Ester resonance

The Claisen condensation is one method of synthesizing (3-dicarbonyl compounds, specifically a (3-keto ester. This reaction begins with an ester and occurs in two steps. In the first step, a strong base, such as sodium ethoxide, removes a hydrogen ion from the carbon atom adjacent to the carbonyl group in the ester. (Resonance stabilizes the anion formed from the ester.) The anion can then attack a second molecule of the ester, which begins a series of mechanistic steps until the anion of the (3-dicarbonyl compound forms, which, in the second reaction step (acidification), gives the product. 

Base-mediated ester hydrolyses have a high driving force. This is because of the acid/base reaction between the carboxylic acid formed in the reaction, and the base used as the reagent. The resonance stabilization of the carboxylate is approximately 30 kcal/mol, which means a gain of about 16 kcal/mol compared to the starting material, the carboxylic ester (resonance stabilization 14 kcal/mol according to Table 6.1). Accordingly, the hydrolysis equilibrium lies completely on the side of the carboxylate. 

Ireland-Claisen rearrangements obviously occur under much milder conditions than the classical Claisen rearrangements of Figures 14.46 and 14.47. Among other things, this is due to product development control. The rearranged product of a Claisen-Ireland rearrangement is an a-allylated silyl ester, and its C=0 bond is stabilized by ester resonance (=14 kcal/mol... 

Conjugation with an oxygen atom has much the same effect—formate esters resonate at about 8 p.p.m.—but conjugation with 7t bonds does not. The simple conjugated aldehyde below and myrte-nal both have CHO protons in the normal region (9-10 p.p.m.). 

The facile reduction of the -COOH group by BHj THF or BH3 SMej has been employed for chemoselective reductions of the carboxyl group in the presence of ester or lactone functionalities using a stoichiometric quantity of the borane. The carbonyl group in triacylboranes resembles the reactivity of an aldehyde or a ketone more than of an ester (ester resonance) due to electron delocalization from the acyl oxygen into the p orbital of boron. 

Indeed, triethyl phosphonoglyoxylate was easily prepared by this approach using propylene oxide as the oxygen donor (100 % by NMR, 84 % isolated), with refluxing benzene (16 hr) as the solvent [79]. The course of the reaction was conveniently monitored by 31P-NMR (diazo ester resonance, 610.8 ppm, ketone ester resonance <5 -2 ppm). Workup is simple both side products (dinitrogen and propylene) are gases at room temperature, and excess propylene oxide and solvent is removed in vacuo. On reuse, recovered rhodium catalyst was found to retain its activity. When 0.02 eq. (tenfold more) catalyst was used, the reaction was complete in approximately 2 h. 

At 30°C, the protons of the unsubstituted ring in mono-t-butyluranocene resonate at 0.51 ppm lower field and those in the mono-ester resonate at 0.43 ppm higher field than the ring protons in uranocene. These differences are small but real and were established independently by observing the spectrum of mixtures of these compounds. 

Aspirin is a phenyl ester. Phenyl esters arc considerably more susceptible to hydrolysis than ordinary esters arc. for two reasons Delocalization of a lone electron pair from the phenol oxygen into the carbonyl group (ester resonance, sec Section 20-1) is diminished, because this lone pair is also in resonance with the benzene ring. The result is a greater 8 on the carbonyl carbon, which facilitates nucleophilic attack. The... 

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