Ascorbyl Radicals as Reducing Agents in Copper-Catalyzed Redox Reactions.

Radical-based redox reactions are greatly influenced by their surrounding environment, with the solvent playing a pivotal yet sometimes underestimated role. In this study, we examined how copper catalysts and the choice of solvent impact the reductive power of ascorbyl radicals. Our study used the reduction of 4-nitrophenyl azide as a model and further extended it to other azides and aldehydes. The results reveal a striking difference in radical stability and reductive efficiency, with higher conversions in methanolic solutions compared to acetonitrile. This difference was attributed to the formation and persistence of ascorbyl radicals in methanolic solutions as in acetonitrile; the copper complexes were fully reduced to their copper(I) forms, and the ascorbyl radicals were barely detectable via EPR spectroscopy. Conversely, in methanol, DMPO-trapped ascorbyl radicals persisted for extended periods, indicating that these radicals were the primary reducing agents. Theoretical calculations supported this hypothesis, indicating that these findings suggest that optimizing solvent and copper catalyst selection is crucial for enhancing the reductive power of ascorbyl radicals, with implications for other metal-mediated reductions.