Analyzing the Hydricity and Mechanistic Insights of Pyryliums and Xanthyliums as Thermal Deoxygenation Catalysts

While the photocatalytic properties of pyryliums have been explored, their use as thermal catalysts, specifically for the defunctionalizations of sugars and other polyoxygenated molecules, is under explored, with limited literature focused on related acridiniums and benzothiazoliums as thermal catalysts. Preliminary experiments in our lab demonstrate the aptitude of pyrylium and xanthylium catalysts for the deoxygenation of organic molecules, including simple ketals, acetals, and ethers and several complex sugars. This paper examines the Lewis acidity of various xanthyliums and pyryliums via the proxy of hydricity and hydride affinity, the relationship between hydricity and catalytic speed, and the energetics of the heterolytic splitting of silane onto several xanthyliums. Additionally, this paper begins to probe the mechanistic underpinnings of the thermal deoxygenation capabilities of pyryliums and xanthyliums and assess which mechanistic route is the most probable via UV-Vis kinetic analysis and DFT calculations. UV-Vis kinetic data supports the notion that the rate of silane activation is first order in silane and catalyst and zero order in THP, suggesting the substrate may not be necessary for silane activation. Additionally, a DFT calculated reaction coordinate diagram suggests pyryliums and xanthyliums may follow a BCF-like pathway with an SN1-like hydride delivery to the product. Lastly, based on the reaction coordinate diagram, the rate determining step for 9-phenylxanthylium and 9-(3,5-trifluoromethyl)phenylxanthylium is likely the hydride delivery to the products, while both steps may play a role in the rate for 2,4,6-tris[trifluoromethyl)phenyl]pyrylium, given the similarity in activation energy for the two steps.