# Divergent functionalization of aldehydes photocatalyzed by neutral eosin Y with sulfone reagents

## TL;DR

A new green method uses light and a non-metal catalyst to generate acyl radicals from aldehydes, enabling various chemical transformations.

## Contribution

A metal-free, green photocatalytic method for generating acyl radicals from aldehydes using neutral eosin Y and light.

## Key findings

- Eosin Y photocatalysis enables the generation of acyl radicals from aldehydes under mild conditions.
- The method allows for multiple aldehyde C–H functionalizations, including fluoromethylthiolation and arylthiolation.
- The process is metal-free, redox-neutral, and scalable using continuous-flow technology.

## Abstract

While aldehydes represent a classic class of electrophilic synthons, the corresponding acyl radicals are inherently nucleophilic, which exhibits umpolung reactivity. Generation of acyl radicals typically requires noble metal catalysts or excess oxidants to be added. Herein, we report a convenient and green approach to access acyl radicals, capitalizing on neutral eosin Y-enabled hydrogen atom transfer (HAT) photocatalysis with aldehydes. The generated acyl radicals underwent SOMOphilic substitutions with various functionalized sulfones (X–SO2R’) to deliver value-added acyl products. The merger of eosin Y photocatalysis and sulfone-based SOMOphiles provides a versatile platform for a wide array of aldehydic C–H functionalizations, including fluoromethylthiolation, arylthiolation, alkynylation, alkenylation and azidation. The present protocol features green characteristics, such as being free of metals, harmful oxidants and additives; step-economic; redox-neutral; and amenable to scale-up assisted by continuous-flow technology.

Acyl radicals represent a reactive species that allow for aldehyde subunits to be nucleophilic instead of their typical electrophilic behavior; however, these species are difficult to access in mild conditions. Here the authors show a method to generate acyl radicals using only an organic photocatalyst and light, and these species are shown as competent nucleophiles in a variety of couplings.