The electrochemical approach to concerted proton—electron transfers in the oxidation of phenols in water

Abstract

Establishing mechanisms and intrinsic reactivity in the oxidation of phenol with water as the proton acceptor is a fundamental task relevant to many reactions occurring in natural systems. Thanks to the easy measure of the reaction kinetics by the current and the setting of the driving force by the electrode potential, the electrochemical approach is particularly suited to this endeavor. Despite challenging difficulties related to self-inhibition blocking the electrode surface, experimental conditions were established that allowed a reliable analysis of the thermodynamics and mechanisms of the proton-coupled electron-transfer oxidation of phenol to be carried out by means of cyclic voltammetry. The thermodynamic characterization was conducted in buffer media whereas the mechanisms were revealed in unbuffered water. Unambiguous evidence of a concerted proton-electron transfer mechanism, with water as proton acceptor, was thus gathered by simulation of the experimental data with appropriately derived theoretical relationships, leading to the determination of a remarkably large intrinsic rate constant. The same strategy also allowed the quantitative analysis of the competition between the concerted proton-electron transfer pathway and an OH ؊ -triggered stepwise pathway (proton transfer followed by electron transfer) at high pHs. Investigation of the passage between unbuffered and buffered media with the example of the PO4H2 ؊ /PO4H 2؊ couple revealed the prevalence of a mechanism involving a proton transfer preceding an electron transfer over a PO4H 2؊ -triggered concerted process.electrochemistry ͉ phenol oxidation ͉ proton-coupled electron transfer I n the active attention that is currently devoted to protoncoupled electron transfers (PCET), in which proton and electron transfers involve different molecular centers, the oxidation of phenols has played a prominent role in view of its relevance to reactions occurring in natural systems, particularly, but not exclusively, to the oxidation of tyrosine in photosystem II (1-3). Photosystem II is the most prominent example (4-6), but evidence has been gathered that similar processes are involved in the functioning of several other biochemical systems (7). Another aspect of the oxidative PCET chemistry of phenols is related to other biological roles, notably their antioxidant properties (8-10). Oxidative dehydrodimerization of phenols is also an important class of reactions, being involved in the first stages of natural processes such as lignin formation (11, 12). Last but not least, phenol oxidation has important synthetic applications (13).Unraveling the mechanisms of PCETs is an important task from a fundamental viewpoint and is also in conjunction with the relevance of these reactions to natural and synthetic processes.As sketched in Scheme 1, in the PCET process that precedes dimerization, the stepwise pathways form a square reaction scheme that may involve electron transfer first, followed by proton transfer (EPT pathway) or, conversely, proton transfer first foll...

Ultrafast real-time visualization of active site flexibility of flavoenzyme thymidylate synthase ThyX

“…In both cases, the radical pair can be subsequently stabilized by proton transfer from the residue. Simultaneous proton and electron transfer, avoiding high-energy charge-separation intermediates, has been observed for phenol oxidation reactions in aqueous solutions (33,34). Though we cannot fully exclude that such simultaneous processes (and hence motions between Tyr-91 and potential proton acceptors) could play a role in FAD* quenching in ThyX, we consider it unlikely in view of the evidence from other flavoproteins and the fact that the high-driving force generated by FAD* formation (see above) energetically allows formation of the FAD − Tyr°+ intermediate by electron transfer only.…”

Kinetic isotope effects in proton coupled electron transfer

“…Costentin and co-workers have extensively developed a suitable model for investigating the role of electrolyte composition on PCET mechanisms [25,26] which we summarize as succinctly as possible below. We utilize this model with the intention of providing insight into kinetic isotope effects as they apply to electrochemical PCET systems (i.e.…”

The Electrochemical Reaction Mechanism and Applications of Quinones

“…electrons and protons are transferred individually in kinetically distinct steps. However, there are increasing reports of concerted transfer of electron and proton, 8,35,36 and k app has been modified to take the concerted pathway into account.…”