Esters of tertiary alcohols

The acid chlorides are generally more reactive than the corresponding acid anhydrides. In fact, the alcoholysis of acid chlorides is probably the best laboratory method for preparing esters. Frequentiy, basic materials are added during the course of the reaction to neutralize by-product hydrochloric acid. When the basic material is aqueous caustic, the procedure is referred to as the Schotten-Baumann procedure (73). Esterification of tertiary alcohols by acid chlorides is described in Reference 74. Esters of tertiary alcohols can also be formed through an intermediate /-butyl thioate group (75) ... 

Reduction of esters by trichlorosilane in tetrahydrofuran in the presence of tert-butyl peroxide and under ultraviolet irradiation gave predominantly ethers from esters of primary alcohols, while esters of tertiary alcohols were cleaved to acids and hydrocarbons. Esters of secondary alcohols gave mixtures of ethers and acids/hydrocarbons in varying ratios. 1-Adamantyl trimethylacetate, for example, afforded 50-100% yields of mixtures containing 2-42% of 1-adamantyl neopentyl ether and 58-98% of adamantane and trimethylacetic acid [1033]. 

Carboxylic esters are produced by the addition of carboxylic acids to olefins, a reaction that is usually acid-catalyzed (by proton or Lewis acids182) and similar in mechanism to 5-4. Since Markovnikov s rule is followed, hard-to-get esters of tertiary alcohols can be prepared from olefins of the form R2C=CHR.183 When a carboxylic acid that contains a double bond in the chain is treated with a strong acid, the addition occurs internally and the product is a y- and/or a 8-lactone, regardless of the original position of the double bond in the chain, since strong acids catalyze double bond shifts (2-2).184 The double bond always migrates to a position favorable for the reaction, whether this has to be toward or away from the carboxyl group. Carboxylic esters have also been prepared by the acyloxymercuration-demercuration of olefins (similar to the procedures mentioned in 5-2 and 5-4).185... 

Esters of tertiary alcohols are cleaved so rapidly that the protonated esters cannot be observed, their solutions showing only the signals for the protonated acids and tertiary carbonium ions, even at — 80°C. 

The rate of hydrolysis of a carboxylic ester in strong sulphuric acid generally shows one of the three types of dependence on acid concentration illustrated in Fig. 1. The simplest behaviour, a continuous increase in hydrolysis rate with increasing acid concentration, is shown by esters of tertiary alcohols, which are hydrolyzed very rapidly even in moderately concentrated acid, and by phenol esters, which are somewhat less reactive, but are hydrolyzed much faster than esters of simple primary and secondary alcohols with above about 60% H2S04. Substituted phenyl acetates behave very much like the parent compound, the p-chlorophenyl ester being hydrolyzed at almost the same rate as the unsubstituted compound, while p-nitrophenyl acetate is somewhat less reactive at low acid concentrations, but more reactive in above 70% sulphuric acid. 

Tertiary alcohols, tertiary ethers, or carboxylic acid esters of tertiary alcohols can undergo El eliminations, but only in the presence of Bronsted or Lewis acids. Anyone who has prepared a tertiary alkoxide by a Grignard reaction and treated the crude reaction mixture with HC1 and obtained the alkene knows that tertiary alcohols can be converted into alkenes even with dilute hydrochloric acid. 

This reaction has wide application for the preparation of esters. The difficulties encountered in method 285 because of a reversible reaction are avoided. Esters of tertiary alcohols and phenols are best prepared in this way. The formation of tertiary halides from tertiary alcohols is prevented by carrying out the reaction in the presence of powdered magnesium or dimethylaniline which react with the hydrogen chloride as it is formed. The esterification of phenols is effected in the same manner. Magnesium or pyridine is added to combine with the hydrogen halide. Pyridine has replaced aqueous alkali formerly used for this purpose... 

