Characterization of cation radical reactions. Aminium salt-catalysed Diels–Alder reactions

Abstract

Kinetic studies of the Diels–Alder cycloadditions of a series of trans-stilbenes (2) to 2,3-dimethylbuta-1,3-diene (3), catalysed by tris(4-bromophenyl)aminium hexachloroantimonate (1˙⁺), reveal a curved Hammett–Brown plot, consistent with a mechanistic transition from reversible to rate determining ionization of 2 to 2˙⁺ as the substituents become progressively more electron donating. Further, substituent effects in mono- and di-substituted stilbenes are essentially multiplicative, describing a distribution of positive charge which is symmetrical with respect to both aryl rings. The symmetry of the charge development is inconsistent with an electrophilic addition to the substrates, proceeding via a carbocation. The combination of the magnitude and symmetry of the positive charge distribution provides strong evidence for a cation radical mechanism.