Oxoammonium-Catalyzed Ether Oxidation via Hydride Abstraction: Methodology Development and Mechanistic Investigation Using Paramagnetic Relaxation Enhancement NMR

Hydride abstraction represents a promising yet underexplored approach in the functionalization of C-H bonds. In this work, we report the oxidation of α-C-H bonds of ethers via oxoammonium catalysis using 3-chloroperbenzoic acid (<i>m</i>CPBA) as the terminal chemical oxidant or by means of electrochemistry. Mechanistic studies revealed intricate equilibria and interconversion events between various catalytic intermediates in the presence of <i>m</i>CPBA, which alone however was incompetent to drive catalytic turnover. The addition of a small amount of strong acid HNTf₂ or weakly coordinating salt NaSbF₆ turned on catalytic turnover and promoted ether oxidation with excellent efficiency. NMR experiments leveraging paramagnetic relaxation enhancement effect allowed for quantification of open-shell catalytic intermediates in real time during the reaction course, which aided the identification of catalyst resting states and elucidation of reaction mechanisms.