C,O-bonded carbonyls

The mechanisms of the Fischer esterification and the reactions of alcohols with acyl chlorides and acid anhydrides will be discussed m detail m Chapters 19 and 20 after some fundamental principles of carbonyl group reactivity have been developed For the present it is sufficient to point out that most of the reactions that convert alcohols to esters leave the C—O bond of the alcohol intact... 

Stereoselective All lations. Ben2ene is stereoselectively alkylated with chiral 4-valerolactone in the presence of aluminum chloride with 50% net inversion of configuration (32). The stereoselectivity is explained by the coordination of the Lewis acid with the carbonyl oxygen of the lactone, resulting in the typ displacement at the C—O bond. Partial racemi2ation of the substrate (incomplete inversion of configuration) results by internal... 

Hi) Pyrazole rings containing carbonyl groups In this subsection compounds with a pyrazole C—O bond will be discussed independently of their aromatic character. In solution the tautomers of pyrazolinones, e.g. (78a), (78b) and (78c), are easily identified by their IR spectra (Figure 18) (76AHC(Sl)l). 

Isotopic labeling experiments have established that C—O bond rupture occurs between the carbonyl carbon and oxygen substitution at the alcohol C—O bond is not involved. 

Three-dimensional potential energy diagrams of the type discussed in connection with the variable E2 transition state theory for elimination reactions can be used to consider structural effects on the reactivity of carbonyl compounds and the tetrahedral intermediates involved in carbonyl-group reactions. Many of these reactions involve the formation or breaking of two separate bonds. This is the case in the first stage of acetal hydrolysis, which involves both a proton transfer and breaking of a C—O bond. The overall reaction might take place in several ways. There are two mechanistic extremes ... 

The initial step of the coupling reaction is the binding of the carbonyl substrate to the titanium surface, and the transfer of an electron to the carbonyl group. The carbonyl group is reduced to a radical species 3, and the titanium is oxidized. Two such ketyl radicals can dimerize to form a pinacolate-like intermediate 4, that is coordinated to titanium. Cleavage of the C—O bonds leads to formation of an alkene 2 and a titanium oxide 5 ... 

The decarbonylation of oxide-supported metal carbonyls yields gaseous products including not just CO, but also CO2, H2, and hydrocarbons [20]. The chemistry evidently involves the support surface and breaking of C - O bonds and has been thought to possibly leave C on the clusters [21]. The chemistry has been compared with that occurring in Fischer-Tropsch catalysis on metal surfaces [20] support hydroxyl groups are probably involved in the chemistry. 

The stereochemistry of the C(3) hydroxy was established in Step D. The Baeyer-Villiger oxidation proceeds with retention of configuration of the migrating group (see Section 12.5.2), so the correct stereochemistry is established for the C—O bond. The final stereocenter for which configuration must be established is the methyl group at C(6) that was introduced by an enolate alkylation in Step E, but this reaction was not very stereoselective. However, since this center is adjacent to the lactone carbonyl, it can be epimerized through the enolate. The enolate was formed and quenched with acid. The kinetically preferred protonation from the axial direction provides the correct stereochemistry at C(6). 

As for the acetyl phosphate monoanion, a metaphosphate mechanism has also been proposed 78) for the carbamoyl phosphate monoanion 119. Once again, an intramolecular proton transfer to the carbonyl group is feasible. The dianion likewise decomposes in a unimolecular reaction but not with spontaneous formation of POf as does the acetyl phosphate dianion, but to HPOj and cyanic acid. Support for this mechanism comes from isotopic labeling proof of C—O bond cleavage and from the formation of carbamoyl azide in the presence of azide ions. 

The keto carbonyl group can be hydrogenated fairly readily and many of the characteristics of aldehyde hydrogenations also apply here. Initially, the alcohol is produced, but overhydrogenation may result in hydrogenolysis of the C-O bond to form the alkane (Fig. 2.23). Acidic media facilitate hydrogenolysis whereas basic media or basic substituents inhibit hydrogenolysis. 

The chemical behavior of metal carbonyls is influenced by the nature of other ligands present. A decrease in C-O bond order results from an increase in M-C bond order. If other ligands are present that cannot accept electron density, more back donation to CO occurs, so the M-C bond will be stronger and substitution reactions leading to replacement of CO will be retarded. If other ligands are present that are good iy acceptors, less back donation to the CO groups occurs. They will be labilized and substitution will be enhanced. 

Acylsilanes are a class of compounds in which a silyl group is directly bound to the carbonyl carbon, and they have received considerable research interest from the point of view of both physical organic and synthetic organic chemistry [15]. Acylsilanes have a structure quite similar to the structure of a-silyl-substituted ethers a silyl group is attached to the carbon adjacent to the oxygen atom, although the nature of the C-O bond is different. Therefore, one can expect /1-silicon effects in the electron-transfer reactions of acylsilanes. 

Although oxidation potentials of aldehydes and ketones are generally very high, silyl substitution at the carbonyl carbon results in a significant decrease in the oxidation potential [16]. The decrease in the oxidation potentials is attributed to the rise of the HOMO level by the interaction of the C Si cr-bond and the nonbonding p orbital (lone pair) of the carbonyl oxygen (Fig. 9). In the case of a-silyl-substituted ethers, the rotation around the C-O bond is free and,... 

Studies on the electrochemical oxidation of silyl-substituted ethers have uncovered a rich variety of synthetic application in recent years. Since acetals, the products of the anodic oxidation in the presence of alcohols, are readily hydrolyzed to carbonyl compounds, silyl-substituted ethers can be utilized as efficient precursors of carbonyl compounds. If we consider the synthetic application of the electrooxidation of silyl-substituted ethers, the first question which must be solved is how we synthesize ethers having a silyl group at the carbon adjacent to the oxygen. We can consider either the formation of the C-C bond (Scheme 15a) or the formation of the C-O bond (Scheme 15b). The formation of the C Si bond is also effective, but this method does not seem to be useful from a view point of organic synthesis because the required starting materials are carbonyl compounds. 

The Wittig reaction consists in the replacement of carbonyl oxygen of aldehydes and ketones by a methylene group with the aid of phosphine-methylenes resulting in the formation of cis or trans olefines. The reaction proceeds through the nucleophilic addition of Wittig reagent (phosphine methylene) across the > C = O bond and formation of an intermediate cyclic. 

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