Electron transfer concerted proton-coupled

For oxidation of G in duplex DNA, Steenken concluded that the proton on N-1 of G shifts spontaneously to N-3 of the cytosine in the normal Watson-Crick base pair to generate [C+(H)/G ]. Consistent with this proposal, calculations indicate that charge transfer in oxidized DNA is coupled with proton transfer from G to Experiments carried out in D2O also reveal a kinetic isotope effect for G oxidation, implicating a concerted proton-coupled electron transfer mechanism. However, density functional theory (DFT) calculations in the gas phase predict that the structure with a proton on G N-1 [C/HG ] is more stable than [C (H)/G ] by 1.4kcal/mol. " ... 

Fig. 19 A Pourbaix diagram for phenol oxidation in water with concentration regimes for favored concerted proton-coupled electron transfer (CPET), proton-first stepwise PCET (PET), or electron-first stepwise PCET (EPT). Figure reprinted with permission from Proc Nat Acad Sci [153]...

To study the mechanism by which HCIO4 catalyzes electron transfer, the kinetic order in acid was measured by performing a series of Stem-Volmer luminescence quenching experiments at various concentrations of HCIO4. These experiments revealed a linear relationship between k t and [HCIO4] (Fig. 42) showing electron transfer from [Ru (bpy)3] " is first order in acid. In this work, the authors proposed a stepwise ET/PT mechanism in which protonation of the ketyl radical anion provides additional thermodynamic driving force which causes an increase in et- This work predated the wide-spread acceptance of concerted proton-coupled electron transfer as an elementary step, however these seminal observations provided the conceptual framework for PCET to be applied further in contemporary synthetic chemistry. 

In 2013, our lab demonstrated the feasibility of concerted proton-coupled electron transfer as a mechanism of substrate activation in the development of a catalytic ketyl-olefin coupling protocol [32]. The thermodynamics associated with ketone reduction make them particularly well suited for activation by concerted... 

For aminophenols, one-electron oxidation and the proton elimination can run together in one stage. This leads to a cation-radical containing O and +NH3 fragments within one and the same molecular carcass (Rhile et al. 2006). Such concerted reactions are classified as proton-coupled electron transfer (Mayer 2004). Proton-coupled electron transfer differs from conventional one-electron redox reaction in the sense that proton motion affects electron transfer. Because the transfers of a proton and an electron proceed in a single step, we can say about the hydrogen-atom transference, (H+ -I- e)=H. It is the fundamental feature of proton-coupled electron-transfer reactions that the proton and electron are transferred simultaneously, but from different places (see Tanko 2006). 

The substrate water molecules are prepared in a stepwise fashion for 0-0 bond formation by binding to the M OjCa cluster and by (partial) deprotonation. The concerted oxidation of the activated substrate occurs then either in two 2e steps or in one concerted 4e reaction step, thus avoiding high-energy intermediates. It is the matrix (protein) and the Ca2+/CF ions that allow for the coupling of proton- and electron-transfer reactions to occur. These features are... 

In the most general sense, the problem is one of proton-coupled electron transfer, as described by Hammes-Schiffer and by Nocera in Volume 1, Chapters 16 and 17, respectively. The two limiting mechanisms described above are the cases of perfect coupling (concerted, one-step hydride transfer) and perfect uncoupling (EPE). 

An H transfer, or hydrogen atom transfer (HAT), reaction has been defined by Mayer as the concerted movement of a proton and an electron. .. in a single kinetic step where both. .. originate from the same reactant and travel to the same product. [18] Mayer considers HAT to be one type of the broad class of proton-coupled electron transfer (PCET) reactions, which also includes reactions where the proton and electron are separated. The distinction is a matter of ongoing discussion [19, 20], and other acronyms have been proposed [19, 21], but all the reactions to be considered in this chapter can be satisfactorily described as H transfer . 

For the first, third, and fourth catalysts in (84) the two tautomers are chemically identical, and the same is true for ions such as HCOJ, HPO4", H2PO2, and H2ASO4, which have been reported to have an abnormally high catalytic activity in some reactions.It is clear that the effectiveness of this kind of catalyst is related to its particular electronic structure rather than to its acid-base properties, and the process is more appropriately described as tautomeric catalysis than as bifunctional or concerted acid-base catalysis. It is of interest that a theoretical treatment of some molecules in which acidic and basic groups form part of the same TT-electron system shows some parallelism between catalytic activity and the coupling constants of the molecular orbital theory moreover, a very general treatment of concerted proton transfers indicates that simple bifunctional acid-base catalysis is likely to be of importance only under very restricted conditions. ... 

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