Hindered acids

In a series of organic acids of similar type, not much tendency exists for one acid to be more reactive than another. For example, in the replacement of stearic acid in methyl stearate by acetic acid, the equilibrium constant is 1.0. However, acidolysis in formic acid is usually much faster than in acetic acid, due to higher acidity and better ionizing properties of the former (115). Branched-chain acids, and some aromatic acids, especially stericaHy hindered acids such as ortho-substituted benzoic acids, would be expected to be less active in replacing other acids. Mixtures of esters are obtained when acidolysis is carried out without forcing the replacement to completion by removing one of the products. The acidolysis equilibrium and mechanism are discussed in detail in Reference 115. 

RCO2H = alkyl, aryl, hindered acids R = Et, rt- and 5-Bu, CH3SCH2. . . ... 

In the case of very hindered acids the yields are poor and formation of the symmetrical anhydride is observed. Useful selectivity can be achieved for a less hindered acid in the presence of a more hindered one. ... 

The reaction between acyl halides and alcohols or phenols is the best general method for the preparation of carboxylic esters. It is believed to proceed by a 8 2 mechanism. As with 10-8, the mechanism can be S l or tetrahedral. Pyridine catalyzes the reaction by the nucleophilic catalysis route (see 10-9). The reaction is of wide scope, and many functional groups do not interfere. A base is frequently added to combine with the HX formed. When aqueous alkali is used, this is called the Schotten-Baumann procedure, but pyridine is also frequently used. Both R and R may be primary, secondary, or tertiary alkyl or aryl. Enolic esters can also be prepared by this method, though C-acylation competes in these cases. In difficult cases, especially with hindered acids or tertiary R, the alkoxide can be used instead of the alcohol. Activated alumina has also been used as a catalyst, for tertiary R. Thallium salts of phenols give very high yields of phenolic esters. Phase-transfer catalysis has been used for hindered phenols. Zinc has been used to couple... 

Esterification of tertiary alcohols poses several problems and expensive catalysts, like dimethylamino pyridine, are recommended. While esterification/transesterification/hydrolysis involving primary and secondary alcohols has been reported both with chemocatalysts and biocatalysts, terf-alcohol based esters have not found success. Recent work of Yeo et al. (1998) reports successful results for /er/-butyl octonoate using a new strain of lipase. This is a significant finding as the production of esters based on fert-alcohols (and reciprocally with hindered acids) may well be possible with biocatalysts, avoiding expensive catalysts and allowing easier separation. 

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) ... 

The protonation of carbenes by acids has been studied by PAC. Using sterically hindered acids to prevent collapse of the ion pair, the heat of reaction can be obtained and used to determine the p as of the resultant carbocations (Fig. 4).56 The results indicate that deprotonation of the carbocations to generate carbenes should be possible with strong hindered bases. This method has only rarely been used in the literature.1-57... 

Reaction of the side chain hydroxyacetone in flumethasone (27-4) with periodic acid leads to cleavage of that function to give carboxylic acid (29-1) with the loss of the carbon atom at C-21. Further reaction of the very hindered acid group requires prior activation. Thus, acylation with diphenyl chlorophosphate leads to the mixed anhydride (29-2) this is not isolated, but treated immediately with methyl mercaptan. The product, tibecasone (29-3), is a quite effective topical anti-inflammatory agent [24]. Cleavage of the ester side chain would lead back to the inactive starting acid (29-1). 

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... 

Typical synthetic procedures include the reaction of alkyl halides with the silver salts of carboxylic acids, the reaction of carboxylate anions in alkali with an excess of a dialkyl sulphate, (especially dimethyl sulphate), and heating tertiary184 or quaternary ammonium salts of carboxylic acids. These routes are particularly valuable for the preparation of esters of seriously sterically hindered acids. For example, Fuson et al.iK made the methyl ester of 2,4,6-triethylbenzoic acid by heating the tetramethyl ammonium salt to 200-250°C, viz. 

Difficulty Esterifiable Acids. The sterically hindered acids, such as 2.6-disubstituted benzoic acids, cannot usually be cslerified by conventional means. Several esters of sterically hindered acids such as 2,4.6-triisopropylbcnzoic acid have been prepared by dissolving 2 g of the acid in 14-20 mL of 100% H SOj. After standing a few minutes at room temperature, when presumably the acylium cation is formed, the solution is poured into an excess of cold absolute methanol. Most of the alcohol is removed under reduced pressure, about 50 ml. of water is added, and the distillation is continued under reduced pressure to remove the remainder of the methanol. The organic mailer is extracted with ether and treated with sodium carbonate solution. The ester is then distilled. Yields of esters made in this manner are 57-81%. 

Bulky groups in the esterifying acid also hinder the reaction. A classic example is 2,4,6-trimethylbenzoic (mesitoic) acid, which cannot be esterified readily under normal conditions because the methyl groups ortho to the carboxyl group make the transition state for formation of the intermediate 10 less favorable relative to the starting acid than would be the case for less hindered acids, such as ethanoic acid ... 

Despite the huge structural diversity of known carboxylic acids, most of these are readily converted into esters or amides. Even sterically hindered acids, for example pivalic, triphenylacetic [1], or 2,6-disubstituted benzoic acids [1, 2], can be converted into suitable acylating reagents for alcohols or amines (Scheme 7.1). Esters of sterically demanding carboxylic acids can, alternatively, also be prepared by O-alkylation of the corresponding carboxylates [3, 4]. 

An unsymmetrical ketone may even lead to the generation of one enoiate in a selective fashion if the lack of symmetry is caused by the substitution patterns in the /3- rather than oc-posi-tions. The difference in the /3-positions may be due to the number or the kind of substituents there. This point is emphasized in Figure 13.13 with cyclohexanones that contain one (D) or two (A) /3-substituents. In this case, deprotonation occurs preferentially on the side opposite to the location of the extra substituent that is, the sterically less hindered acidic H atom reacts. 

Esterification.1 This reagent in combination with a catalytic amount of 4-dimethylaminopyridine (DMAP) is very effective for esterification of carboxylic acids with alcohols or thiols at room temperatures. However, reaction of aromatic and hindered acids requires several days at room temperature. French chemists report that only this method is useful for esterification of the protected baccatin III derivative (2) with (2R,3S)-N-benzoyl-0-(l-ethoxyethyl)-3-phenylisoserine (3) to provide the protected taxol derivative (4). A reaction conducted at 73° for 100 hours with 6 equiv. of 1 and 2 equiv. of DMAP produced 4 in 80% yield. Natural taxol, a cancer chemotherapeutic agent, is obtained by removal of the protective groups at C2 and C7 of 4. 

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