Esters of Aromatic Acids

3 Esters of Aromatic Acids The molecular ion peak of methyl esters of aromatic acids is prominent. As the size of the alcohol moiety increases, the intensity of the molecular ion peak decreases rapidly to practically zero at C5. The base peak results from elimination of 

As the alkyl moiety increases in length, three modes of cleavage become important (1) McLafferty rearrangement, (2) rearrangement of two hydrogen atoms with elimination of an allylic radical, and (3) retention of the positive charge by the alkyl group. 

Appropriately, ortho-substituted benzoates eliminate ROH through the general ortho effect described above under aromatic acids. Thus, the base peak in the spectrum of methyl salicylate is m/z 120 this ion eliminates carbon monoxide to give a strong peak at m/z 92. 

The familiar McLafferty rearrangement pathway gives rise to a peak for the aromatic acid, (ArCOOH)+. The second, similar pathway gives the protonated aromatic acid, (ArCOOH2)+. The third mode of cleavage gives the alkyl cation, R+. 

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In general the method is more satisfactory with esters of aromatic acids than with esters of aliphatic acids. Esters of alcohols other than methyl and ethyl are best treated by first converting them into methyl esters thus Heat together under reflux i ml. of the higher ester, 5 ml. of methanol and 0-2 g. of sodium methoxide. [In place of the sodium methoxide, it suffices to add o i g. of metallic sodium to the methanol.] After refluxing, distil off the excess of methanol (b.p, 65 ). The residue is then heated under reflux with benzylamine as described above. 

B) Benzylamtdes (see (a) above) can often be prepared directly from the ester, particularly if a methyl or ethyl ester. Usually works best with esters of aromatic acids. (M.ps., pp. 543 545.)... 

NOTE. Many esters reduce Fehling s solution on warming. This reduction occurs rapidly with the alkyl esters of many aliphatic acids, but scarcely at all with similar esters of aromatic acids (f.g., ethyl oxalate reduces, but ethyl benzoate does not). Note also that this is a property of the ester itself thus both methyl and ethyl oxalate reduce Fehling s solution very rapidly, whereas neither oxalic acid, nor sodium oxalate, nor a mixture of the alcohol and oxalic acid (or sodium oxalate), reduces the solution. 

N-Benzylamides are recommended when the corresponding acid is liquid and/or water-soluble so that it cannot itself serve as a derivative. Phe benzylamides derived from the simple fatty acids or their esters are not altogether satisfactory (see Table below) those derived from most hydroxy-acids and from poly basic acids or their esters are formed in good yield and are easily purified. The esters of aromatic acids yield satisfactory derivatives but the method must compete with the equally simple process of hydrolysis and precipitation of the free acid, an obvious derivative when the acid is a solid. The procedure fails with esters of keto, sul phonic, inorganic and some halogenated aliphatic esters. 

Cellulose esters of aromatic acids, aUphatic acids containing more than four carbon atoms and aUphatic diacids are difficult and expensive to prepare because of the poor reactivity of the corresponding anhydrides with cellulose Httle commercial interest has been shown in these esters. Of notable exception, however, is the recent interest in the mixed esters of cellulose succinates, prepared by the sodium acetate catalyzed reaction of cellulose with succinic anhydride. The additional expense incurred in manufacturing succinate esters is compensated by the improved film properties observed in waterborne coatings (5). 

The treatment of esters of aromatic acids with l-alkyl-2-pyrrolidones and l-alkyl-2-piperidones is an extremely useful method for the preparation of simple pyrrolines and piperideines, respectively. The l-alkyl-3-aroyl-2-... 

G. Ethyl or Higher Aliphatic Esters of Aromatic Acids... 

Ortho substituent of methyl esters of aromatic acids. Loss of 31 also should be present. 

Esters of aromatic acids are not reduced by this procedure, so an aromatic COOH group can be reduced in the presence of a COOR group. However, it is also possible to reduce aromatic ester groups, by a variation of the trichlorosilane procedure. The o- and p-hydroxybenzoic acids and their esters have been reduced to cresols (HOC6H4CH3) with sodium bis(2-methoxyethoxy)aluminum hydride, NaAlH2(0C2H40Me)2 Red-Al). ... 

Mixed condensations of esters are subject to the same general restrictions as outlined for mixed aldol reactions (Section 2.1.2). One reactant must act preferentially as the acceptor and another as the nucleophile for good yields to be obtained. Combinations that work best involve one ester that cannot form an enolate but is relatively reactive as an electrophile. Esters of aromatic acids, formic acid, and oxalic acid are especially useful. Some examples of mixed ester condensations are shown in Section C of Scheme 2.14. Entries 9 and 10 show diethyl oxalate as the acceptor, and aromatic esters function as acceptors in Entries 11 and 12. 

Esters of aromatic acids in aprotic solvents form radical-anions detected by cyclic voltammetry on a short time scale [144]. Ethyl benzoate has E° = -2,19 V V5. see [145], Follow-up reactions of radical-anions from methyl and ethyl benzoate result from protonation by extraneous water. rm-Butyl benzoate radical-anion undergoes very rapid cleavage of the alkyl-oxygen bond to give benzoate ion and rerf-butyl radical. 

