Unravelling the role of silver in addressing the nitrate radical induced photocatalytic oxidation of olefins to epoxides or carbonyl compounds

Nitrate radicals possess the unique capability to add to unsaturated bonds, leading to the formation of carbon-centred radicals evolving to organic compounds, which cannot be achieved through more common reactive oxygen centred species. Therefore, the possibility of generating nitrate radicals through heterogeneous photocatalysis endows this technology with unprecedented possibilities in the field of green organic synthesis. In particular, previous research showcased the promising potential of nitrate radicals in selectively conducting the photocatalytic oxidative cleavage of limonene to limononaldehyde. However, the role of silver ions, required for the reaction to occur, remained uncertain. This study aims at elucidating the role of silver in determining conversion of limonene and selectivity towards the corresponding oxidized compounds. In particular, it has been demonstrated that silver ions efficiently scavenge photogenerated electrons, thus making molecular oxygen available for the nitrate radical induced production of limononaldehyde, obtained with selectivity values up to 60%. In this study it is shown that, nitrate radical formation is as well boosted by silver nanoparticles, in the absence of silver ions. However, in this case, the consequent oxidation of limonene produced both 1,2-limonene epoxide and limononaldehyde. Notably, in the absence of silver ions, molecular oxygen is the main electron scavenger and its lower availability for the oxidative cleavage mechanism addresses the reaction towards the epoxide. Ab initio calculations, confirmed this hypothesis, showing that, while formation of limononaldehyde is thermodynamically and kinetically favoured, formation of the epoxide is kinetically limited. Therefore, its formation takes place when the formation of limononaldehyde is hindered by factors such as the reduced availability of molecular oxygen. The identification of key parameters governing the process offers possibilities to tune the selectivity of the reaction towards oxidative cleavage or epoxidation. Moreover, it allows to further optimize this reaction, which may become an appealing and versatile method of chemicals green synthesis.