Intramolecular one-electron transfer

Intramolecular one-electron transfer with subsequent dissociation (i.e., homolytic bond cleavage), as described by Reaction 15. This... 

It has been observed that a series of 2,4-alkanedionato adducts of cobalt(III)(salen), salen = bis(salicylideneaminato) dianion, undergo a thermally induced, intramolecular one-electron transfer reaction to cobalt(II)bis(salicylideneaminato) . The concomitant formation in the gas phase of a mixture of the /9-diketone (not more than 50%), methanol, ethanol and acetone has been explained as follows the thermally induced, homolytic fission of the Co—Odik bond gives a /3-diketonato radical which abstracts a hydrogen atom from a second /3-diketonate to form the corresponding diketone, whereas the dehydrogenated /3-diketonato radical decomposes into compounds of lower molecular weight. 

Ascorbic acid is a strong two-electron reducing agent that is readily oxidized in one-electron steps by metal ions and metal complexes in their higher valence states. An inner sphere mechanism for the stoichiometric oxidation of ascorbic acid by ferric ion in acid solution is illustrated by Scheme 1(8). The first step in the reaction is the formation of a monoprotonated Fe(III) complex similar to the monoprotonated ascorbate complexes listed in Table I. The intermediate monoprotonated Fe(III) complex is short-lived and rapidly undergoes an intramolecular one-electron transfer to give a deprotonated Fe(II) complex of the ascorbate radical anion, indicated by 7. This complex dissociates to the free radical anion, which may then combine with a second ferric ion to form the complex 9. Complex 9 in turn undergoes a second intramolecular electron... 

C6F5)2Yb clearly exhibits its reductive properties in the reaction with QF5COOH [28], or with orto- (but not meta- or para-) halogen substituted benzoic acid [30]. After hydrolysis of the reaction products considerable quantities of 0-HQF4COOH [28] and PhCOOH [30] were discovered. To explain their formation the scheme was proposed, which includes intramolecular one-electron transfer from ytterbium to orto- halogen atom ... 

This study indicates that the oxidation of dihydroanthracene in a basic medium involves the formation of a monocarbanion, which is then converted to a free radical by a one-electron transfer step. It is postulated that the free radical reacts with oxygen to form a peroxy free radical, which then attacks a hydrogen atom at the 10-position by an intramolecular reaction. The reaction then proceeds by a free-radical chain mechanism. This mechanism has been used as a basis for optimizing the yield of anthraquinone and minimizing the formation of anthracene. 

In summary, it would appear that the oxidation of a catecholamine probably first involves the formation of a semi-quinone radical (this can be brought about by an one-electron transfer, e.g. from Cu++ ions,14 or by photoactivation 1) which rapidly undergoes further oxidation (e.g. with atmospheric oxygen) to an intermediate open-chain quinone (such as adrenaline-quinone) and then cyclizes by an oxidative nucleophilic intramolecular substitution to the amino-chrome molecule. Whilst the initial formation of a leucoaminochrome by non-oxidative cyclization of the intermediate open-chain quinone in some cases cannot be entirely excluded at the moment (cf. Raper s original scheme for aminochrome formation72), the... 

Subsequent one-electron transfer and intramolecular hydrogen migration lead to radical 102 followed by reaction with 02 to yield hydroperoxide radical 103. Radical 103 is further oxidized to a dihydroperoxide (104), which decomposes to anthra-quinone. Alternatively, 103 may be transformed to a diradical that eventually gives anthracene as a byproduct. The ratio of the two products strongly depends on the solvent used. The highest yield of anthraquinone (85% at 100% conversion) was achieved in 95% aqueous pyridine. 

Enhancement of fluorescence due to the complexation of metal ions with fluoroionophores has been used as a well-precedented technique to analyze for the presence of metal ions [189-191], A number of studies have reported chelating fluorophores whose emission spectra change upon the addition of metal ions [192-198]. One remarkable result of this emission intensity enhancement is shown in Scheme 23, where the chelation of zinc chloride to 9,10-bis(((2-(dimethylamino)ethyl)methylamino)methyl)anthracene drastically enhances the observed fluorescence by a factor greater than 1000-fold [199], In the absence of Zn2+, the singlet excited state of anthracene moiety is strongly quenched by intramolecular photoinduced electron transfer from the amine to the anthracene moiety. The complex formation of Zn2+ with the amine moiety may result in the largely positive shift of the one-electron oxidation potential. Thus, intramolecular photoinduced electron transfer is strongly suppressed by the complexation of the amine moiety with Zn2+,... 

They also form self-assembled /-dimers in non-coordinating solvents probably through coordination of one of the diethylamino nitrogen atoms with the central metal of adjacent molecule. Upon addition of pyridine, they are converted back to the monomeric species. The fluorescence is significantly quenched and the efficiency of singlet oxygen formation is also reduced as a result of intramolecular photoinduced electron transfer, which is inhibited in the dimers. 

A versatile strategy for efficient intramolecular oc-arylation of ketones was achieved by the reaction of silyle enol ethers with PET-generated arene radical cations. This strategy involved one-electron transfer from the excited methoxy-substituted arenes to ground-state DCN [42]. Pandey et al. reported the construction of five- to eight-membered benzannulated as well as benzospiroannulated compounds using this approach (Sch. 20) [42a]. The course of the reaction can be controlled via the silyl enol ether obtained... 

Chromate(VI) has been reported to undergo reduction to Crv as a result of PET between its LMCT excited state and an external electron donor. In the study carried out for several aliphatic alcohols (methanol, ethanol, propan-2-ol, butan-1-ol, butan-2-ol, 2-methyl-propan-2-ol) two pathways of PET were identified one-electron transfer for intermolecular and two-electron transfer for intramolecular systems [96,97]. The intermolecular mechanism of the CrVI excited state quenching was also found for phenol or its derivatives [98], whereas in the case of an anion donor (such as oxalate) an effect of external cations was observed [99],... 

Paquette has reported an intramolecular oxidative coupling using ferric chloride to prepare the intermediate 30 for the synthesis of cerorubenic acid-III. Addition of the dienolate of 28 to FeCls in dmf at —78°C produced the cyclopropane intermediate 29 in 54% yield (equation 16). Although the mechanism of this oxidative cyclization is not discussed in the paper, it is likely that a one-electron transfer pathway is involved. Copper(n) salts have also been utilized for intramolecular enolate coupling, but they proved to be somewhat less effective in the present context. 

NADPH cytochrome P450 reductase, an enzyme containing which a complex flavoenzyme that contains two flavins, one electron is first intramolecularly transferred from FAD to FMN, before the reaction with cytochrome P450 takes place. With FNR, NADP+ first has to bind to the oxidized form, before the very fast one-electron transfer from the specifically interacting reduced ferredoxin (Fdred) occurs (8). Subsequent dissociation of the oxidized ferredoxin (Fdox) is rate-limiting in catalysis. The enzyme semiquinone-NADP" complex then reacts with another reduced ferredoxin molecule to yield the flavin hydroquinone state. In the final steps of the catalytic cycle, the NADP+ is reduced and the NADPH dissociates ... 

This mechanism with coupled one-electron intermolecular electron transfer from the external donor and intramolecular multi-electron transfer from the catalyst to coordinated N2 is, presumably, more efficient than the simpler mechanism considered above with one-electron and multi-electron transfers separated in time. In this mechanism the strongest reductant, which is of necessity the external reducing agent, is used for direct reduction of the substrate, whereas for consecutive one-electron and multi-electron processes its reducing power is used only to prepare the reduced form of the catalyst. 

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