Acridine radical acts as a powerful photoreductant

Reduction, the process of pushing electrons into molecules, is an important transformation for chemical synthesis. But it often requires harsh reagents, like alkali metals, or transformations that can be difficult to scale up, such as reactions that require dissolving metals. Chemists led by the University of North Carolina at Chapel Hill’s David A. Nicewicz have now identified a gentler reagent—an acridine radical (shown), catalytically generated in situ—that can do many of the same reactions when used with ultraviolet light. Nicewicz’s team discovered the photoreductant while working with acridinium salts as photooxidation catalysts. During that work, they found that the acridine radical was indefinitely stable under oxygen-free conditions and had a reduction potential on par with that of elemental lithium. This, the chemists note, makes the acridine radical “one of the most potent chemical reductants reported” (Nature2020, DOI: 10.1038/s41586-020-2131-1). Nicewicz and colleagues used the acridine radical to reduce N-tosyl species, organic bromides, and organic chlorides that can resist reductions. Although there have been a few examples of neutral radicals behaving as compounds that transfer electrons when excited by light, these photoinduced electron-transfer agents are typically transition-metal complexes or closed-shell organic molecules.