One-Electron Oxidation Processes

Pyrroloquinoline quinone (PQQ) (or methoxatin) 6 is a coenzyme, responsible for the oxidation of methanol [7]. It has been found that cyclopropanol 4 inactivates the enzyme from M. methanica [8], the dimeric methanol dehydrogenase and the monomeric enzyme from a Pseudomonas PQQ-dependent methanol dehydrogenase [9] by forming adducts such as 7, through a one-electron oxidation process and the ready ring opening of a cyclopropyloxonium radical, Eq. (3) [8,9]. 

A flavoprotein oxidase, which is also a methanol oxidizing enzyme, was inhibited by cyclopropanol 4 through the formation of a N-5 flavin adduct with a ring opened cyclopropyloxy radical [10]. 

The N-benzyl-l-methylcyclopropylamine 7 is an irreversible inhibitor of the mitochondrial flavoenzyme monoamine oxidase (MAO). It was suggested that 

MAO oxidizes amine substrates also by a one-electron route via the cyclopropylamine radical cation 8 which undergoes ready ring opening to the iminium radical cation 9 [11]. Then capture by a flavin radical, may cause the enzyme inactivation [12]. This mechanism was established by labeling experiments, Eq.(4) [13]. 

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Of particular interest is an electrochemistry study on related Mn(CNR) complexes, and on several carbonyl species 154). The one-electron oxidation process in acetonitrile (Mn(CNR)6+ Mn(CNR)s +) is substantially... 

The first attempt to construct a dendrimer with an electroactive Ru-polypyridi-ne core was based on the reaction of Ru(bpy)2Cl2 with a branched polyether-substituted phenanthroline ligand (11) [27]. In the potential window +2/-2V, this compound shows a one-electron oxidation process and three distinct one-electron reduction processes that, by comparison with the behavior of the... 

In larger structures, the number of equivalent units becomes huge. In the docosanuclear dendrimer made of an Os(II)-based core and 21 Ru(II)-based units, a one-electron oxidation process, assigned to the Os(II)-based unit, is followed by a 12-electron process, due to the simultaneous oxidation of the 12 equivalent and non-interacting peripheral Ru(II)-based units [36]. 

Since the reaction is shown to be a one-electron oxidation process, the reaction may proceed via a perfluoro carbocation intermediate as is seen with nonfluorinated iodides (Eq. 15). 

It has been briefly reported that in acetonitrile this complex exhibits a first reversible one-electron oxidation process (Eor = +1.0 V vs. SCE), corresponding to the transformation Mn2niMn2lv/MnmMn3IV, and a second irreversible process (Ep = + 1.2 V).83... 

As illustrated in Figure 79, just like triferrocene, it displays three well separated (one-electron) oxidation processes, all possessing features of chemical reversibility on the cyclic voltammetric time scale. They are clearly attributed to the step-by-step electron removals Fen3/ Feni3(Eo/+ — +0.56 V vs. SCE EV,2 + = +0.93 V E2 + 13 + = + 1.25 V).122... 

Cyclic voltammetry revealed four reversible one-electron oxidation processes. The ground state structures for HOC and HOC " are aromatic . The structures for HOC" and HOC " are doublets and show Jahn-Teller distortions. The structures for HOC " in the solid is a Jahn-Teller distorted, closed-shell singlet with a canonical structure [37 ], as represented by cyanine/p-phenylenediammonium fragments. The one-to-one complex of HOC" TCNE" was indeed synthesized but found to be antiferromagnetic. 

The cyclization of the lithium amide 29a to 2,5-disubstituted pyrrolidine shown in Scheme 17 is clearly a one-electron oxidation process. This suggests that the radical 29c is not an oxidizable species at the applied potential and thus... 

The head-to-head trans-cyclorimer 92 of 9-vinylcarbazole (see Section II,C,2) undergoes C—C bond fission by a one-electron oxidative process on treatment with cerium ammonium nitrate or tris-(p-bromophenyl)ammo-niumyl hexachloroantimonate in methanol giving 93. ... 

A simple one-electron oxidation process appears to be excluded by the lack of a correlation between the observed reactivities and the half-wave oxidation potentials of the olefins from the literature. What is the role of the sacrificial aldehyde The answer to this question is a good entry point into the mechanism of the transformation. We... 

The related dirhodium(II) a-caprolactamate (cap) complex [Rh2(p--cap)4] undergoes a one-electron oxidation process at quite a lower potential (11 mV) than the acetate complex (1170 mV). In agreement with the Kochi hypothesis, the a-caprolactamate complex has recently been found to be an exceptional catalyst for the allylic oxidation of alkenes under mild conditions. A wide range of cyclohexenes, cycloheptenes, and 2-cycloheptenone (Eq. 5) are rapidly converted to enones and enediones in 1 h with only 0.1 mol % of [Rh2( x-cap)4] and yields ranging from 60 to 90%, in the presence of potassium carbonate [34]. 

The CV of (SS,i / )- Re(NO)(PPh3)Cp 2(p-C=CC=C) (96/Ph) also displays two reversible oxidation waves. Samples enriched in the SR,RS diasteromer give identical electrochemical results.248,249,251 Racemic mixtures of 96/tol, which bears the P(tol)3 ligand, displayed two reversible one-electron oxidation processes in the CV which were approximately 230 mV more facile than the corresponding processes in 96/Ph.251... 

Group 8. The iron complexes [ Cp L2Fe 2(p-C=CC=C)] [L2 = dppe (107), dippe (99)] are characterized by a series of three one-electron oxidation processes (L2 = dppe,258,269,270 dippe258). Substitution of the dppe ligand for dippe results in positive shifts of the three electrode potentials by 0.29, 0.14, and 0.14 V respectively, and that the difference between these processes increases with increasing electron density at the metal centers. 

In a classical flash study, Kok et al. showed that the 02-evolving complex of photosystem II is oxidized sequentially in a four-quantum, four one-electron oxidative process, the steps being named S0-S4, with S0 representing the totally reduced form (186). In the S state model (Scheme 1), S4 is only transiently stable, rapidly converting to S0 with concomitant 02 evolution. 

Initially a simple reversible one-electron oxidation process is examined [see (19)], such as the oxidation of ferrocene to the ferricinium cation in acetonitrile/0.1 M (C4H9)4NC104 (Sharp et al., 1980 Kadish et al., 1984). In (19), initially only A is present in solution. At the usual macrodisc electrode (radius in the millimetre range), material reaches the electrode by linear diffusion which is perpendicular to its surface (x-direction), and the concentrations of A and B may be obtained as a function of time by solving Pick s second law of diffusion as applied to species A and B, (30) and (31). 

Table 4 Boundary conditions for a simple reversible one-electron oxidation process examined by cyclic voltammetry.

Fig. 15 Typical cyclic voltammogram obtained for an electrochemically irreversible one-electron oxidation process.

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