Over the past decade, photoredox catalysis has received a fast and growing interest from the world of synthetic chemistry. By combining visible light with a photocatalyst (PC), a large variety of efficient and selective transformations have been achieved under mild conditions, during which the excited state PC is involved in a single electron transfer (SET) with a substrate or a co-catalyst. However, SET with conventional photocatalyst is usually limited to reduction potential down to -2.0 V vs Saturated Calomel electrode (SCE). In recent years, several elegant reports have shown that open shell doublet radicals, generated in situ either electrochemically or photochemically, can act as potent photoreducing agent. However, recent reports are suggesting that alternative mechanism may be at play, and that most organic radical are not suited to undergo PET. Despite several convincing reports, the synthesis and isolation of a photoactive radical that can shed light on the viability of radical based photocatalysis remains elusive. We have recently reported the synthesis and characterization of such an isolated neutral helicene radical. Herein we report the use of this neutral helicene radical as a photo redox active species with an estimated excited state oxidation potential of -3.31 V vs SCE. The strongly photoreducing nature was experimentally confirmed by the demonstration of well studied photo-dehalogenation, photo-Arbuzov, photo-borylation and C-C bond formation reactions. Our DMQA platform offers a unique opportunity to study the photoactivity of a neutral radical species, thanks to a close shell singlet nPr-DMQA+ active in both red and blue light in junction with the isolable and stable open shell doublet nPr-DMQA radical only active under blue and green light.

Photoredox behavior of a neutral organic radical

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.