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

<table border="0" width="100%" cellspacing="0" cellpadding="5" bgcolor="#FFFFFF" data-ogsb="" data-ogab="#FFFFFF"> <tbody> <tr bgcolor="#FFFFFF" data-ogsb="" data-ogab="#FFFFFF"> <td><span>While the photocatalytic properties&nbsp;<span class="markk0ovprtt6" data-markjs="true" data-ogac="" data-ogab="" data-ogsc="" data-ogsb="">of</span>&nbsp;pyryliums have been explored, their use as thermal catalysts, specifically for the defunctionalizations&nbsp;<span class="markk0ovprtt6" data-markjs="true" data-ogac="" data-ogab="" data-ogsc="" data-ogsb="">of</span>&nbsp;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&nbsp;<span class="markk0ovprtt6" data-markjs="true" data-ogac="" data-ogab="" data-ogsc="" data-ogsb="">of</span>&nbsp;pyrylium and xanthylium catalysts for the deoxygenation&nbsp;<span class="markk0ovprtt6" data-markjs="true" data-ogac="" data-ogab="" data-ogsc="" data-ogsb="">of</span> organic molecules, including simple ketals, acetals, and ethers and several complex sugars. This paper examines the Lewis acidity <span class="markk0ovprtt6" data-markjs="true" data-ogac="" data-ogab="" data-ogsc="" data-ogsb="">of</span>&nbsp;various xanthyliums and pyryliums via the proxy&nbsp;<span class="markk0ovprtt6" data-markjs="true" data-ogac="" data-ogab="" data-ogsc="" data-ogsb="">of</span>&nbsp;hydricity and hydride affinity, the relationship between hydricity and catalytic speed, and the energetics&nbsp;<span class="markk0ovprtt6" data-markjs="true" data-ogac="" data-ogab="" data-ogsc="" data-ogsb="">of</span>&nbsp;the heterolytic splitting&nbsp;<span class="markk0ovprtt6" data-markjs="true" data-ogac="" data-ogab="" data-ogsc="" data-ogsb="">of</span> silane onto several xanthyliums. Additionally, this paper begins to probe the mechanistic underpinnings <span class="markk0ovprtt6" data-markjs="true" data-ogac="" data-ogab="" data-ogsc="" data-ogsb="">of</span>&nbsp;the thermal deoxygenation capabilities&nbsp;<span class="markk0ovprtt6" data-markjs="true" data-ogac="" data-ogab="" data-ogsc="" data-ogsb="">of</span>&nbsp;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&nbsp;<span class="markk0ovprtt6" data-markjs="true" data-ogac="" data-ogab="" data-ogsc="" data-ogsb="">of</span>&nbsp;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.</span></td> </tr> </tbody> </table>