New Reactivity of Amine Radical Cations and Their Related Species

Nitrogen-centered radicals, including amine radical cations, provide unique and exceptional reactivity that distinguishes them from closed-shell nitrogen species. They enable many highly synthetically valuable transformations, such as addition to π bonds and hydrogen atom abstraction, which are difficult to achieve by closed-shell nitrogen species. Despite their vast potential, they have remained underutilized until recently. However, the rapid advancement in photoredox catalysis has fueled a rise of the number of methods enabled by these species. For nitrogen-centered radicals, the impact of photoredox catalysis is profound and multifold. The versatility in quenching the excited photocatalyst via reductive or oxidative PET or PCET allows structurally diverse nitrogen-containing compounds to serve as precursors. Moreover, visible light photoredox catalysis unleashes some unique reactivity, such as performing redox-neutral reactions and anti-Markovnikov addition to π bonds that would otherwise be challenging. Finally, the barrier to merge visible light photocatalysis with another type of catalysis is significantly lower than dual catalysis using traditional radical methods, which allows a synergistic catalysis to maximize nitrogen-centered radicals’ utility. In this chapter, three types of reactions, namely, formal [3+2] cycloadditions, hydrogen atom abstraction, and hydroamination/hydroaminoalkylation, will be discussed in detail.