Large low-field magnetic enhancement in photoredox hydrodechlorination of <i>N</i> -heteroarene chlorides restricted by poor cage escape

Abstract The efficiency of photoredox catalysis has long been hampered by rapid back electron transfer (BET) within the radical ion pairs (RIPs). While external magnetic fields can modulate the spin dynamics of RIPs to suppress BET, achieving a large magnetic field effect (MFE) at low-field strengths has remained a formidable challenge. Herein, we report a remarkably large MFE, with a quantum yield enhancement exceeding 130% and a yield enhancement up to 160%, in the photoredox hydrodechlorination of N-heteroaryl chlorides under a weak magnetic field. This enhancement arises from the effective magnetic-field modulation of the hyperfine-coupling-driven spin dynamics within triplet-born RIPs, particularly for systems with poor cage escape. We successfully deconvolve complex reaction pathways and quantitatively reproduce the intrinsic magnetic sensitivity, reconciling experimental observations with theoretical predictions in terms of half-saturation field, by reversely applying a kinetic modeling developed in our previous work. This work provides a new paradigm for photoredox transformation through spin modulation and demonstrates practical synthetic utility at high conversions, offering a powerful strategy for enhancing catalytic efficiency through accessible magnetic fields.