The present procedure4 is an especially effective method for the synthesis of esters of aromatic acids and hindered tertiary alcohols or of acid-labile alcohols such as 2,2-diphenylethanol. The yields are excellent, and the reaction procedure is simple. The method is illustrated by the preparation of /-butyl p-toluate, a compound that could not be prepared by a conventional method0 of esterification involving the acid chloride and /-butanol in the presence of dimothylaniline. Examples of esters prepared by this method are illustrated in Table I. 

Although there is not a direct simultaneous consumption of the base, even this possibility was admitted, assuming that alkylation of the amide group by the ester may occur (23). This seems however not to be correct, as the alkylation of amides produced by esters is limited to esters of aromatic acids only (73). Nevertheless, the esters might be accounted for a loss of basicity from the point of view of condensation reactions producing water and acidic products. 

We have considered the addition of three radicals, Cl, H- and CH3, as models for bases. In the Ziegler-Natta reaction, Lewis bases involved are esters of aromatic acids and hindered amines. Bases are supposed either to interact with the support (MgCl2) or to coordinate with Ti [40, 41]. 

It is not possible to prepare biaryls containing a free carboxyl group directly by the diazo reaction. No biaryl is formed when (a) diazotized aniline and sodium benzoate, (b) diazotized anthranilic acid and aqueous sodium benzoate, or (c) diazotized anthranilic acid and benzene are used as components in the reaction.13 On the other hand, the reaction proceeds normally if methyl benzoate is used in reaction (a) or when methyl anthranilate replaces the anthranilic acid in (b) and in (c). The success of the diazohydroxide reaction appears to lie in the ability of the non-aqueous liquid to extract the reactive diazo compound from the aqueous layer.4 However, esters and nitriles can be prepared from esters of aromatic amino acids and cyanoanilines and also by coupling with esters of aromatic acids, and from the products the acids can be obtained by hydrolysis. By coupling N-nitrosoacetanilide with ethyl phthalate, ethyl 4-phenylphthalate (VIII) is formed in 37% yield. 

The C—C(=0)—O band of saturated esters, except for acetates, shows strongly in the 1210-1163 cm"1 region. It is often broader and stronger than the C=0 stretch absorption. Acetates of saturated alcohols display this band at 1240 cm"1. Vinyl and phenyl acetates absorb at a somewhat lower frequency, 1190-1140 cm"1 for example, see Figure 2.25. The C—C(=0)—O stretch of esters of a,/3-unsaturated acids results in multiple bands in the 1300-1160 cm"1 region. Esters of aromatic acids absorb strongly in the 1310-1250 cm"1 region. 

Cellulose esters, of aromatic acids, I, 320 of dibasic acids, I, 320 of higher aliphatic acids, I, 319 industrial applications, I, 322 mixed, I, 317... 

Finally, esters of aromatic acids cannot enolize but are less reactive than ordinary esters because of conjugation from the aromatic ring. These compounds may still be useful as we shall see. 

Esters of aromatic acids are used rather less frequently in this manner because they are considerably less reactive than carbonates or formates. This simple example works quite well—admittedly the ketone is very enolizable. 

Treatment of esters of aromatic acids with l-alkyl-2-pyrrolidones and l-methyl-2-piperidones is an useful method for the preparation of simple pyrrolines and piperidines, respectively. The l-alkyl-3-aroyl-2-pyrrolidones (110, n = 1) and l-alkyl-3-aroyl-2-piperidones (110, n = 2) thus formed are cleaved by the action of concentrated hydrochloric acid to give l-methyl-2-aryl-2-pyrrolines453 (111, n = 1) and l-alkyl-2-aryl-2-piperidines454 (111, n = 2), respectively. After blocking the secondary nitrogen atom in a lactam by means of acylation, one can prepare 2-aryl-1-pyrrolines (112, n = 1) and 2-aryl-1 -piperidines4 55-457 (112, n = 2). 

The only reported (59JOC1478) examples (142) of these compounds were synthesized by condensative cyclization of 2-hydrazino-l, 3-benzoselenazole (141) with aliphatic orthoesters or esters of aromatic acids. 

Esters of aromatic acids are not reduced by this procedure, so an aromatic COOH group can be reduced in the presence of a COOR group. However, it is also... 

Thiol esters of aromatic acids, ArC(0)SR, are cathodically reduced, with fission of the S-CO bond generating the arylcarbonyl radical and thiolate as in Eq. (25) [101]. The thiol may be obtained in good yield [102]—for example, L-cysteine in 83% yield from S-benzoyl-L-cysteine at a mercury cathode in DMF-water-tetramethylammonium chloride. The electroreduction of aryl thioesters [containing -C(0)SR, -C(S)OR, and -C(S)SR functions] has been studied, and some interesting products have been formed [103-105], as in Eqs. (26) through (29). Trithiocarbonates are reduced to the tetraalkyltetrathioethylenes [106]. 

Esters of aromatic acids and alcohols are usually on reduction cleaved to aryl car-boxylate and alkyl radical, which may be reduced to the anion and protonated to the hydrocarbon [43,44]. The influence of EGB should be considered when an acid is used as protecting group. Acyl derivatives of phenols as a rule cleaves between the carbonyl group and oxygen to phenolate and acyl radical, and may thus be used for protection of a phenol. The fate of the acyl radical is not clear a dimerization to a diketone has been suggested [45,46]. Such a ketone would be more easily reduced than the ester and possibly attacked by the EGB [47]. The radical anion of some easily reducible esters (e.g., esters of 4-nitrobenzoic acid) cleaves very slowly, but the dianion usually cleaves fast [46]. 

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