Photocatalytic carboxylation of C-H bonds promoted by popped graphene oxide (PGO) either bare or loaded with CuO

Popped graphene oxide (PGO), either bare or loaded with CuO (or Cu2O), has been characterized for its photoelectrochemical properties and used for the first time as photocatalyst for CO2 insertion into C-H bonds, a challenging reaction of great industrial interest. PGO joins, thus, the restricted number of modified polymeric C-species (fullerenes and carbon nanotubes-CNT) having photochemical activity. Using acetylacetone as test molecule and bare PGO, two isomeric carboxylic acids were formed with ca. 3.5% yield under white (solar) light irradiation that was enhanced to over 10% after deposition of CuO on PGO. Under the reaction conditions, CuO alone gave only minor traces of the carboxylated products. Therefore, the two components together produce a synergistic effect that reinforces the photocatalytic activity of each of them. Interestingly, both PGO and CuO@PGO do not have the correct potential for the generation of the radical anion CO2 center dot-, therefore the reaction mechanism of formation of the carboxylated products cannot imply the formation of C-C bond via radical coupling. Most likely, the one-hole oxidation of acetylacetone to the relevant radical is the starter of the carboxylation process that implies the transfer of two electrons to CO2. The possible implicacy of HCO2H in the carboxylation has been demonstrated by the presence of such compound in the photocatalytic reaction mixture under CO2 and by the direct carboxylation of acac with formic acid under dinitrogen in the same operative conditions as with CO2.