Reductive transformation

One of these alternate models, postulated by Gunter Wachtershanser, involves an archaic version of the TCA cycle running in the reverse (reductive) direction. Reversal of the TCA cycle results in assimilation of CO9 and fixation of carbon as shown. For each turn of the reversed cycle, two carbons are fixed in the formation of isocitrate and two more are fixed in the reductive transformation of acetyl-CoA to oxaloacetate. Thus, for every succinate that enters the reversed cycle, two succinates are returned, making the cycle highly antocatalytic. Because TCA cycle intermediates are involved in many biosynthetic pathways (see Section 20.13), a reversed TCA cycle would be a bountiful and broad source of metabolic substrates. 

Triazolo[l,2-a]triazolediylium salts 7 can be reductively transformed to the corresponding 1,3,5,7-tetrazocine system.26... 

Peijnenburg WJGM, MJ t Hart, HA den Hollander, D van de Meent, HH Verboom, NL Wolfe (1992) QSARs for predicting reductive transformation constants of halogenated aromatic hydrocarbons in anoxic sediment systems. Environ Toxicol Chem 11 301-314. 

The reductive transformation of arene carboxylates to the corresponding aldehydes under aerobic conditions has already been noted. In addition, aromatic aldehydes may undergo both reductive and oxidative reactions, with the possibility of decarboxylation of the carboxylic acid formed ... 

The metabolism of pentafluoro-, pentachloro-, and pentabromophenol by Mycobacterium fortuitum strain CG-2 is initiated by a monooxygenase that carries out hydroxylation at the para position (Uotila et al. 1992). Cell extracts of Rhodococcus chiorophenoiicus Mycobacterium chlorophenolicunt) strain PCP-1 in the presence of a reductant transformed tetrafluoro-, tetrachloro-, and tetrabromohydroquinone to 1,2,4-trihydroxybenzene by reactions that clearly involve both hydrolytic and reductive loss of fluorine (Uotila et al. 1995). 

Tas DO, SG Pavlostathis (2005) Microbial reductive transformation of pentachloronitrobenzene under metha-nogenic conditions. Environ Sci Technol 39 8264-8272. 

Abstract Recent advances in the metal-catalyzed one-electron reduction reactions are described in this chapter. One-electron reduction induced by redox of early transition metals including titanium, vanadium, and lanthanide metals provides a variety of synthetic methods for carbon-carbon bond formation via radical species, as observed in the pinacol coupling, dehalogenation, and related radical-like reactions. The reversible catalytic cycle is achieved by a multi-component catalytic system in combination with a co-reductant and additives, which serve for the recycling, activation, and liberation of the real catalyst and the facilitation of the reaction steps. In the catalytic reductive transformations, the high stereoselectivity is attained by the design of the multi-component catalytic system. This article focuses mostly on the pinacol coupling reaction. 

This article mostly focuses on the catalytic pinacol coupling and related reductive transformations via one-electron transfer. On the other hand, the corresponding methods for catalytic oxidative transformations via one-electron oxidation have been scarcely investigated and remain to be developed. Both methods are complementary and useful for generating radical intermediates. 

The proceeding of common methods of data analysis can be traced back to a few fundamental principles the most essential of which are dimensionality reduction, transformation of coordinates, and eigenanalysis. 

Reductive and oxidative transformations of small ring compounds form the basis of a variety of versatile synthetic methods which include functionalization and carbon skeleton construction. Redox mechanisms of organotransition metal compounds play an important role in inducing or catalyzing specific reactions. Another useful route in this area is based on one-electron redox reactions. The redox tautomerism of dialkyl phosphonate also contributes to the efficiency of the reductive transformation of small ring compounds. This review summarizes selective transformations which have a high potential for chemical synthesis. 

Redox reactions are considered as being able to provide versatile and efficient methods for bringing about ring transformations. Transition metal complexes in particular are able to induce or catalyze oxidative or reductive transformations of small ring compounds. Organometallics, such as metal-lacycles derived by the insertion of metal atoms into rings, are involved as key intermediates in many cases, allowing subsequent functionalization or carbon-carbon bond formation. 

Radicals are versatile synthetic intermediates. One of the efficient procedures for radical generation is based on one-electron oxidation or reduction with transition metal compounds. An important feature is that the redox activity of transition metal compounds can be controlled by appropriate ligands, in order to attain chemoselectivity in the generation of radicals. The application to small ring compounds provides useful methods for organic syntheses. Reductive transformation are first reviewed here. 

