Mesitoic acid esters

The use of mesitoic acid esters has again been successfully employed by Burrows and Topping (1975) in the elucidation of intramolecular carbon acid participation. Under basic aqueous conditions, 2-acetylphenyl mesitoate [41] hydrolyses to yield mesitoic acid and 2-hydroxyacetophenone, reacting with intramolecular catalysis via the monoanion of the ketonic hydrate (see p. 192). However, in 47.5% aqueous ethanol containing potassium hydroxide, the reaction products from l-acetyl-2-naphthyl mesitoate [45] were found... 

The kinetics of the sulphuric acid-catalysed decarboxylation of a range of alkyl substituted benzoic acids have been measured by Schubert et a/.634,635. The variation of rate coefficient with temperature for mesitoic acid is given in Table 206 and the value for the methyl ester shows that, at this acid concentration, the... 

The other mechanism involving acyl cleavage is the AacI mechanism. This is rare, being found only where R is very bulky, so that bimolecular attack is sterically hindered, and only in ionizing solvents. The mechanism has been demonstrated for esters of 2,4,6-trimethylbenzoic acid (mesitoic acid). This acid depresses the freezing point of sulfuric acid four times as much as would be predicted from its molecular weight, which is evidence for the equilibrium... 

Exactly the same considerations apply to the esterification of hindered acids (182) in the reverse direction. It will be noticed that this mechanism requires protonation on the less favoured (cf. p. 240) hydroxyl oxygen atom (185) to allow the formation of the acyl carbocationic intermediate (184). Apart from a number of R3C types, a very well known example is 2,4,6-trimethylbenzoic (mesitoic) acid (186), which will not esterify under ordinary acid-catalysis conditions—and nor will its esters (187) hydrolyse. Dissolving acid or ester in cone. H2S04 and pouring this solution into told alcohol or water, respectively, is. found to effect essentially quantitative esterification or hydrolysis as required the reaction proceeds via the acyl cation (188) ... 

In some systems the acylium ion is formed reverisbly and to a detectable extent while in others, to be discussed in the next chapter, it is a hypothetical intermediate. The first evidence for a stable acylium ion was the fourfold depression of the freezing point of sulfuric acid by mesitoic acid.176 The presence of the acylium ion in the sulfuric acid solutions of carboxylic acids with fourfold depressed freezing points is confirmed chemically by the high yield of ester obtained on pouring the... 

One of the characteristics of the acid-catalyzed hydrolysis of esters, that is shared by ester formation also, is that substituent effects on the rate coefficients are small, and not simply related to a values (see below, p. 131). The data in Table 14 show that this is also true for the, sO-exchange reaction of substituted benzoic acids. This is borne out by the relative constancy of the ratio khyJkexch for the different substituted acids it was not possible to obtain a meaningful p value from the data of Table 14, because of the small number of points and the large amount of scatter evident on the Hammett plot. Mesitoic acid is highly unreactive, compared with the m- and p-substi-tuted esters used, as is its methyl ester towards alkaline hydrolysis138, and presumably reacts by the seriously hindered Aac2 route. 

Preparation and saponification of hindered esters Mesitoic acid can be es-... 

Quantitative ester eation of carboxylic acids. To a solution of 10 mmole of mesitoic acid in 25 ml. of IIMFT in a separatory funnel was added 20 mmole of sodium hydroxide in a 25% aqueous solution. After shaking for 5 min., 40 mmole of methyl... 

Burrows, H. D., Topping, R. M. Intramolecular participation by enolate anions in the cleavage of aryl esters of mesitoic acid. Carbon-carbon bond formation in aqueous and alcoholic solvents. J. Chem. Soc., Perkin Trans. 21975, 571-574. 

Alkyl esters such as n-butyl mesitoate (87) resist the acyl addition reaction because the acyl carbon is sterically hindered by the ortho methyl groups. When 87 reacted with phenylmagnesium iodide, for example, the products were mesitoic acid (88) and iodobutane. Since acyl addition is inhibited by the presence of... 

As in ester hydrolysis. Bender suggested that exchange takes place hy the addition of water to form a tetrahedral intermediate. The very slow exchange of mesitoic acid was attributed to steric hindrance or an ortho-effect by the ormethyl groups. The rate constant hydroi for the hydrolysis of the ethyl esters of these acids is two to four times greater than cxcii> fhe rate constant for 0 -exchange of the free acid. The difference is presumed to be due to the fact that the acid is more resonance-stabilized than the esters, although this is almost certainly not the only factor involved. 

Sodium salts of carboxylic acids, including hindered acids such as mesitoic, rapidly react with primary and secondary bromides and iodides at room temperature in dipolar aprotic solvents, especially HMPA, to give high yields of carboxylic esters.679 The mechanism is Sn2. Another method uses phase transfer catalysis.680 With this method good yields of esters have been obtained from primary, secondary, benzylic, allylic, and phenacyl halides.681 In another procedure, which is applicable to long-chain primary halides, the dry carboxylate salt and the halide, impregnated on alumina as a solid support, are subjected to irradiation by microwaves in a commercial microwave oven.682 In still another method, carboxylic acids... 

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