Fe(III)/Pyridine N-oxide LMCT Photocatalysis for Unactivated C(sp³)-H Functionalizations

Heteroatom-centered radical-mediated hydrogenatom transfer (HAT) has emerged as a powerful tool for C-H functionalization, yet selective activation of unactivated C(sp³)-H bonds, especially at the primary sites, remains a significant challenge. Here, we report an Fe(III)/pyridine N-oxide (PNO) catalytic system that harnesses ligand-to-metal charge transfer (LMCT) excitation to access highly electrophilic cationic N-oxy radicals from readily available PNOs, enabling diverse HAT-mediated functionalization of unactivated C(sp³)-H bonds. This LMCT system enables the catalytic generation of reactive cationic N-oxy radicals, such as the pentachloropyridine N-oxy radical, which are inaccessible by conventional outer-sphere photoredox catalysis. The synthetic application of the Fe(III)/PNO LMCT system was highlighted by using pentachloropyridine N-oxide as a selective HAT agent for primary C(sp³)-H bonds, affording up to 20:1 regioselectivity in hydrazination of alkanes, aliphatic ketones/esters/nitriles, protected amines/alcohols, and amino acids. Computational and kinetic studies were performed to elucidate the reaction mechanisms, revealing that HAT by cationic N-oxy radicals is highly facile and reversible, effectively leveling the intrinsic energy barrier among various C-H bonds.