The synthesis of new 11-deoxyprostaglandin analogs with a cyclopentane fragment in the oo-chain, prostanoid 418, has been accomplished by a reaction sequence involving nitrile oxide generation from the nitromethyl derivative of 2-(oo-carbomethoxyhexyl)-2-cyclopenten-l-one, its 1,3-cycloaddition to cyclopenten-l-one and reductive transformations of these cycloadducts (459). Diastereoisomers of a new prostanoid precursor 419 with a 4,5,6,6a-tetrahydro-3aH-cyclopent[d isoxazole fragment in the oo-chain have been synthesized. Reduction of 419 gives novel 11-deoxyprostanoids with modified a- and oo-chains (460). 

Ring-reduction transformations were also reported in CHEC-II(1996) <1996CHEC-II(8)421 >, and some novel similar transformations also appeared during the recent years. These reactions are shown in Scheme 19. 

A carbohydrate based approach to the synthesis of polyketide acid unit present in nagahamide A has been reported. Reductive transformation of 183 followed by cyclization allowed to prepare 184 (Fig. 58).75... 

Schwarzenbach et al. (1990) have shown that reductive transformations of a series of monosubstituted nitrobenzenes and nitrophenols in aqueous solutions containing reduced sulfur species occur readily in presence of small concentrations of an iron prophyrin as an electron transfer catalyst. 

Controlled one-electron reductions transform l,2,3,4-tetraphenyl-l,3-cyclopentadiene or 1,2,3, 4,5-pentaphenyl-l,3-cyclopentadiene into mixtures of the dihydrogenated products and the corresponding cyclopentadienyl anions (Famia et al. 1999). The anion-radicals initially formed are protonated by the substrates themselves. The latter are thermodynamically very strong acids because of their strong tendency to aromatization. As with the cyclopentadiene anion-radicals, they need two protons to give more or less stable cyclopentadienes. The following equations represent the initial one-electron electrode reduction of l,2,3,4,5-pentaphenyl-l,3-cyclopentadiene (CjHAtj) and explains the ratio and the nature of the products obtained at the expense of the further reactions in the electrolytic pool ... 

Scheme 6.27 considers other, formally confined, conformers of cycloocta-l,3,5,7-tetraene (COT) in complexes with metals. In the following text, M(l,5-COT) and M(l,3-COT) stand for the tube and chair structures, respectively. M(l,5-COT) is favored in neutral (18-electron) complexes with nickel, palladium, cobalt, or rhodium. One-electron reduction transforms these complexes into 19-electron forms, which we can identify as anion-radicals of metallocomplexes. Notably, the anion-radicals of the nickel and palladium complexes retain their M(l,5-COT) geometry in both the 18- and 19-electron forms. When the metal is cobalt or rhodium, transition in the 19-electron form causes quick conversion of M(l,5-COT) into M(l,3-COT) form (Shaw et al. 2004, reference therein). This difference should be connected with the manner of spin-charge distribution. The nickel and palladium complexes are essentially metal-based anion-radicals. In contrast, the SOMO is highly delocalized in the anion-radicals of cobalt and rhodium complexes, with at least half of the orbital residing in the COT ring. For this reason, cyclooctateraene flattens for a while and then acquires the conformation that is more favorable for the spatial structure of the whole complex, namely, M(l,3-COT) (see Schemes 6.1 and 6.27). 

This conversion is directed so as to create the most favorable conditions for the delocalization of the nnpaired electron within the aromatic nucleus. It is worth noting here that thermal treatment (150—190°C) also initiates isomerization of the initial neutral molecule of norcara-diene into the benzotropylidene system. At the same time, the reductive transformation of Scheme 6.33 proceeds smoothly even at negative temperatures. Under comparable reaction conditions (25°C), the rate of conversion of the neutral molecule is 15 orders lower than that of the anion-radical. 

Table 13.4 Reductive transformation known to occur in natural reducing environments (Larson and Weber 1994)...

Subsurface environments under anoxic conditions may contain high levels of Fe(II) on the solid phase or dissolved within immobile pore water or groundwater. The role of Fe(II) species in reductive transformation reactions of organic and inorganic contaminants in the subsurface was reviewed by Haderlein and Pecher (1988). A major finding of current studies is that Fe(II) associated with solid phases is much more reactive than Fe(II) present in dissolved forms (e.g., Erbs et al. 1999 Hwang and Batchelor 2000). 

Klupinski et al. (2004) conclude that the reduction of nitroaromatic compounds is a surface-mediated process and suggest that, with lack of an iron mineral, reductive transformation induced only by Fe(II) does not occur. However, when C Cl NO degradation was investigated in reaction media containing Fe(II) with no mineral phase added, a slow reductive transformation of the contaminant was observed. Because the loss of C Cl NO in this case was not described by a first-order kinetic model, as in the case of high concentration of Fe(II), but better by a zero-order kinetic description, Klupinski et al. (2004) suggest that degradation in these systems in fact is a surface-mediated reaction. They note that, in the reaction system, trace amounts of oxidize Fe(II), which form in situ suspended iron oxide... 

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