Proton-Coupled Electron Transfer from Hydrogen-Bonded Phenols to Benzophenone Triplets

Phenols with intramolecular hydrogen bond between a pendant base and the phenolic OH group react differently in polar and non-polar environments with electron/proton acceptors. This was demonstrated by using time resolved chemically induced dynamic nuclear polarization (TR CIDNP) and theoretical calculations. In benzene, those phenols undergo a concerted electron-proton transfer (EPT) that yields neutral ketyl and phenoxyl radicals. In polar acetonitrile, the reaction mechanism turns into an electron transfer from the phenol to the triplet ketone, accompanied by the shift of a proton from the phenolic OH group to the nitrogen atom of the pendant base to form a distonic radical cation. This behavior is similar to that of tyrosine H-bonded to basic residues in some radical enzymes. This solvent-induced mechanism switch in proton-coupled electron transfers is important in different biological systems, in which the same metabolites and intermediates can react differently depending on the specific local environments.