Carbon-oxygen bond reductive

Oxidation of carbon corresponds to an increase in the number of bonds between carbon and oxygen or to a decrease in the number of carbon-hydrogen bonds Conversely reduction corresponds to an increase in the number of carbon-hydrogen bonds or to a decrease in the number of carbon-oxygen bonds From Table 2 4 it can be seen that each successive increase m oxidation state increases the number of bonds between carbon and oxygen and decreases the number of carbon-hydrogen bonds Methane has four C—H bonds and no C—O bonds car bon dioxide has four C—O bonds and no C—H bonds... 

The carbon-oxygen bond is strong, like the normal carbon-fluorine bond, it is difficult to reduce However, some structural features facilitate its reductive cleavage Aryl esters of perfluoroalkanesulfomc acids can be cleaved in good yield by... 

Hydrogenolysis, without ring reduction, of the carbon-oxygen bond in phenols cannot be depended on, but by conversion of the phenol to a better leaving group, such as is formed by interaction of the phenol with 2-chlorobenzoxazole, l-phenyl-5-chlorotetrazole, phenylisocyanate,... 

The carbon-oxygen bond formation follows the same pathway. For both nitrogen-carbon and oxygen-carbon bond formation, a competing reaction is 13-hydride elimination (if a hydride is present at the heteroatom fragment), which lowers the yield and the reduced arene is obtained after reductive elimination. Reductive elimination of the C-N or C-0 fragments should be faster than 13-hydride elimination in order to avoid reduction of the aryl moiety. The side-reaction is shown at the bottom of Figure 13.25. 

Carbon-Oxygen Bond Formation The cathodic reduction of some nitrocarhonyl compounds in aqueous acidic medium gives the hydroxylamino derivatives that can undergo a ring-closure reaction affording anthrandic compounds or isoxazolones [102-104] (Schemes 70 and 71). 

As you will learn in this chapter, the modern definitions for oxidation and reduction are much broader. The current definitions are based on the idea of electron transfers, and can now be applied to numerous chemical reactions. In Unit 1, you saw the terms oxidation and reduction used to describe changes to carbon-hydrogen and carbon-oxygen bonds within organic compounds. These changes involve electron transfers, so the broader definitions that you will learn in this chapter still apply. 

Creation of the synthon from the retron. The user carries out on the retron structure the corresponding changes to elaborate the synthon. For example, if we want to convert, in retrosynthetic direction, an alcohol into a carbonyl (the equivalent to a reduction reaction), an alcohol group has to exist in the retron, and the user must indicate "the formation (retrosynthetic) of a carbon-oxygen bond". 

Catalysts suitable specifically for reduction of carbon-oxygen bonds are based on oxides of copper, zinc and chromium Adkins catalysts). The so-called copper chromite (which is not necessarily a stoichiometric compound) is prepared by thermal decomposition of ammonium chromate and copper nitrate [50]. Its activity and stability is improved if barium nitrate is added before the thermal decomposition [57]. Similarly prepared zinc chromite is suitable for reductions of unsaturated acids and esters to unsaturated alcohols [52]. These catalysts are used specifically for reduction of carbonyl- and carboxyl-containing compounds to alcohols. Aldehydes and ketones are reduced at 150-200° and 100-150 atm, whereas esters and acids require temperatures up to 300° and pressures up to 350 atm. Because such conditions require special equipment and because all reductions achievable with copper chromite catalysts can be accomplished by hydrides and complex hydrides the use of Adkins catalyst in the laboratory is very limited. 

Carbon-oxygen bonds adjacent to an aromatic ring or an alkene function can be cleaved by reduction at very negative potentials [1]. The process is often followed by reduction of the activating group as in 1. In these processes, the reduction potential of the activating group controls the electrode potential required. Thus an... 

Use has been made of the bond cleavage processes initiated by an adjacent carbonyl function for the modification of steroidial ketols such as 18 [97], Reduction in ethanol eliminates the hydroxyl function and in the same reaction, the carbonyl function is reduced to a secondary alcohol. In compound 19 where there are several groups to act as electrophores, carbon-oxygen bond cleavage is initiated from the most easily reduced dienone function [98], Cleavage of the carbon-oxygen bond in an a-acetoxycarbonyl function is achievable in good yields from multifunctional compounds such as the sesquiterpene taxol [99]. 

Apart from the carbon-halogen bond, the carbon-oxygen one is rather active toward the reductive cleavage due to its polarity, so different types of compounds bearing a carbon-oxygen bond are able to undergo this reaction. 

A rather more complex tertracyclic indole based compound lowers blood pressure by selective blockade of a 1-adrenergic receptors. Reaction of the anion from indole (72-1) with butyrolactone (72-2) leads to the scission of the carbon-oxygen bond in the reagent and the formation of the alkylated product (72-3). The acid is then cyclized onto the adjacent 2 position to give the ketone (72-4) by treatment with a Lewis acid such as polyphosphoric acid. Reaction with bromine then leads to the brominated ketone (72-5). This is subjected to reductive alkylation with ethylene... 

A facile synthesis of cyclobutylmethanols has been devised by reacting 2-ethoxy-5-alkyl-3,4-dihydro-2H-pyrans with aluminum alkyls (Scheme 149) (80TL4525). When (648) is reacted with triisobutylaluminum the cyclobutylmethanol (649) is formed quantitatively. While several mechanisms have been proposed for this process, initial rupture of the carbon-oxygen bond of the pyran ring to form an aluminum enolate, which then undergoes ring closure and reduction, appears to be most likely. 

The retrosynthetic analysis of 2,4,6-triphenylpyrylium tetrafluoroborate (86), involving an initial reduction followed by a disconnection of one carbon-oxygen bond (cf. disconnection of 2,5-dimethylfuran, Section 8.3.1, p. 1146), reveals the substituted 1,5-dicarbonyl compound (89). Further rational disconnection then reveals acetophenone and l,3-diphenylprop-2-en-l-one (chalcone) clearly the latter may originate from acetophenone and benzaldehyde (cf. Section 6.12.2, p. 1032). 

There has been no change in oxidation state in going from reactants to products, and the reaction is neither oxidation nor reduction. The number of carbon-oxygen bonds does not change in this reaction. 

Alcohols are oxidized to give ketones or aldehydes, depending on the substitution pattern on the alcohol. Aldehydes and ketones are reduced to alcohols. Notice that, when going from an alcohol to an aldehyde or ketone, two hydrogen atoms (shown in blue in Figure 11.39) are removed, while, at the same time, the carbon-oxygen bond order increases from a single bond to a double bond. In reduction, the reverse is true. 

The reductive lithiation of cyclic benzofused ethers, for example, 510, with 4,4 -di-/< rt-butylbiphenyl (DTBB) and lithium gives intermediate organolithiums, for example, 511 and 512, that can be quenched with a variety of electrophiles to give general products 513 and 514 (Scheme 92). The process is not synthetically useful for 4H-chromene as carbon-oxygen bond cleavage occurs in both directions <2002TL4907>. 

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