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A study was made of the ability of cation-exchanged montmorillonites to catalyze some typical Diels-Alder cyclization reactions in organic solvents. Although Cr3+- and Fe3+-clays were effective catalysts, samples exchanged with nontransition metals, such as Na+, Mg2+, and Al3+, were not. The differences between the activities, for example, of Fe3+- and Al3+-montmorillonites suggest that the clays did not act as Lewis acid Diels-Alder catalysts nor as proton donors, but rather that a one-electron transfer was involved, leading to a radical cation-catalyzed process. The Cr3+- and Fe3+-clay samples were of comparable activity to the Fe3+-exchanged, acid-treated montmorillonite (K10, Süd-Chemie) suggested recently as a useful Diels-Alder catalyst. Under comparable conditions, for example, Fe3+-K10, Fe3+-, and Cr3+-montmorillonites all produced >90% reaction between cyclopentadiene and methyl vinyl ketone in 20 min at 22°C in dichloromethane solvent. The clay-based catalysts gave rapid and stereoselective reactions and were easy to use. The type of activity displayed in these reactions emphasizes that ion-exchanged montmorillonites are not merely solid sources of protons, but can with suitable pretreatment be effective catalysts in a range of organic syntheses.

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