Site‐selective Photoredox‐Catalyzed Late‐stage Benzylic Hydrogen Isotope Exchange

A highly regioselective visible light photoredox‐catalyzed hydrogen isotope exchange (HIE) of benzylic positions in both simple and complex molecules is reported. The process follows a dual catalytic approach using an acridinium photocatalyst in combination with a thiol‐based hydrogen atom transfer catalyst, while the use of D2O as an isotope source ensures operational simplicity and cost‐effectiveness. High reactivity has been achieved for electron‐rich benzylic positions. Moreover, targeted radical formation enables unprecedented selective HIE on intramolecular competing benzylic and alpha to heteroatom positions with moderate to excellent deuterium incorporation. The utility of the reaction was demonstrated on the late‐stage HIE of several natural compounds and drug derivatives. Experimental studies and density functional theory (DFT) calculations suggested a single electron transfer (SET) mechanism followed by deprotonation to generate the benzylic radical, and revealed the importance of halogenated solvents or additives. Upon a weak complexation of the halogenated species to the substrate, an oxidation potential lowering effect is induced, as well as a stabilization of the radical‐cation species through spin delocalization.