Re‐Examination of Self‐Decay Chemistry of Phthalimide‐N‐oxyl Redox‐Organocatalyst for Free‐Radical CH‐Functionalization – Puzzle Begins to Come Together

Abstract

Phthalimide‐N‐oxyl radicals (PINO) are catalytically active hydrogen abstracting species generated from N‐hydroxyphthalimide (NHPI) — one of the most efficient and widely used organocatalysts for free‐radical oxidative CH‐functionalization processes. The self‐decay of PINO is one of the main limiting factors of NHPI usage, but currently there is no consensus on the mechanism of this important process. In the present work, quantitative EPR and NMR monitoring of PINO generation and decay along with decay product analysis and control experiments allowed us to build a consistent picture of PINO decay chemistry and put together previously contradictory results. PINO yields achievable employing various oxidants were measured and compared for the first time. At least two PINO decay pathways were revealed: “trimerization” (with partial fragmentation, favored by high PINO concentrations), and medium oxidation favored by low PINO concentrations. The main PINO decomposition product, the “trimer” can easily be recycled back to NHPI.