Switchable quaternary ammonium transformation leading to salinity-tolerance-conferring plant biostimulants

Since their discovery by Menshutkin in 1890, quaternary ammonium salts have been synthesized by alkylating tertiary amines, with the development of new synthetic methods having received limited attention. Here we describe a photoredox-catalyzed method for synthesizing quaternary ammonium salts that involves reacting α-haloalkylammonium salts with olefins. This chemistry enables selective and switchable alkylations and alkenylations that afford a variety of structurally new quaternary ammonium salts. The key to success is attributable to the photocatalytic generation of distonic α-ammonium radicals under both oxidative and reductive quenching conditions. Furthermore, during the course of our reaction-development-based chemical-screening campaign, we serendipitously discovered that the synthesized quaternary ammonium salts confer plants with salinity tolerance. The discovery of a novel class of salinity-tolerance-conferring molecule is expected to impact agrochemical development as salinity damage increasingly becomes a global problem.