and efficiently promote the reaction of (−)-menthyl acrylate with cyclopentadiene. The clays silylated with trimethylchlorosilane are worse catalysts, due to the decrease in the number of acid sites. This effect is particularly important when the silylation is carried out under argon atmosphere. On the other hand, rehydration of the silylated clay gives rise to the appearance of Brønsted acid sites, able to promote the reaction. The elimination of Lewis acidity by silylation shows the role of the hydroxyl groups of the clay. The use of anisole as a solvent improves the catalytic activity of the silylated clays, but the endo/exo and the diastereofacial selectivities are lower, which is due to a radical cation mechanism. Given that the Zn(II)-exchanged clay also shows this effect, the structural Fe(III) of the clay must contribute to this radical cation mechanism. The results obtained show how different treatments modify the properties of the clays.