Silica Gel-Supported Cation Radical Catalyzed Diels-Alder Reaction

Abstract

The discovery of the cation-radical catalyzed Diels-Alder reaction opened a field of cation radical chemistry. The inefficiency of reactions involving neutral or electron-rich dienophiles imposes a major constraint on the Diels-Alder reaction. This reactivity constraint might be relieved by using cation radical which converts these dienophiles to their corresponding electron-deficient species. Bauld has reported that tris(p-bromophenyl) aminium hexachloroantimonate 1 was efficient for certain Diels-Alder reactions. The successful development of homogeneous catalysts has been often followed by attempts to attach them onto an insoluble polymeric support. The attachment of homogeneous catalysts to an insoluble support gives advantages over traditional solution phase chemistry. The immobilized catalysts can be separated from reaction mixture by simple filtration and then is easily regenerated for reuse. The method would be attractive from an economical viewpoint as well as a simplified work-up. Our interest in the area led us to prepare a silica gel-supported aminium cation radical. Herein, we describe the synthesis of silica gel-supported triphenylamine cation radical 2 and its application in DielsAlder reaction. As shown in Scheme 1, we prepared triphenylamine monomer 6 containing a reactive vinyl group which can be immobilized on a modified silica gel. The Friedel-Crafts reaction of triphenylamine with acetyl chloride provided 3 in 70% yield. Treatment of 3 with NBS gave 4 in 81% yield. Reduction of 4 with lithium aluminum hydride afforded 5 in 91% yield. Attempts to drive the next step to 6 by treatment of 5 with conventional dehydrating reagents such as H2SO4, KHSO4, Al2O3 and POCl3/pyridine failed due to too low yield. In the case of TsCl/t-BuOK, the formation of the