Abstract:

Access to carbon-centered radicals under mild conditions (i.e., room temperature in the absence of harsh/toxic initiators) is very desirable, as it opens the door to greener construction of C-X (X = C, N, O etc.) connections, which is a cornerstone of synthetic chemistry. Thiol desulfurization by phosphines yields carbon radicals, and modern photocatalytic strategies allow the reaction to occur under mild conditions. However, all reports of photocatalytic thiol desulfurization utilize homogeneous photocatalysts, which are difficult to separate from reaction products, and the majority of reported catalysts rely on rare metals like iridium. Previously, we reported that visible-light thiol desulfurization by triphenylphosphine (TPP) occurs in the absence of an explicit photocatalyst through the coordination of TPP with triphenylphosphine oxide (TPPO) formed in-situ; although, elevated temperatures are required for an appreciable reaction rate via this strategy. Here, we show TiO₂ as a cheap and easily recyclable heterogenous photocatalyst for visible-light driven thiol desulfurization by TPP under mild conditions. Furthermore, we use UV-Visible spectroscopy (UV-Vis), Ti K-edge X-ray absorption spectroscopy (XAS), P K-edge XAS, and electron paramagnetic resonance spectroscopy (EPR) in conjunction with catalytic controls to build a detailed mechanistic picture of TiO₂’s photocatalytic action. UV-Vis revealed the formation of charge transfer complexes upon adsorption of the desulfurization reagents onto the TiO₂. Also, UV-Vis after excitation of the complexes showed oxygen-vacancy formation on the TiO₂ surface via oxidation of the thiol and TPP by surface lattice oxygen. Ti K-edge XAS probed the changes to local Ti-O bonding at and near the particle surface upon excitation of the complexes, and helped confirm the oxygen vacancy formation. P K-edge XAS showed a unique response to visible-light of TPP adsorbed onto TiO₂ by itself compared to TPP co-adsorbed with TPPO. A UV-Vis difference spectrum suggested interaction between the co-adsorbed TPP and TPPO through the appearance of an absorption at ~530 nm, with catalytic controls utilizing green LEDs supporting this observation. Finally, EPR ties everything together by direct observation of the charge separated intermediates at the TiO₂ surface directly after visible-light excitation, culminating in the construction of a molecular scale catalytic mechanism. With such a detailed picture of this transformation, it should be possible to not only improve upon photocatalytic thiol desulfurization, but develop analogous light-driven synthetic schemes.  

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