Photochemical reduction of aryl chlorides, bromides, and iodides

Photoredox catalysis has traditionally required sophisticated catalyst design, multi-photon systems, or photoelectrochemical strategies to reach the reducing potentials necessary for aryl chloride activation. In a surprising departure from these paradigms, we find that a simple, bench-stable guanidine base, TBD, functions as a powerful photoreductant under visible light, promoting the reduction of aryl iodides, bromides, and even unactivated aryl chlorides-as well as Birch-type dearomatization of polyarenes. Mechanistic interrogation reveals an unusual ternary EDA complex formed between a TBD dimer and the aryl halide, which upon excitation engages in an electron transfer, generating aryl radicals. These intermediates enable hydrodehalogenation, borylation, and radical cyclization pathways, demonstrating broad downstream reactivity. This discovery establishes guanidine bases as a new class of photoactive reductants and highlights aggregation-driven EDA activation as a powerful and underexplored strategy for highly reducing photochemical processes.