Journal of Photocatalysis - Current Issue

From an industries and academic perspective, there is a need for a method for producing 3-nitro-4-aryl-2H-chromen-2-ones from aryl alkynoate esters that is both economic and environmental benign. In this context, superoxide ion-assisted radical cascade reaction can be an efficient and greener protocol.

Herein, we have demonstrated an unprecedented methylene blue (MB) visible light photocatalysis for the production of a series of 3-nitro-4-aryl-2H-chromen-2-ones from readily available aryl alkynoate esters and a nitrating agent in solution.

Synthesis of 3-nitro-4-aryl-2H-chromen-2-ones has been performed in the presence of aryl alkynoate ester, TBAN, DIPEA, solvent, catalyst and molecular oxygen under visible light irradiation at room temperature. The products were purified by column chromatography using silica gel, and the mixture of ethyl acetate/petroleum ether as an eluting solvent and characterized by IR, NMR and mass spectroscopic analysis.

A series of aryl alkynoate esters were successfully nitrated into corresponding 3-nitro-4-aryl-2H-chromen-2-ones with good isolated yields by this protocol, in which the key NO2-radicals formed by the action of superoxide ion (O2−∙).

In contrast to the literature-reported methods of synthesis of 3-nitro-4-aryl-2H-chromen-2-ones, the process described here for making 3-nitro-4-aryl-2H-chromen-2-ones uses methylene blue visible light photocatalysis, is inexpensive, mild, does not require a metal precursor or high temperatures, and is successful when using the direct sunlight.

Photocatalysis is of particular importance in the oxidation of alcohols to aldehydes to increase the conversion of benzyl alcohol oxidation to benzaldehyde at high selectivity, which could be useful for the pharmaceutical and perfumery industries.

The oxidation of benzyl alcohol over P25 was investigated in various solvents (water, benzotrifluoride, toluene and acetonitrile).

The reaction was performed in an isothermal slurry batch reactor in the presence and absence of UV-light. The products were analysed using GC-FID; the deposits formed on the catalyst was analysed using TGA and FTIR.

In the presence of light, the reaction was very selective for the formation of benzaldehyde (e.g., 99% selectivity at 53% conversion using acetonitrile as a solvent), whereas, in the absence of light, the formation of higher molecular weight products was observed (e.g., 22% selectivity at 1.7% conversion using acetonitrile as a solvent). It was observed that the activity in the absence of oxygen was initially high, but it dropped rapidly from initially 0.4 to 0 mmol g^-1 h^-1 after 2-4 h (using acetonitrile as a solvent). This was attributed to the activity of the few oxidized sites present on P25.

Acetonitrile appears to be the most effective solvent, as it seems to interact least with the catalytically active sites. The photocatalytic oxidation of benzyl alcohol over P25 does not only yield products in the solution, but also deposits on the surface. The deposits can be removed in an oxidative environment or an inert environment.