Optimization of WO ₃ Crystal Facet for Photoelectrochemical Oxidation of Trifluoromethylation

Direct oxidative C─H trifluoromethylation utilizing inexpensive trifluoroacetic acid (TFA) is a highly desirable but thermodynamically challenging transformation, primarily due to the exceptionally high oxidation potential of the trifluoroacetate anion (CF ₃ COO ⁻ ). In this work, we report a photoelectrocatalytic (PEC) strategy that leverages rational crystal facet engineering of tungsten trioxide (WO ₃ ) photoanodes to overcome this kinetic bottleneck. By systematically modulating the growth conditions, we demonstrate that increasing the proportion of the (200) facet in the WO ₃ photoanode enhances photoactivity, facilitating the conversion of electron‐rich arene into its mono‐trifluoromethylated derivative, with a maximum yield of 80%. To better understand this observation, we analyzed the reaction progression by examining the active surface on the photoanode, as well as the reactant, intermediate, and product. Our studies reveal that the activation and oxidation of trifluoroacetate anion (CF ₃ COO ^– ) represent key barriers in this transformation. The (200) facets of WO ₃ appear to favor CF ₃ COO ^– adsorption, thereby facilitating its conversion. This study demonstrates the application of a PEC system for C─H trifluoromethylation and provides valuable insights into the relationship between crystal facets and the performance of organic transformations. It offers a new PEC platform for controlled trifluoromethylation reactions, which could inspire further advancements in related organosynthesis fields.