Electron-transfer mechanism isolation

The pale blue tris(2,2 -bipyridine)iron(3+) ion [18661-69-3] [Fe(bipy)2], can be obtained by oxidation of [Fe(bipy)2]. It cannot be prepared directiy from iron(III) salts. Addition of 2,2 -bipyridine to aqueous iron(III) chloride solutions precipitates the doubly hydroxy-bridged species [(bipy)2Fe(. t-OH)2Fe(bipy)2]Cl4 [74930-87-3]. [Fe(bipy)2] has an absorption maximum at 610 nm, an absorptivity of 330 (Mem), and a formation constant of 10. In mildly acidic to alkaline aqueous solutions the ion is reduced to the iron(II) complex. [Fe(bipy)2] is frequentiy used in studies of electron-transfer mechanisms. The triperchlorate salt [15388-50-8] is isolated most commonly. 

Since the inosose phenylhydrazones could be acylated even in the presence of the strong base and isolated, it was concluded that the radical anions must arise following the abstraction of an acidic imino proton of the phenylhydrazone group by base. On interaction with oxygen the resulting anions (by an electron-transfer mechanism (29)) give... 

Finally, it should be stressed that organic electron transfers only rarely occur as isolated steps because of the high chemical reactivity of odd-electron species. Normally, they are part of multi-step mechanisms together with other types of elementary reaction, such as bond forming and breaking. In organic electrochemistry a useful shorthand nomenclature for electrode mechanisms denotes electrochemical (= electron transfer) steps by E and chemical ones by C, and it is appropriate to use the same notation for homogeneous electron-transfer mechanisms too. Thus, an example of a very common mechanism would be the ECEC sequence illustrated below by the Ce(IV) oxidation of an alkylaromatic compound (14-17) (Baciocchi et al., 1976,... 

Photolysis of FL at a silica gel/air interface leads to the generation of 9-fluorenone (FLO) as the only isolable product. No dark reactions were observed and singlet molecular oxygen is not involved in the reaction. Transient spectroscopy shows that both the triplet state and the radical cation of FL are formed, thus indicating that an electron transfer mechanism is involved. Loss of a proton from the radical cation and subsequent reaction with molecular oxygen yields FL peroxide radicals, leading to the formation of 9-hydroxyfluorene. The latter readily photolyzes on the silica surface to produce FLO. 

A single electron transfer mechanism is involved in the phototransformation of the enones (38) into (39) in the presence of phosphites. The reactions are carried out in acetonitrile and proceeded by the triplet enone to which an electron is transferred from the phosphite to give the radical cation/radical anion pair (A). Collapse of radical cation component of (A) gives (B) which then reacts by addition to the enone radical anion. The products (39) are isolated after hydrolysis of the corresponding silyl ethers. The influence of ring size and substituents was also examined and these results are given in Scheme... 

The reaction of ( )-bis-(2,4,6-tri-t-butylphenyl)diphosphane (16) with a four equivalent excess of tetrachloro-o-benzoquinone (17) gave (19). Evidence was presented to suggest that the reaction occurred via (18) with cleavage of the P=P bond and indeed, with two equivalents of (17) a low yield of (18) was isolated. The structures of the products were determined by X-ray crystallography and it was also suggested that the formation of (18) involved an electron transfer mechanism (Route A or Route B, Scheme 1) via a phosphinium radical cation and the radical anion of (17). ... 

We have developed a mathematical model simulating four different EET cases for EABs (1) diffusion-based EET (2) conduction-based EET (3) isolated-dual EET and (4) interacting-dual EET. Our simulations clearly demonsttate that differentiating these or other mechanisms will critically depend on a variety of measured parameters values. That is one of the reasons why this book is focused on providing protocols to measure critical parameters in EABs. We believe that by the time researchers advance and replicate experimental measurements under different conditions, new tools will be provided for the modelers to fiilly address the questions surrounding electron transfer mechanisms. 

The copper-mediated 1,4-addition reactions are thought to proceed through complex electron-transfer mechanisms. The first isolable intermediate is an enolate ion, which can be trapped by alkylating species, as shown in Section 18-4. Conjugate addition followed by alkylation constitutes a useful sequence for a,jS-dialkylation of unsaturated aldehydes and ketones. 

Numerous bacteria that can generate electricity have been isolated and investigated. Studies on these pure cultures provide us valuable knowledge about the properties of exoelectrogens, as well as the exocellular electron transfer mechanisms. Some examples of exoelectrogenic active bacteria are listed in... 

On the basis of these results it seems to the present author that inner and outer complexes can reasonably be assumed for the electron transfer to the diazonium ion, but that an outer-sphere mechanism is more likely for metal complexes with a completely saturated coordination sphere of relatively high stability, such as Fe(CN) (Bagal et al., 1974) or ferrocene (Doyle et al., 1987 a). Romming and Waerstad (1965) isolated the complex obtained from a Sandmeyer reaction of benzenediazonium ions and [Cu B ]- ions. The X-ray structural data for this complex also indicate an outer-sphere complex. 

Although single-electron-transfer (SET) processes would be expected to be important in reactions that use metals as reagents, this type of process has also been recognized in the reduction of carbonyl groups that involve 1,4-dihydronicotinamide derivatives . Recent work by Oae and coworkers" has shown that an SET process is operative in the reduction of dibenzothiophene S-oxide by l-benzyl-l,4-dihydronicotinamide when the reaction is catalyzed by metalloporphins. The reaction is outlined in equation (18), but the study gave results of much more mechanistic than synthetic value. This type of study is relevant to understanding biochemical mechanisms since it is known that methionine sulphoxide is reduced to methionine by NADPH when the reaction is catalyzed by an enzyme isolated from certain yeasts . 

As a 3-step mechanism, the electron-transfer paradigm provides a pair of discrete intermediates [D, A] and D+, A for the prior organization and the activation, respectively, of the donor and the acceptor. The quantitative evaluation of these intermediates would allow the overall second-order reaction (k2) to be determined. Although the presence of [D, A] does not necessarily imply its transformation to D+, A-, a large number and variety of donor/ acceptor couples showing transient charge-transfer absorptions associated with [D, A] have now been identified. In each case, the product can be predicted from the expected behavior of the individual ion radicals D+ and A-. Consider for example, the labile 1 1 benzene complex with bromine that has been isolated at low temperatures and characterized crystallographically (Chart 9).256... 

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