Four-electron oxidation

As shown in equation 12, the chemistry of this developer s oxidation and decomposition has been found to be less simple than first envisioned. One oxidation product, tetramethyl succinic acid (18), is not found under normal circumstances. Instead, the products are the a-hydroxyacid (20) and the a-ketoacid (22). When silver bromide is the oxidant, only the two-electron oxidation and hydrolysis occur to give (20). When silver chloride is the oxidant, a four-electron oxidation can occur to give (22). In model experiments the hydroxyacid was not converted to the keto acid. Therefore, it seemed that the two-electron intermediate triketone hydrate (19) in the presence of a stronger oxidant would reduce more silver, possibly involving a species such as (21) as a likely reactive intermediate. This mechanism was verified experimentally, using a controlled, constant electrochemical potential. At potentials like that of silver chloride, four electrons were used at lower potentials only two were used (104). 

Nakabayashi, S., Yagi, 1., Sugiyama, N., Tamura, K. and Uosaki, K. (1997) Reaction pathway of four-electron oxidation of formaldehyde on platinum electrode as observed by in situ optical spectroscopy. Surf. Sci., 386, 82-88. 

Very recently a new kind of electrocatalyst has been propounded using the dinuclear quinone-containing complex of ruthenium (25).492,493 Controlled-potential electrolysis of the complex at 1.70 V vs. Ag AgCl in H20 + CF3CH2OH evolves dioxygen with a current efficiency of 91% (21 turnovers). The turnover number of 02 evolution increases up to 33,500 when the electrolysis is carried out in water (pH 4.0) with an indium-tin oxide(ITO) electrode to which the complex is bound. It has been suggested that the four-electron oxidation of water is achieved by redox reactions of not only the two Run/Ruin couples, but also the two semiquinone/quinone couples of the molecule. 

The use of a chemical mediator can alter the chemoselectivity of an electrochemical reaction. In the reaction illustrated in Scheme 2, -methylstyrene was oxidized using both direct electrolysis and mediated conditions [10]. The current density, amount of charge passed, temperature, and other variables were all kept constant. The only difference was the addition of 6.4 mole percent of tris(4-bomophenyl)amine to the mediated reaction. The direct electrolysis tended to afford the product of a four-electron oxidation (7). When 3.5 F mole of electricity was passed, a 55% yield of (7) was obtained along with a 45% yield of (6). With additional current (6.4 F mole ), a 75% yield of (7) was obtained. The mediated process led to a preponderance of the product from the two-electron oxidation. When 3.5 F mole of electricity was passed in the experiment using the triarylamine mediator, a 93% yield of (6) was obtained along with only 6% of the four-electron oxidation product. 

Anodic amide oxidations have also proven useful for selectively converting amides into enamides that have in turn been used to functionalize the carbon beta to nitrogen. For example, the four-electron oxidation of carbamates was used to introduce a carbonyl beta to a nitrogen (Scheme 21) [50]. In this example, the starting carbamate was oxidized at a carbon... 

In photosystem II an intermediate tyrosyl radical is formed which then repetitively oxidizes an adjacent manganese cluster leading to a four-electron oxidation of two water molecules to dioxygen. In broad detail, the model compounds" described above were demonstrated to undergo similar reactions on photochemical excitation of the respective ruthenium centers. 

The oxidation of water to dioxygen occurs as the consequence of Photosystem 11-dependent generation of a very strong oxidant. Protons liberated by the water-oxidation reaction then contribute to the thylakoid transmembrane electrochemical gradient that drives ATP synthesis. Brudvig et aL describe how flash-induced proton-release measurements have resolved key steps that provide insights on how the 02-evolving center of PSll mediates this four-electron oxidation of water. 

Pyridine and substituted pyridine species such as trans- u 0) y) f, Ru(0)2(py)2X2 and Ru3(0)g(py) are covered here. The pyridine species are unusual in that some are aerobic catalysts (albeit inefficient ones) for the oxidation of alcohols. The oxoruthenate(Vl) complexes so far considered are two electron oxidants, the metal being reduced to Ru(IV) the (py) complexes however, and probably those of (bpy) and (phen), are effectively four electron oxidants, being reduced to Ru(II). This probably arises from the strong 71-acceptor properties of these N-donor ligands, which will prefer a d" metal configuration to maximise metal-to-ligand back bonding. 

RCH20H-h2[0] RCOOH -t H,0 This is a four-electron oxidation. 

A large amount of work has been devoted to N-binding macrocyclic complexes of Ni, Cu and Fe, which yield imine ligand products. Bidentate amine ligands, mainly ethylenediamine (en), have been used with Ru and Os complexes. The oxidation of coordinated ethylenediamine and related ligands stops at the diimine stage and does not continue to the dinitrile. The a,a -diimine entity -N=C-C = N- formed in the four-electron oxidation is particularly stable (93). 

Anodic oxidation of 4-methoxybenzo[6]thiophene in 1% methanolic KOH at 20 °C afforded (318 21%) and (319 36%). In refluxing methanol, only the latter was formed (78%) (79CC1172). The overall four-electron oxidation probably goes through a two-electron oxidation to (317), loss of methanol to give (318) and further oxidation to (319). Hydrolysis of (319) gives the quinone. 

Although catalytic water oxidation (dark reaction) is the first and important reaction of the electron flow in the photosynthesis represented by Fig. 19.1 whereby water is used as the source of electrons provided to the whole system, its catalyst and reaction mechanism are not yet established.10-13) In the photosynthesis Mn-protein complex works as a catalyst for the difficult four-electron oxidation of two molecules of water to liberate one 02 molecule (Eq. (19.2)). It is inferred that at least four Mn ions are involved in the active center, but its structure is not yet completely elucidated. 

Ruthenium tetroxide is a four-electron oxidant which directly transforms alkenic compounds into oxidative cleavage products, i.e. carbonyl compounds and carboxylic acids.288 The reaction can be visualized as proceeding according to a [4 + 2] cycloaddition of the cis-dioxo moiety with the alkene, resulting in the formation of a RuVI cyclic diester which decomposes to ruthenium(IV) dioxide and oxidative cleavage products (equation 114).288 This reaction can be made catalytic... 

Fig. 7.29 Theoretical voltammograms in staircase cyclic voltammetry (a and b) and differential staircase voltammetry (c and d) at disc electrodes for the four-electron oxidation of bis (l,2-diferrocenyldithiolene)nickel in [NBu4][PF6]/CH2C12 solution (ETj — = 120mV,...

In the four-electron oxidation of water to 02, the polymanganese system acts as (i) an electron reservoir, accumulating charge in an exactly controlled fashion at physiologically high redox potential and (ii) as a non-3Orretaining catalyst. 

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