Visible-Light Driven Reduction of C-X Bonds

Aryl halides are fundamental and extremely useful building blocks in synthesis. Recent advances in photoredox catalysis have proposed solutions to promote the reduction of very recalcitrant substrates, including fluoro- and chloro-arenes. Many popular photocatalysts are cationic species, and, consequently, the reduction potential of the corresponding excited species hit a ceiling at around -2 V. However, it has been recently shown that suitable photoexcited anions could push this limit, enabling one to access reduction potential up to ca – 3 V vs SCE. Two recent examples from the literature involve indeed the activation of an indole-thiolate anion1 and of a benzo[b]phenothiazine anion.2 We wondered whether a new, more potent, super reductant could have been accessed by triggering the activation of a di-anionic species. We report that the di-anion of diphenyl acetic acid can be activated by blue light and the corresponding photoexcited species can trigger single electron transfer (SET) events. The process can lead to the activation of electron-rich aryl chlorides and fluorides. The resulting radical anion can then collapse, affording a reactive aryl radical that is smoothly reduced by THF, which is used as solvent (Figure 1). This method is practical and convenient because the super reductant species is generated from the DPA, which is a commercial, affordable and easily available molecule (DPA = 0.21 $/mmol). The strategy can be applied to several products, including commercial drugs and natural compounds, showing ample functional group tolerance. Moreover, electron rich fluorides that are usually poor candidates for existing methods can be readily reduced, too.