Evidence for Carbocation Intermediates in the TiO₂-Catalyzed Photochemical Fluorination of Carboxylic Acids

Laser flash photolysis/transient absorbance spectroscopy was used to determine the mechanism of photo-Kolbe fluorination of carboxylic acids, RCOOH → RF, at colloidal TiO₂ suspensions in acetonitrile. Transient absorption spectra of Ph₃C⁺, Ph₃C^·, Ph₂CH^· and Ph₂CH⁺ were observed from the photooxidation of Ph₃CCOOH and Ph₂CCOOH at TiO₂ using 355-nm excitation. Transient decays, monitored in the presence and absence of fluoride ions, showed that the carbocations reacted rapidly with fluoride, but the neutral radicals did not. By varying the laser intensity, it was found that the photooxidation of Ph₃CCOOH to Ph₃C^· at TiO₂ occurred via a single-photon process, while the formation of Ph₃C⁺ required two photons. This finding is in agreement with the parabolic light intensity dependence of initial reaction rates in bulk photolysis experiments. Although fluoride is strongly adsorbed on the TiO₂ surface in acetonitrile solution, the oxidizing power of photogenerated holes could be increased by coordinating HF to F⁻, and therefore the threshold for oxidative photochemical fluorination was extended to more positive potentials. In this way less easily oxidized carboxylic acids RCOOH could be converted to RF.