Air-Sensitive Photoredox Catalysis Performed under Aerobic Conditions in Gel Networks

Abstract

In this work, we demonstrate that useful C-C bond-forming photoredox catalysis can be performed in air using easily prepared gel networks as reaction media to give similar results as are obtained under inert atmosphere conditions. These reactions are completely inhibited in homogeneous solution in air. However, the supramolecular fibrillar gel networks confine the reactants and block oxygen diffusion, allowing air-sensitive catalytic activity under ambient conditions. We investigate the mechanism of this remarkable protection, focusing on the boundary effect in the self-assembled supramolecular gels that enhances the rates of productive reactions over diffusion-controlled quenching of excited states. Our observations suggest the occurrence of triplet-sensitized chemical reactions in the gel networks within the compartmentalized solvent pools held between the nanofibers. The combination of enhanced viscosity and added interfaces in supramolecular gel media seems to be a key factor in facilitating the reactions under aerobic conditions.

Metal- and Oxidant-Free Photoinduced Aromatic Trifluoromethylation Performed in Aerated Gel Media: Determining the Effects on Yield and Selectivity

“…It is important to emphasize that the comparisons between reactions in solution and in gel media were performed exactly under the same conditions in terms of reaction volume, time, temperature, light irradiation, and stoichiometry. These results are in very good agreement with our previous observations [16,17], and suggest a stabilizing effect on the reactive intermediates by the surrounding supramolecular network.…”

“…Importantly, due to the air-sensitivity of the excited reactive intermediates, the reaction is carried out in solution under inert atmosphere. Inspired by our previous investigations [16,17], we explored the feasibility of this process in supramolecular gel media, which should facilitate the reaction under aerobic conditions.…”

“…Recently, we have demonstrated the potential of physical gels as confined microenvironments to perform photochemical reactions under conditions milder than in solution [16,17]. Specifically, photoreduction of aryl halides by low-energy light irradiation of a suitable donor/acceptor pair embedded in a physical gel [16], as well as photoinduced C–C cross-coupling reactions (i.e., functionalization of aryl halides, trifluoromethylation of arenes) [17], were successfully carried out under aerobic conditions in spite of the high air-sensitivity of these processes.…”

“…Recently, we have demonstrated the potential of physical gels as confined microenvironments to perform photochemical reactions under conditions milder than in solution [16,17]. Specifically, photoreduction of aryl halides by low-energy light irradiation of a suitable donor/acceptor pair embedded in a physical gel [16], as well as photoinduced C–C cross-coupling reactions (i.e., functionalization of aryl halides, trifluoromethylation of arenes) [17], were successfully carried out under aerobic conditions in spite of the high air-sensitivity of these processes. The results derived from these extensive studies suggest that triplet sensitized chemical reactions could take place either inside the gel nanofibers [16] and/or within the solvents pools compartmentalized in between the fibers [17], where the volume/surface ratio and viscosity changes dramatically compared to homogeneous solution.…”

“…Specifically, photoreduction of aryl halides by low-energy light irradiation of a suitable donor/acceptor pair embedded in a physical gel [16], as well as photoinduced C–C cross-coupling reactions (i.e., functionalization of aryl halides, trifluoromethylation of arenes) [17], were successfully carried out under aerobic conditions in spite of the high air-sensitivity of these processes. The results derived from these extensive studies suggest that triplet sensitized chemical reactions could take place either inside the gel nanofibers [16] and/or within the solvents pools compartmentalized in between the fibers [17], where the volume/surface ratio and viscosity changes dramatically compared to homogeneous solution. The exact location of photocatalytic species in the gel system seems to be influenced by the hydrophobic/hydrophilic balance between the photocatalyst and the surrounded medium (e.g., nanofibers, liquid).…”