Million-Fold Acceleration of a Diels−Alder Reaction due to Combined Lewis Acid and Micellar Catalysis in Water

Abstract

The effect of micelles of sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), dodecyl heptaoxyethylene ether (C 12 E 7 ), and copper and zinc didodecyl sulfate (M(DS) 2 ) on the Diels−Alder reaction of 3-(para-substituted phenyl)-1-(2-pyridyl)-2-propen-1-ones 1a − g, containing neutral, cationic, or anionic substituents, with cyclopentadiene ( 2 ) has been studied. In the absence of catalytically active transition-metal ions, micelles invariably retard the reaction. This can be rationalized on the basis of different binding locations of the reaction partners in the micelle. These binding sites have been probed using solubilizate-induced aromatic shifts in the 1 H NMR spectrum of the surfactant and paramagnetic counterion-induced relaxation enhancements of the 1 H NMR signals of the solubilizate. In contrast to SDS, CTAB, and C 12 E 7, Cu(DS) 2 micelles catalyze the Diels−Alder reaction between 1 and 2 with extremely high efficiency, leading to rate enhancements up to 1.8 × 10 6 compared to the uncatalyzed reaction in acetonitrile. This results primarily from the essentially complete complexation of 1 to the copper ions at the micellar surface. Analysis of substituent effects and endo/exo ratios of the Diels−Alder adducts indicates that the reaction experiences a waterlike environment.