# Photochemical reduction of aryl chlorides, bromides, and iodides via ternary EDA complexes with guanidine bases

## TL;DR

A simple guanidine base called TBD can act as a powerful photoreductant, enabling the reduction of various aryl halides under visible light.

## Contribution

The discovery that guanidine bases like TBD can function as photoactive reductants through ternary EDA complexes is novel.

## Key findings

- TBD promotes the reduction of aryl iodides, bromides, and unactivated aryl chlorides under visible light.
- A ternary EDA complex formed between TBD and aryl halides enables electron transfer and generates aryl radicals.
- The method supports diverse reactivity pathways like hydrodehalogenation, borylation, and radical cyclization.

## Abstract

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.

Commercially available TBD was found to be an effective photoreductant for a variety of aryl halides. The reactivity is proposed to proceed via formation of a ternary EDA complex composed of two TBD molecules and one aryl halide.