Halogen-Substitution of Benzotriazolium Cations Unlocks Enhanced Lewis Acidity

Abstract

Nitrenium cations have seen a surge of interest for catalytic applications as they are inexpensive, robust, and have high tunability. Thus far, this tunability has focused on modification of the wingtip, N-bound substituents or the employment of alternative aryl scaffolds. Here, we report the systematic modification of the benzotriazolium cation by installing a variety of halogen substitution patterns to the aryl backbone. Computational investigations suggested that these modifications would increase the Lewis acidity of the nitrenium center, which was supported experimentally by the Gutmann–Beckett test. The series of novel cations were employed as Lewis acid catalysts in a model reaction, and the most Lewis acidic derivative exhibited significantly enhanced catalytic performance relative to the parent, unsubstituted benzotriazolium cation. We further examined the relative stabilities of the neutral radical forms of these derivatives through a combined experimental and computational approach.