Esters from carboxylic acid salts

You can t make esters from carboxylic acids and alcohols under basic conditions because the base deprotonates the carboxylic acid (see p. 288). However, you can reverse that reaction and hydrolyse an ester to a carboxylic acid (more accurately, a carboxylate salt) and an alcohol. 

Formation of the 3j8-acetoxyeti-5-enic esters has been used to obtain optically pure samples of (+)- and (—)-tran5-verbenoP and to resolve an alcohol intermediate in the synthesis of the witchweed seed germination stimulant (+)-strigol. A general synthesis of thiol esters from carboxylic acids, exemplified by the formation of the n-propylthio-, isopropylthio-, and t-butylthio-esters of cholic acid, comprises reaction with diethyl chlorophosphate-triethylamine, followed by the thallium(i) salt of the appropriate thiol. 

The formation of esters from carboxylic acids by using Ar-alkyl-2-halopyridinium salts, such as 2-chloro-l-methylpyridine iodide (1) and alcohols, was first reported by Mukaiyama and, thus, the reaction is defined as the Mukaiyama esterification. The active ester (2) forms esters in good yield under mild conditions when subjected to alcohols. 

Olofsson and coworkers have developed a general and high-yielding synthesis of various diaryl ethers 681 using the reaction of diaryliodonium salts with phenols under basic conditions (Scheme 3.273) [874]. The scope of products includes bulky orf/io-substituted diaryl ethers, which are difficult to obtain by metal-catalyzed protocols. A similar procedure has been used for the metal-free synthesis of aryl esters from carboxylic acids and diaryliodonium salts [875]. 

Carboxylic acid trityl esters from carboxylic acids via tbeir salts... 

A-Acyl-4-pyridones can serve as useful acyl group transfer reagents for the esterification of alcohols in >80% yields.Two notable points about these reagents are their effectiveness in the preparation of formate esters (c/. ref. 157), and their much greater rate of reaction with primary alcohols, (c/. ref. 147). Thioesters and amides can also be prepared from such pyridones by reaction with thiols or amines, respectively. Other useful reagents for the direct formation of esters from carboxylic acids and alcohols are 2-fluoropyridinium salts in the presence of caesium fluorideand certain benzoisothiazole derivatives. Primary alcohols can be esterified with 2,2 -bipyridyl-6-yl carboxylates and CsF. Selective esterification of a primary alcohol in the presence of a secondary alcohol can be achieved in acceptable yields with this reagent (c/. ref. 144). 

Halogenopyridine methiodideJtri-n-butylamine o-Halogenocyclimmonium salts as condensing agents Carboxylic acid esters from carboxylic acids and alcohols... 

Synthesis of Esters from Carboxylic Acids, Esterification (eq 1) proceeds by activation of the carboxyl component using a 2-halopyridinium salt (2 X = Cl, Y = I or X = F, Y = OTs) and (1) (2,4 equiv) in the presence of an alcohol. The best solvents for this process are CH2CI2 or MeCN, but a range of other solvents may also be used. 

Acyl cations are now well-established chemical species. They can be prepared in quantity in solution, and several have been isolated as their crystalline salts. Recent papers have described their formation from carboxylic acids and esters under strongly acidic conditions81214, but they are most conveniently available from the reactions of acyl halides with L.ewis acids21 well-defined Lewis acid-base complexes are formed, which decompose in a second stage to the acyl cations21-23, viz. 

Monocationic acyl ions are readily prepared as persistent species in solutions of low nucleophile strength.68 These acyl ions have been thoroughly characterized by IR and NMR spectroscopy, and several acyl ion salts have been characterized by X-ray crystallography. The monocationic acyl ions are often prepared in situ from carboxylic acids, esters, or anhydrides, by the action of a strong Brpnsted acid, or the ions can be prepared from ionization of an appropriate acid halide with a strong Lewis acid. Both methods have been used to prepare acyl-centered dications, some of which can be considered distonic superelectrophiles. As described previously, dicarboxylic acids cleave to the bis-acyl ions in superacid (FSChH-SbFs) provided that the acyl cations are separated by at least three methylene units (eq 54).55 The first bis-acyl dications were reported by Olah and Comisarow, being prepared by the reactions of dicarboxylic acid fluorides with superacidic SbFs (eq 72).69... 

Sulphonic acids are usually esterified with diazomethane [121] by methods analogous to those used for the preparation of esters of carboxylic acids (see p. 54). If the reaction starts from the salts, it is necessary to convert them into acids either by direct acidification or with the aid of a cation-exchange resin in the H+ form. 

If we copy Nature rather more exactly, the Claisen ester condensation can be carried out under neutral conditions. This requires rather different reagents. The enol component is the magnesium salt of a malonate mono-thiol-ester, while the electrophilic component is an imidazolide—an amide derived from the heterocycle imidazole. Imidazole has a pK of about 7, Imidazolides are therefore very reactive amides, of about the same electrophilic reactivity as thiol esters. They are prepared from carboxylic acids with carbonyl diimidazole (CDI). 

Radical nucleophile oxidation based on one-electron oxidation, known as the Minisci reaction, is employed for the functionalization of /V-heterocycles with acidic hydrogen peroxide in the presence of iron(II) salts (Figure 3.112).472 A range of A-heterocycles (pyridines, pyrazines, quinolines, etc.) which are activated towards attack by nucleophilic radicals when protonated are suited to this chemistry. The Minisci reaction is suitable for the preparation of carboxylic amides (from formamide), carboxylic esters (from pyruvic esters via a hydroxyhydroperoxide), aldehydes (from 1,3,5-trioxane) and alkylated pyridines (either from carboxylic acids or from alkyl iodides in dimethyl sulfoxide).473 The latter reaction uses dimethyl sulfoxide as the source of methyl radical (Figure 3.112). 

The major subgroups of anionic surfactants include the alkali carboxylates (soaps), sulfates, sulfonates, and to a smaller degree, phosphates. The esterification of alcohol with sulfuric acid yields probably the best-studied surfactant, sodium dodecylsulfate or SDS. SDS, a sulfate ester, is an extremely effective emulsifier because of its high-electrostatic repulsion. Other sulfates are, for example, sulfated esters from fatty acids, sulfated ethers, and sulfated fats and oils. Sulfonates stem from the reaction of sulfonic acid with suitable substrates. Members of the class of sulfonates are, for example, sulfonic acid salts or aliphatic sulfonates. Other anionic surfactants include substances such as carboxylated soaps and esters of phosphoric acid. 

The six common groups derived from carboxylic acids are, in decreasing priority after carboxylic acids salts, anhydrides, esters, acyl halides, amides, and nitriles. 

In addition to carboxylic acid salts, other effective lubricity additives that the chemist will work with are esters of hydroxyfatty acids, non-ionic surface agents, dibasic acids, and additives from substituted fatty acids. Although this is not a complete list, its contents are the most effective from a cost perspective and are typically used in the industry. The oleate of ricinoleic acid dimer is an example of a hydroxyfatty acid prepared from ricinoleic acid oligomer with acid chloride in the presence of pyridine, as shown in Figure 2.4. 

---