Direct activation of strong bonds in readily available, benchtop substrates offer a straightforward simplification, albeit in most cases existing catalytic systems are limited to unlock such activation. Photoexcitation of neutral organic radical manifests a powerful manifold for previously challenging organic transformations. In recent years, a surge of in-situ generated organic radicals that can act as potent photoinduced electron transfer (PET) agents have appeared as potential gateway for the activation of remarkably stable bonds. Indeed, direct photoarylation reaction offers simplified, efficient and elegant transformations while it remains significantly underexplored. In addition to that, the mechanism of such radical photocatalytic systems remain not fully understood. In this context, we have documented the use of N,N′-di-n-propyl-1,13-dimethoxyquinacridine (nPr-DMQA radical), an isolated and stable neutral helicene radical, as a highly photoreducing species. The strongly photoreducing nature of the nPr-DMQA radical was experimentally confirmed by the demonstration of photo activation of electron rich aryl bromides and chlorides. Dehalogenation, photo-Arbuzov, photo-borylation and C-C bond formation reactions with aryl chlorides and bromides are reported herein, as well as the α-arylation of carbonyl using cyclic ketones. The latter transformation exhibits the facile synthesis of α-arylated cyclic ketones as critical feedstock chemical for diverse useful molecules, especially in the biomedical enterprises. Our ability to isolate the doublet state open shell nPr-DMQA radical offers a unique opportunity to shed light on the mechanism behind PET reactions of organic radicals. Preliminary mechanistic study revealed that several reaction pathways can be at play, including a consecutive photoexcitation mechanism, halogen and hydrogen abstraction mechanisms, as well as the involvement of photoactive singlet state neutral intermediates.

Isolated neutral [4]helicene radical highlights the complexity of organic radical based photocatalysis

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ACS Fall 2022

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