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

Article abstract of DOI:10.1021/jo00058a019

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 Ph2CHCOOH 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 wa s found that the photooxidation of Ph3CCOOH to Ph3C(*) at TiO2 occured via a single-photon process, while the formation of 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 extented to more positive potentials.In this way less easily oxidized carboxylic acids RCOOH could be converted to RF.