Resonance stabilisation esters

The carbonyl oxygen of an ester group, (e.g., in acrylates or vinyl esters), is more basic than a vinyl group and it captures protons (or other cations) from the catalytic system to give a resonance-stabilised cation which does not involve the reaction site, namely the olefinic double bond. Hence, acrylates and vinyl esters do not polymerise cationically. 

The amide group is resonance-stabilised to a greater extent than the ester group, as shown by the extensive evidence of X-ray analysis of the peptide bond10 and the carbon-nitrogen bond distance in amides8. 

There is a useful analogy between resonance-stabilised anion 3.39a,b and an ester enolate anion. Note that in both cases the negative charge can be delocalised onto a heteroatom. 

The utility of thiol esters and carbonates as protecting groups is limited by their vulnerability to hydrolysis, The poor overlap between the non-bonded electrons on the sulfur atom (3p) and the n-system of the carbonyl (2p) precludes or diminishes resonance stabilisation of the type enjoyed by normal esters thereby raising their ground state energy, The carbonyl group of the thioester is more electrophilic than a normal ester and hence more reactive. Thiocarbonate derivatives are marginally more stable. The 5-benzoyl derivative of cysteine is 95% hydrolysed in 30 minutes with 2 M ammonia whereas the 5-benzyloxycarbonyl derivative is only 20% hydrolysed in 30 minutes under the same conditions.54... 

These carbanions can be formed (Figure 5.8) by proton abstraction from ketones resulting in aldol condensations, by proton abstraction from acetyl CoA, leading to Claisen ester condensation, and by decarboxylation of p-keto acids leading to a resonance-stabilised enolate, which can likewise add to an electrophilic centre. It should be noted that the reverse of decarboxylation also leads to formation of a carbon—carbon bond (this is again a group transfer reaction involving biotin as the carrier of the activated CO2 to be transferred). 

Esters with a-hydrogen atoms can be deprotonated (like aldehydes and ketones) to form resonance-stabilised enolates, which can act as nucleophiles. 

Of course, the carboxylate anion is an unactivated, low-energy resonance-stabilised system that is even less susceptible to nucleophilic attack. However, if the hydroxyl group is first converted to a phosphate ester expulsion will occur readily because the leaving group is now a resonance-stabilised phosphate anion which is... 

The alkyne is protonated by the acid, to give a resonance stabilised cation intermediate, which is intercepted by water to give an enolic intermediate. Tautomerisation gives an activated oxonium cation, which is attacked by water. This initiates ester hydrolysis, leading to expulsion of ethanol, giving the carboxylic acid product. 

Deprotonation of the a-protons of the ester is easily achieved by NaOEt to give an enolate, which is resonance stabilised, and which is also a good nucleophile. Esters readily undergo nucleophilic addition-elimination. 

Esters (RCO2R) can be hydrolysed in aqueous acid (see Section 9.4.2) or base. In basic solution, this is known as a saponification reaction and this has an important application in the manufacture of soaps from vegetable oils or fats. Whereas acid hydrolysis is reversible, base hydrolysis is irreversible due to the formation of the resonance-stabilised carboxylate ion. 

Primary antioxidants also include phenolic acids, in particular the substituted cinnamic acids and their esters (depsides), glycosides and amides. The phenoxyl radical of very effective 4-hydroxysubstituted cinnamic acids is stabilised by resonance (Figure 11.8). Many flavonoid substances are also primary antioxidants. Particularly effective compounds are chalcones, which provide resonance stabilised radicals (Figure 11.9). 

Steric effects on both the amide and the acyloxyl side chain are similar. Tert-butyl and adamantyl groups on the amide side chain in 29v, 29x, 29c, and 29e (Table 2 entries 53 and 54, 63 and 65) result in lower stretch frequencies that, on average, are only 40 cm-1 higher than their precurser hydroxamic esters. Streck and coworkers have suggested that such changes in dialkyl ketones can be ascribed to destabilisation of resonance form II through steric hindrance to solvation which, in the case of tert-butyl counteracts the inductive stabilisation.127... 

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. 

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. 

P-keto ester enolate (anion stabilised by resonance)... 

Ketones are more reactive than esters due to the stabilising resonance delocalisation in the ester, and the nitrogen of hydroxylamine is more nucleophilic than oxygen, since nitrogen is less electronegative. 

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