Sequential oxidation and reduction

In the anaerobic photoheteio iophRhodobacter(Rhodopseudomonas) spheroides, a molybdenum enzyme in which the metal is coordinated by two equivalents of a pyranopterin cofactor is involved in sequential oxidations and reductions involving Mo(VI), Mo(V), and Mo(IV) (Cobb et al. 2005). 

Castro CE, DM Riebeth, NO Belser (1992b) Biodehalogenation the metabolism of vinyl chloride by Methylosinus trichosporium OB-3b. A sequential oxidative and reductive pathway through chloroethylene oxide. Environ Toxicol Chem 11 749-755. 

Figure 7.35 Stereoinversion of 2-hydroxy acids using sequential oxidation and reduction...

The isomerization of allylic alcohols provides an enol (or enolate) intermediate, which tautomerizes to afford the saturated carbonyl compound (Equation (8)). The isomerization of allylic alcohols to saturated carbonyl compounds is a useful synthetic process with high atom economy, which eliminates conventional two-step sequential oxidation and reduction.25,26 A catalytic one-step transformation, which is equivalent to an internal reduction/oxidation process, is a conceptually attractive strategy due to easy access to allylic alcohols.27-29 A variety of transition metal complexes have been employed for the isomerization of allylic alcohols, as shown below. 

The TTF-porphyrin dyad 3 was described by the group of Odense.11 The fluorescence of 3 is significantly quenched by the photoinduced electron transfer process. Notably, the fluorescence intensity of dyad 3 increases largely after addition of Fe3 + that oxidizes TTF into TTF" +. Successive reduction of TTF" + is not reported. Nevertheless, it is anticipated that the fluorescence of dyad 3 can be reversibly modulated by redox reactions. In fact, the fluorescence of the supramolecule 4, formed between Zn-tetraphenylporphyrin and a pyridine-substituted TTF (TTF- ), can be reversibly tuned by sequential oxidation and reduction of the TTF moiety in 4.12 It should be noted in this context that the synthetically challenging system associating a porphyrin ring fused to four TTFs (5) was also reported.13... 

FIGURE 9.12 Sequential oxidative and reductive reactions during SOD catalysis. 

The oxidation of ethylene to acetaldehyde using PdCb and CuCb as catalysts undo- an oxygm atmosphere is well known as the Wacker process (Scheme 1), and is one of the most important industrial processes employing transition metal catalysts.This industrial oxidation reaction of ethylene involves the following three stoichiometric reactions. These sequential oxidation and reduction reactions constitute a catalytic cycle. 

TABLE 13. Formal electrode potentials (vs. SCE) and relative separations for the sequential oxidations and reductions of the binuclear ruthenium complexes [112] in CH2CI2 solution as a function of the spacer... 

In open flow systems, it is possible that re-oxidation of sulfide may occur far removed from the site of the sulfate reduction. This appears to be the case in Flysch waters in Czechoslovakia. Sulfur isotope data obtained by Smejkal et al. (1971) provided evidence of sequential oxidations and reductions of the form ... 

That is why the reaction of its decomposition may be presented as sequential oxidizing and reduction H ... 

Scheme 19.7 Synthesis of (a) mono- or (b) di- and (c) per-hydroxylated pillar[S]arenes using deprotection strategies using BBrs and sequential oxidation and reduction reactions, respectively...

A double bond isomerization in alkenes possessing hydroxy groups can occur to generate ketones. Such a process represents a more economical approach to redox chemistry than processes involving sequential oxidation and reduction. Isomerization of allyl alcohols to the saturated ketones 219 can be catalyzed by the cyclopentadienyl- or indenylruthenium complexes 89a or 220 in the presence of triethylammonium hexafluorophosphate under mild reaction conditions (Scheme 99). Some representative results of the isomerization of allyl al-... 

The final stages of the successful drive towards amphotericin B (1) are presented in Scheme 19. Thus, compound 9 is obtained stereoselectively by sodium borohydride reduction of heptaenone 6a as previously described. The formation of the desired glycosida-tion product 81 could be achieved in dilute hexane solution in the presence of a catalytic amount PPTS. The by-product ortho ester 85 was also obtained in approximately an equimolar amount. Deacetylation of 81 at C-2, followed sequentially by oxidation and reduction leads, stereoselectively, to the desired hydroxy compound 83 via ketone 82. The configuration of each of the two hydroxylbearing stereocenters generated by reduction of carbonyls as shown in Scheme 19 (6—>9 and 82->83) were confirmed by conversion of 83 to amphotericin B derivative 5 and comparison with an... 

Ferrocene has been widely investigated as an electron donor and its electron donating ability can be tuned by redox reactions. As anticipated, when a ferrocene unit is covalently connected to an electron acceptor moiety that shows intrinsic fluorescence, the fluorescence of the acceptor moiety would be largely quenched because of the photoinduced electron transfer between ferrocene and the fluorescent acceptor. For instance, triad 15 that contains perylene diimide flanked by two ferrocene moieties, shows rather weak fluorescence due to the photoinduced electron transfer between perylene diimide and ferrocene units. Either chemical or electrochemical oxidation of ferrocene unit lead to fluorescence enhancement. This is simply because the electron donating ability of ferrocene is reduced after oxidation and accordingly the photoinduced electron transfer is prohibited. In this way, the fluorescence intensity of 15 can be reversibly modulated by sequential electrochemical oxidation and reduction. Therefore, a new redox fluorescence switch can be established with triad 15.25... 

The mechanism of the catalytic cycle is outlined in Scheme 1.37 [11]. It involves the formation of a reactive 16-electron tricarbonyliron species by coordination of allyl alcohol to pentacarbonyliron and sequential loss of two carbon monoxide ligands. Oxidative addition to a Jt-allyl hydride complex with iron in the oxidation state +2, followed by reductive elimination, affords an alkene-tricarbonyliron complex. As a result of the [1, 3]-hydride shift the allyl alcohol has been converted to an enol, which is released and the catalytically active tricarbonyliron species is regenerated. This example demonstrates that oxidation and reduction steps can be merged to a one-pot procedure by transferring them into oxidative addition and reductive elimination using the transition metal as a reversible switch. Recently, this reaction has been integrated into a tandem isomerization-aldolization reaction which was applied to the synthesis of indanones and indenones [81] and for the transformation of vinylic furanoses into cydopentenones [82]. 

A second basic interaction pathway between transition metal complexes and organic substrates is SET (Path B). The overall processes can involve one individual or several sequential SET steps. For the latter, timing and direction of SET steps determine the reaction outcome significantly. The catalyzed reaction can proceed either as redox-neutral processes, in which oxidative and reductive SET steps are involved in the catalytic cycle, or as overall oxidative or reductive catalytic reactions, where two oxidative or reductive SET steps occur consecutively in the catalytic cycle. The third pathway (Path C) consists of a direct atom or group abstraction by the metal complex, which is possible for a weak R-X bond. 

Alternatively, a one-pot, single-step deracemization of sec-alcohols has been achieved by employing two different microorganisms in a single reaction vessel. However, the number of examples of this type is limited and the oxidation and reduction steps are usually performed sequentially in a one-pot, two-step procedure. For instance, racemic mandeUc acid was deracemized in the presence of whole cells of Pseudomonas polycdor and Micrococcus freudenreichii [14]. Separate experiments showed that P. polycolor was responsible for the oxidation, while M. freudenreichii was needed for reduction of the corresponding a-keto acid. After 24h, (R)-mandelic acid 4 was isolated in a 60% yield and 99% e.e. [14],... 

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