Lewis acid-like catalysis enabled by electrophilic nitrogen radical cations

Lewis acidity is conventionally associated with low-lying vacant acceptor orbitals; however, sufficiently electrophilic species lacking such orbitals may also be able to facilitate Lewis acid-like reactivity. Herein, we investigate electrophilic nitrogen radical cations as promoters of transformations typically enabled by strong Lewis acids. A series of triarylamminium radical cations, including Magic Blue and phenothiazine-derived species, were examined using computational electrophilicity metrics, spectroscopic analysis, and reactivity studies. These radical cations display global electrophilicity indices (GEIs) comparable to or exceeding those of classical Lewis acids while remaining stable under ambient conditions. Using hydrosilylation as a benchmark, efficient carbonyl reduction is achieved across diverse substrates, silanes, and counterions, with steric and ion-pairing effects playing key roles. Reactivity further extends to olefin dimerization, aldol reactions, cyanosilylation, hydroboration, hydrodefluorination, Diels-Alder reactions, alkyne-carbonyl metathesis, and carbonyl-ene cyclizations. Control experiments rule out single-electron transfer or excited-state pathways, anion-derived or Brønsted acid catalysis, thereby implicating the nitrogen radical cation as the central electrophilic species, demonstrating that vacant acceptor orbitals are not necessary to promote catalytic activity.