Micellar Catalysis of Diels−Alder Reactions:  Substrate Positioning in the Micelle

We have studied the kinetics of the Diels−Alder reactions of cyclopentadiene, sorbyl alcohol, and sorbyltrimethylammonium bromide with a series of N-substituted maleimides in micellar media. Micellar rate constants have been determined and were found to be 20−40 times lower than the respective aqueous rate constants. Nevertheless, it was found that upon addition of sodium dodecyl sulfate the observed rate constants could be enhanced up to a factor of about 4.5. The low micellar rate constants can be attributed to the relatively apolar (water-poor) region of the micelle, in which the reactions take place. NMR experiments indicate that the reactants usually reside near the α- or β-CH₂ groups of the surfactant molecules in the micelle. Comparison of the micellar rate constants with rate constants in water/1-propanol mixtures suggests a concentration of water of 10−15 M in the micellar region where the diene and dienophile react.