Mechanistic aspects of aminium salt-catalyzed Diels-Alder reactions: The substrate ionization step

Abstract

Substituent effects in the aminium salt catalyzed Diels-Alder reactions of 2,3-dimetnyl-1,3-butadiene with a series of meta and para substituted β-methylstyrenes are used to probe detailed mechanistic aspects of these reactions. Kinetic studies were carried out using two different aminium salt catalysts and also electrochemically, using anodic potentials corresponding to the oxidation potentials of the aminium salts. Substituent effects in the equilibrium oxidations of the styrene substrates to the corresponding cation radicals were also studied, via oxidation potential measurements. The results indicate rate determining one electron oxidation of the sytrenes to their cation radicals via an outer sphere electron transfer. © 1996 by John Wiley & Sons, Ltd.