Selective modification of carbohydrates using photo-activated nanocatalysts – CARBOPHOTON

With tha aim to develop this new one-pot method, we need to understand the photo-induced mechanisms that govern the reaction. This fundamental study is essential to propose a robust explanation of the reaction mechanism and guide both the design of photocatalysts and the choice of reaction conditions, and is the subject of the first WP. At the same time, the results obtained from different experimental conditions (choice of photocatalyst, concentration of sugar, oxidant, pH, etc.) allow, in addition to optimizing the reaction, to add to the scientific discussion concerning the mechanism and are the subject of the WP2.

On the one hand, the mechanistic study of the photo-oxidation reaction, combining several approaches : experimental (filtration of incident light, study of the influence of different parameters: photocatalyst, solvent, oxidant, etc.), and spectroscopic (spectroscopy of absorption, fluorescence, electronic paramagnetic resonance etc.) allowed to collect data concerning the lifetimes and charge transfers of the excited species and to highlight the complexity of the reaction mechanism. All of this work has enabled us to elucidate the role of each of the species and to propose a reaction mechanism, in accordance with the experimental data of WP2.
Furthermore, due to reproducibility and stability problems, several synthetic methods for the obtaining of photocatalysts (deposition-precipitation; sol-immobilization; photo-reduction) were used and their properties were compared. The sol-immobilization method allowed to obtain stable catalysts in a completely reproducible manner and independently of the support used, but that do not exhibit any photocatalytic property. In parallel, a small library of new materials has been synthesized. None of these catalysts demonstrated greater photocatalytic efficiency than the original photocatalyst, but this provided information concerning the role of the different partners in the reaction mechanism.
Finally, the development of the reaction conditions for the one-pot oxidation / esterification and oxidation / amidation syntheses, initially planned at the end of the project, has been anticipated. Several synthetic strategies were possible and have been explored. It is finally a «one-pot / two-steps« reaction which allowed to obtain the best results: in 1h-3h30 depending on the sugar used, the free ose is converted quantitatively and with a selectivity> 80% in the corresponding 1,4 lactone. The corresponding sugar amide can for its part be obtained with a yield of 94% in less than 4 hours. These conditions were applied to various sugars and very good conversions were obtained for monosaccharides of the hexose and pentose type.

The reaction conditions that allowed to obtain the corresponding lactones / amides were optimized in the absence of light, the photocatalysts developed being, above all, surface catalysts. New photocatalysts are still being synthesized or under study to optimize the properties of the material and allow a photocatalytic approach (taking advantage of the bandgap of semiconductors).
In addition, for the disaccharides, notable differences are observed. A theoretical study is underway to understand this phenomenon. The mechanistic study of the one-pot reaction will also be carried out very soon by EPR, fluorescence spectrometry, etc. to provide data on the lifetimes and the nature of the excited states.

1. Golonu, S.; Pourceau, G.; Quehon, L.; Wadouachi, A.; Sauvage, F. Insight on the Contribution of Plasmons to Gold-Catalyzed Solar-Driven Selective Oxidation of Glucose under Oxygen. Solar RRL, 2020, 4(8), 2000084. onlinelibrary.wiley.com/doi/pdf/10.1002/solr.202000084

Carbohydrates represent a biomass that can be used in a wide range of applicative field from detergents to food or pharmaceuticals industries. Nevertheless, their modification by conventional glycochemistry does often not satisfy the principles of green chemistry. CARBOPHOTON aims at developing a photocatalytic procedure for the selective transformation of free sugars into their corresponding ester or amide derivatives. A preliminary study has allowed us to highlight the possibility of selectively and very rapidly oxidizing a wide variety of mono- and oligosaccharides through the use of photoactivatable nanocatalysts. In this project, both aspects of fundamental and applied research will be studied: (1) by investigating the charge transfer processes that governs the mechanism of this reaction and (2) by using these informations to design the photocatalytic process of one-pot modifications in order to ultimately obtain molecules with high potential to replace petro-sourced substances.