A Mechanistic Exploration of Pyryliums and Xanthyliums as Thermal Deoxygenation Catalysts (2024)

Undergraduate: Pascale Hunter

Faculty Advisor: Michel Gagné
Department: Chemistry

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Approximately ninety-six percent of all organic chemical products originate from petroleum. However, there are numerous organic products which could instead be formed by defunctionlizing waste biomass using biorefinery technology, and finding new catalysts to defunctionalize sugars and other over-functionalized biomass can result in faster synthetic pathways both to get to pre-existing—and to generate novel—high value low volume chemicals, many of which are useful for pharmaceutical development. Pyryliums and xanthyliums have shown promise as selective thermal deoxygenation catalysts. Recent experiments in our lab by William Hearne demonstrate their ability to selectively deoxygenate ketals, acetals, ethers and several sugars. Pyryliums and xanthyliums also offer the benefits of being metal free and bench stable, and they require milder conditions than many other catalysts.
This project delves into the initial step in the mechanism for xanthylium and pyrylium catalyzed deoxygenation—hydride transfer between silane and catalyst—to gain a better understanding of the hydride transfer process for these catalysts and investigate how this mechanistic step relates to catalyst reactivity. It was found that xanthylium catalysts tend to form adducts to undergo hydride transfer with silane—a silane-xanthylium adduct with the hydride residing predominately with silane, and a silylium-xanthene adduct with the hydride residing predominately with the catalyst. The secondary silylium-pyran adduct does not seem to form as an intermediate for pyrylium catalysts, although further research is required to confirm this.