At IFEH RAS, researchers developed a photocatalyst for waste-free oxidation of sulfides, paving the way for new pharmaceutical production.

The pressing task facing modern chemistry is the development of new methods for transforming organic substrates, which are crucial for the advancement of organic synthesis. The use of photocatalysts allows for the elimination of chemical oxidizers. This means, first, that oxidation reactions can occur under mild conditions. Second, the reaction products are not contaminated by the decomposition products of oxidizers, which is particularly important in the production of pharmaceuticals and ultra-pure substances.

Oxidation of sulfur-containing compounds (sulfides) to sulfoxides is of significant interest. Sulfoxides are valuable as intermediate products in organic synthesis. They are frequently encountered natural biologically active compounds and are actively used in the early stages of drug synthesis.

A new pyrazinoporphyrin 2H-1 has been synthesized using an original method. By selecting aromatic meso-substituents, it was possible to achieve sufficient solubility of the substance in organic solvents, high reactivity, and chemical stability at all stages of synthesis. The new pyrazinoporphyrin contains an anchor group on the periphery of the macrocycle, which will allow the substance to adhere to the surface of an inert substrate and enable the creation of hybrid materials based on it.

“To successfully develop hybrid materials with photoactive catalysts, it is essential to pre-determine their application areas and potential limitations. This work was dedicated to systematically investigating the photophysical properties of the synthesized pyrazinoporphyrin. We tested its photocatalytic activity on various sulfide substrates,” said one of the authors of the work, leading researcher of the laboratory, Doctor of Chemical Sciences Kirill Birin.

In previous studies, the scientists of the Laboratory demonstrated that new functionalized pyrazinoporphyrins with free bases can completely convert thioanisole and its analogs when irradiated with low-power blue light. This study revealed that the efficiency of photocatalysis depends on the choice of solvent. The solvent can influence both the stability of the photosensitizer and the lifetime of reactive oxygen species, which determines the reaction rate.

The researchers found that for photocatalysis to be successful, methanol must be used as the solvent. Its amount in the mixture with another solvent significantly affects both the conversion (the fraction of the substance that participated in the reaction) and selectivity (the formation of only the target product).

To verify the photostability (the ability to maintain properties under prolonged light exposure) of the catalyst, it was irradiated with blue and red light. Degradation under red light was found to be minimal. Under blue light, discoloration occurred fairly quickly. “Experiments showed that in a carbon tetrachloride solution, after four hours of blue light irradiation, the pyrazinoporphyrin was completely decolorized. The lifetime of singlet oxygen in this solution is quite long. It is likely that the degradation of pyrazinoporphyrin occurred due to prolonged exposure. In methanol, the lifetime of active oxygen is much shorter,” said laboratory researcher, Candidate of Chemical Sciences Daria Polivanovskaya.

In a mixture of methanol and carbon tetrachloride, the catalyst exhibited greater light resistance. The conversion was also very high: with the addition of a small amount of catalyst, it was possible to almost completely convert all sulfides to sulfoxides. The oxidation reaction proceeds selectively, oxidizing the heteroatom without affecting the double bond for sulfides with unsaturated aliphatic substituents or the cyclic fragments of cyclic sulfides. Such selective reactions are of great importance for use in organic synthesis.

“The study showed that pyrazinoporphyrin is a versatile and effective photocatalyst for the oxidation of organic substrates. Due to the presence of anchor groups, this class of compounds could serve as a promising basis for the development of new heterogeneous photocatalysts,” concluded Kirill Birin.

This work was carried out with financial support from the Russian Science Foundation.