Esters resonance structures

Principal resonance structures of the anion of a 3 keto ester... 

A hydrogen attached to the a-carbon atom of a p-keto ester is relatively acidic. Typical P-keto esters have values of about 11. Because the a-carbon atom is flanked by two electron-withdrawing carbonyl groups, a carbanion formed at this site is highly stabilized. The electron delocalization in the anion of a p-keto ester is represented by the resonance structures... 

The mechanism involves the dissociation of the coordinated borane 15 to generate a monoborane intermediate 16. Coordination of the alkene would generate the alkene borane complex. A /3-borylalkylhydride with B-H stabilization is certainly an important resonance structure of 17. An intramolecular reaction would extrude the alkyl boronate ester product and coordination of HBcat would regenerate the monoborane intermediate. 

Whereas in the nitro group the two resonating structures are equivalent, they are made nonequivalent in the carboxyl group and its esters, becoming equivalent again in the corresponding ions ... 

The primary electronic effect is due to the delocalization of electron pairs between the ether oxygen and the carbonyl group of the ester function as expressed by the resonance structure 2. 2, and 2- Resonance structures 1 and 2 show the delocalization of an electron pair between the carbon and oxygen of the carbonyl group H 2) and resonance structures 2 and 3 show that the ether oxygen has one electron pair delocalized towards the same central carbon (2 3), The primary electronic effect can therefore be viewed as the result of two n-v interactions. The three atoms involved can be considered to be sp3 hybridized and on that basis, the three-dimensional representations 4 and 5 correspond respectively to the Z and E forms of the ester function. 

However, the acidity of the a proton gets increased if it is flanked by two carbonyl groups rather than one, for example, 1, 3-diketones ((i-di ketones) or 1,3-diesters ([i-keto esters). This is due to the fact that the negative charge of the enolate ion can be stabilised by both carbonyl groups which results in three resonance structures (Following fig.). For example, the pKa of 2, 4-pentanedione is 9. 

Theoretically, this resonance could also take place in acid chlorides, acid anhydrides, and esters to give resonance structures (Fig.F). However, the process is much less important because oxygen and chlorine are less nucleophilic than nitrogen. In these structures, the positive charge ends up on an oxygen or a chlorine atom. 

These atoms are more electronegative than nitrogen and less able to stabilise a positive charge. These resonance structures might occur to a small extent with esters and acid anhydrides, but are far less likely in acid chlorides. This tend also matches the trend in reactivity. 

Fig.F. Resonance structures for (a) an acid chloride (b) an acid anhydride (c) an ester...

Metal rf-inline complexes with various transition metals [1-10] and lanthanides [11,12] are well known in the literature. Early transition metal if-imine complexes have attracted attention as a-amino carbanion equivalents. Zirconium rf-imine complexes, or zirconaaziridines (the names describe different resonance structures), are readily accessible and have been applied in organic synthesis in view of the umpolung [13] of their carbons whereas imines readily react with nucleophiles, zirconaaziridines undergo the insertion of electrophilic reagents. Accessible compounds include heterocycles and nitrogen-containing products such as allylic amines, diamines, amino alcohols, amino amides, amino am-idines, and amino acid esters. Asymmetric syntheses of allylic amines and a-amino acid esters have even been carried out. The mechanism of such transformations has implications not only for imine complexes, but also for the related aldehyde and ketone complexes [14-16]. The synthesis and properties of zirconaaziridines and their applications toward organic transformations will be discussed in this chapter. 

The equilibrium is much more favorable to polymer formation in the production of nylons than polyesters. This can be explained as being due to the greater stability of amide as compared to ester linkages. Resonance structures can be written for both groups as... 

The hydrolysis of a thioester is thermodynamically more favorable than that of an oxygen ester because the electrons of the C=0 bond cannot form resonance structures with the C—S bond that are as stable as those that they can form with the C—O bond. Consequently, acetyl CoA has a high acetyl potential (acetyl group-Pansfer potential) because Pansfer of the acetyl group is exergonic. Acetyl CoA carries an activated acetyl group, just as ATP carries an activated phosphoryl group. 

Older literature always presents the initial radicals in equivalent resonant positions with equal probability of forming hydroperoxides. The three resonant positions for linoleic acid or its ester and the three hydroperoxides resulting from these are shown in Reaction 41 (224). Comparable resonant structures have been published for oleate, linolenate, and arachidonate (222, 225). 

A heteroatom in a C=X bond can use its lone pair to react with a Lewis acid to give a product for which a carbocationic resonance structure can be drawn. Protonation of a carbonyl compound belongs in this category. One of the lone pairs on O coordinates to H+ to give a compound with two major resonance structures, one of which is carbocationic. The carbocationic resonance structure is not the best structure, but it tells the most about the reactivity of the ion. If the carbonyl C has heteroatoms directly attached (e.g., esters, carboxylic acids, and amides), more resonance structures can be drawn. Reactions of imines (Schiff bases) often begin by protonation of N. 

Common error alert The reaction of esters, amides, and carboxylic acids with electrophiles occurs on the carbonyl O. Many more resonance structures can be drawn when the carbonyl O is protonated than when the noncarbonyl heteroatom is protonated. 

The ease of carbanion formation from the various carbonyl compounds decreases in the order aldehydes, ketones, esters, amides, and acids. This order is understandable if we consider the nature of Y. The more electron-releasing Y becomes, the less will the carbonyl group be able to withdraw electrons from the a-carbon atom (XI). Thus in comparing aldehydes and ketones, hyperconjugation in Y permits another resonance structure which does not place a positive charge on the carbonyl carbon atom (XIV). Consequently, both stabilization of the anion... 

XII and XIII) and permanent polarization (XI) are reduced. Similarly esters, amides, and carboxylate ions have the resonance structures XV, XVI, and XVII which tend to decrease the case of carbanion formation ... 

The reason is straightforward. When we consider the electronic properties of an ester compared to an alcohol, then we observe an important difference. The carboxyl group can pull electrons from the neighbouring oxygen to give the resonance structure shown in Fig. 7.7 Since the lone pair is involved in such an interaction, it cannot take part so effectively in a hydrogen bond. 

Oxygen esters are stabilized by resonance structures not available to thioesters. 

Radicals substituted a to the amide linkage, 24, have been used in several studies to control stereochemistry in radical transformations, while radicals substituted a to esters, 25, and ethers, 26, have been used on a few occasions. Resonance structures for each of these radicals (A and B) can be written as shown in 24-26, with stabilization resulting from delocalization of the odd electron into the adjacent functional group. This resonance delocalization also restricts the geometry of these radicals, maximum delocalization being obtained when overlap between the radical and adjacent group is highest. 

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