Catalysis of Diels-Alder Cycloaddition Reactions by Ion-Exchanged Montmorillonites
J. M. Adams, K. Martin, R. W. McCabe, 1985. "Catalysis of Diels-Alder Cycloaddition Reactions by Ion-Exchanged Montmorillonites", Proceedings of the International Clay Conference Denver, 1985, Leonard G. Schultz, H. van Olphen, Frederick A. Mumpton
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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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Proceedings of the International Clay Conference Denver, 1985
The papers included in this proceedings volume are representative of the research on clays being conducted in all parts of the world at the time of publication. Many of the subjects treated are controversial, and although some ideas expressed may not necessarily represent the views of the editors, the referees, or the publisher, they deserve to be brought to the attention of the international clay community.