PCET

Fig. 10 Reaction scheme for PCET. Figure taken from reference [78]...

Comproportionation between cA-RuIV(bpy)2(py)02 + and cis- Run(bpy)2(py)(H20)2+ takes place by proton-coupled electron transfer (PCET) and exhibits a KIE of 16.1. Other PCET reactions of these and related ruthenium and osmium complexes also feature large KIEs. For example, oxidations of H202 by RuIV(bpy)2 (py)O2 + and by Ruin(bpy)2(py)OH2 + have KIEs of22.1 and 16.7, respectively. Oxidation of benzyl... 

The oxidation72 of OsIV(Tp)(Cl)2 NP(H)Et2 by quinone (Q) takes place by PCET and involves the P-H bond ... 

Scheme 2 PCET mechanism in the formation of quinone methides from ferrocenyl phenols...

The tris-carbene ligand family with fac geometry points its three wingtip groups downwards around the metal shielding it effectively from the approach of any but small substrates. Its main application is therefore the activation of small molecules, including the activation of dioxygen and proton coupled electron transfer (PCET), a reaction normally performed by certain enzymes [70,71],... 

The original reaction is between a MnOH species and a tyrosine radical forming a MnO moiety. The process is known as a proton coupled electron transfer (PCET) and this reaction step is modelled by the process depicted in Figure 3.131. 

In 2001, Itoh and coworkers reexplored the mechanism of ortAo-phenol hydroxylations using [ Cu°(L ) 2(02)] " (7, Figure 10). This complex contains a deutero-benzyl-amine moiety, and can undergo hgand auto-oxidation, forming benzaldehyde, through a proton-coupled electron transfer (PCET) reaction with the peroxo ligand. 

Figure 10 Reactions of phenolates with /u.-ri ri -peto o complex (7) yields an ort/ro-hydroxylation reaction reminiscent of Tyr, while reactions of phenols by both (7) and his-/r-oxo complex (8) results in ort/ro-phenol coupled dimers through an apparent PCET pathway...

Figure 12 [ Cu (MePY2) 2(02)] (10), which exists as a mixture of both bis-/x-oxo and /u.-ri ri -petoxo complexes in solution, wiU oxidatively N-deaUcylate /lara-substituted dimethylanilines through either a PCET or a HAT pathway...

In an attempt to address the PCET versus HAT issue, Shearer, Karlin, and coworkers investigated the mechanism of dimethylaniline oxidations by [ Cu(MePy2) 2(02)] + (10, Figure 12). Here, the complexes are not pure peroxo or bis-/u.-oxo complexes, but instead are mixtures. The R -para pyridyl ligand donor substituent produces more of the bis-/u.-oxo tautomer as the R group is made more electron donating (7-30% bis-/u.-oxo). Based upon comparisons of both infra- vs intermolecular KIEs and KIE profiles of para-substituted dimethylaniline oxidations (R-DMA, Figure 12), it was determined that both PCET and HAT reactions occur. For the more easily oxidizable R-DMAs, a PCET reaction is preferred, while for the more difficult to oxidize R-DMAs a HAT pathway is favored. Also, it was observed that the more bis-/tr-oxo isomer that is in solution, the more likely it is that a HAT reaction will occur. 

Since the stmctures and spectroscopic correlations for both oxy and red forms of He, Co, and Tyr are well understood, much of the future synthetic modeling work will focus on the reactivity of CU2O2 species. Tyr activity, o-phenol hydroxylation, seems to occur via an electrophihe aromatic substitution, but the broader scope of reaction for side-on /r- -peroxodicopper(II) complexes should be further explored. Much remains to be accomphshed in determining the detailed mechanism of catechol oxidase catalysis (i.e. HAT vs. PCET). The differential reactivity of peroxo versus bis-/u.-oxo tautomers is still largely unknown. Although there is as yet no... 

RNRs catalyze the reduction of ribonucleotides to deoxyribonucleotides, which represents the first committed step in DNA biosynthesis and repair.These enzymes are therefore required for all known life forms. Three classes of RNRs have been identified, all of which turn out to be metalloenzymes. The so-called class I RNRs contain a diiron site (see Cobalt Bn Enzymes Coenzymes and Iron-Sulfur Proteins for the other two types of RNRs). As diagrammed in Figure 5, these enzymes generate first a tyrosyl radical proximal to the diiron site in the protein subunit labeled R2, and then a thiyl radical in an adjacent subunit (Rl) that ultimately abstracts a hydrogen atom from the ribonucleotide substrate. This controlled tyrosine/thiol radical transfer must occur over an estimated distance of 35 A, and a highly choreographed proton-coupled electron transfer (PCET) mechanism across intervening aromatic residues has been proposed. Perhaps, even more remarkably,... 

Proton-coupled electron transfer (PCET) reactions play a vital role in a wide range of chemical and biological processes. For example, PCET is required for the conversion of energy in photosynthesis [1] and respiration [2], In particular, the coupling between proton motion and electron transfer is involved in the pumping of protons across biological membranes in photosynthetic reaction centers [1] and in the conduction of electrons in cytochrome c [3]. In addition to biological processes, PCET is also important in electrochemical processes [4, 5] and in solid state materials [6]. 

The formulation of a theory for PCET is particularly challenging because of the disparate timescales involved. The description of the solute must accurately incorporate the quantum mechanical behavior of the solute electrons involved in both proton transfer (PT) and electron transfer (ET) and the transferring proton(s). The description of the solvent must include the effects of both electronic and inertial... 

The non-adiabatic limit of ET is most relevant to typical PCET reactions. In the non-adiabatic limit, application of the Golden Rule to the diagonal elements of the VB matrix given in Eq. 6 leads to the rate expression [16, 17, 25, 26],... 

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