Concerted proton-electron transfer in the oxidation of hydrogen bonded phenol-base compounds.

摘要

This work focuses on the one-electron oxidation of phenols with various pendant, hydrogen-bonded bases (HOAr-B) in MeCN. The product of chemical and electrochemical oxidation is the phenoxyl radical in which the phenolic proton has transferred to the base, · OAr-BH+, a proton-coupled electron transfer (PCET) process. Thermochemical arguments, kinetic isotope effects (KIEs), and DeltaDeltaG&Dagger/DeltaDelta G° indicate a concerted proton-electron transfer (CPET) mechanism. The coupled proton-transfer results in lower oxidation potentials and slower reaction kinetics compared to simple electron transfer from aromatic compounds at the same driving force. Substantial variations in rate constants of various phenols at the same DeltaG° have been observed and correlate with changes in hydrogen bond strength. A phenol-pyridine where the rings are separated by -CH2- has a weaker hydrogen bond and slower CPET rates compared to the conjugated phenol-pyridine, in agreement with computational predictions. In a study of nine phenol-imidazoles Delta G° was the primary determinant of reaction rate, despite changes in hydrogen-bonding indicated by 1H NMR as well as calculated vibrational spectra and geometries. A computational study of CPET kinetics and KIEs in which the transferring proton is treated quantum-mechanically indicates that reactivity is influenced by promoting vibrations along the proton-transfer coordinate leading to more facile proton tunneling, however, energetic terms dominate relative reactivities. The influence of proton-transfer distance on CPET was probed in two phenol-amines where the oxygen-nitrogen distance is varied by > 0.1A. CPET kinetics are similar in the two compounds, but show different driving force dependencies. Reduced KIEs correlate with longer proton transfer distance, but are also dependent on Delta G°. These results are in sharp contrast with predictions from simple proton tunneling models. Finally, CPET was studied in phenol-pyridines where substituents vary the basicity by nearly 10 pK a units. Differences in reactivity observed with iron(III)- tris-bipyridine or -phenanthroline oxidants are ascribed to electronic coupling effects. Less basic pyridines have weaker intramolecular hydrogen bonds and react with slower rates at the same driving force. This result is not predicted by preliminary calculations and may in part be influenced by electronic coupling effects.