Cobalt‐Mediated Photochemical C−H Arylation of Pyrroles

Abstract

Precious metal complexes remain ubiquitous in photoredox catalysis (PRC) despite concerted efforts to find more earth‐abundant catalysts and replacements based on 3d metals in particular. Most otherwise plausible 3d metal complexes are assumed to be unsuitable due to short‐lived excited states, which has led researchers to prioritize the pursuit of longer excited‐state lifetimes through careful molecular design. However, we report herein that the C−H arylation of pyrroles and related substrates (which are benchmark reactions for assessing the efficacy of photoredox catalysts) can be achieved using a simple and readily accessible octahedral bis(diiminopyridine) cobalt complex, [1‑Co](PF6)2. Notably, [1‑Co]2+ efficiently functionalizes both chloro‐ and bromoarene substrates despite the short excited‐state lifetime of the key photoexcited intermediate *[1‑Co]2+ (8 ps). We present herein the scope of this C−H arylation protocol and provide mechanistic insights derived from detailed spectroscopic and computational studies. These indicate that, despite its transient existence, reduction of *[1‑Co]2+ is facilitated via pre‐assembly with the NEt3 reductant, highlighting an alternative strategy for the future development of 3d metal‐catalyzed PRC.