Another method for the conversion of alkyl hahdes to carboxylic esters is treatment of a halide with nickel carbonyl Ni(CO)4 in the presence of an alcohol and its conjugate base. When R is primary, RX may only be a vinylic or an aryl halide retention of configuration is observed at a vinylic R. Consequently, a carbocation intermediate is not involved here. When R is tertiary, R may be primary alkyl as well as vinylic or aryl. This is thus one of the few methods for preparing esters of tertiary alcohols. Alkyl iodides give the best results, then bromides. In the presence of an amine, an amide can be isolated directly, at least in some instances. 

According to Menschutkin, the esters of tertiary alcohols are split up by water into acid and unsaturated hydrocarbons instead of being saponified into the corresponding alcohol and acid. Thus the amyl ester of acetic acid forms acetic acid and amylene according to the equation... 

Fig. 9.6. Scattergrams of the absolute value of C(0)-0-C-C torsion angle t vs. O-C-C angle a in acyclic esters found in the CSD release of July 1991. Exclusion criteria as in Figure 9.5 except that only structures with a reported mean error in C-C bond length as0.005 A were included. Top 74 esters of tertiary alcohols. Every fragment contributes three points to the scattergram. Middle 31 esters of secondary alcohols. Every fragment contributes two points to the scattergram. Bottom 395 esters of primary alcohols...

The ease of addition of organic carboxylic acids to a carbon-carbon double bond increases with activation of the latter by neighboring substituents and with the acidity of the acid. Esters of tertiary alcohols are formed with particular... 

Brewster and Ciotti Jr. prepare esters of tertiary alcohols by adding / -toluenesulfonyl chloride to the acid and the alcohol in pyridine an anhydride is formed as intermediate, which reacts at once with the alcohol. 

This method is always successful in replacing one primary alcohol residue by another or by the residue of a secondary alcohol of higher boiling point, but it cannot be used for preparation of esters of tertiary alcohols. Esters of tertiary alcohols can be prepared by Verley s method,865 namely, mutual transesterification of two carboxylic esters ... 

Mixed anhydrides can be obtained by those of the above methods in which acid chlorides are used. Further, formation of anhydrides from carboxylic acids and acetic anhydride involves mixed anhydrides as intermediates, and in some cases these can be isolated, e.g., the mixed formic acetic anhydride.977 This anhydride can replace the unstable formic anhydride itself, for only the reactive formyl group is transferred in its reactions this fact has been utilized in the preparation of numerous formic esters, which in turn can be transformed into esters of tertiary alcohols that are otherwise difficult of access (see page 376). 

Esters of tertiary alcohols may not be prepared from carboxylic acids containing acidic a-protons using this modified procedure, since deprotonation and subsequent condensation, competes. However, the use of stoichiometric 1,8-Diazabicyclo[5.4.0]-undec-7-ene as base has been shown to provide good yields of i-butyl esters even for acids with acidic a-protons (eq 3). This procedure was unsuccessful for pivaUc acid or for IV-acyl-a-amino acids. 

One limitation in substrate construction is common for both types of substrates the remaining hydrogen atom at the chiral center must not be replaced, since a,a,a-trisubstituted carboxylates and esters of tertiary alcohols are usually too... 

Why are the esters of tertiary alcohols prepared by reaction with acid chlorides rather than by the Fischer esterification method ... 

Predominantly racemic alcohol is formed by the Iqrdrolysis of optically active methylethylisohexyl carbinylacetate in an aqueous solution of dioxane. The optically active alcohol itself is very slowly racemized under these conditions (medium, acid). Esters of tertiary alcohols, for example, tert-butyl acetate in aqueous solutions, are hydrolyzed by the. 4al1 mechanism. This is related to a relative stability of terti-... 

The preparation of esters of tertiary alcohols is accomplished by the reaction of an olefin with a carboxylic acid. For example, isobutylene condenses with malonic acid to give 58-60% of di-/cr/-butyl malonate, with monoethyl maleate to give 53-58% of ethyl tert-butyl malonate, and with acrylic acid to give 47% tert- >vAy acrylate. 

In most cases, tertiary alcohols do not react. A few lipases and esterases have a larger active site and react slowly with esters of tertiary alcohols [13]. Esters of straight chain carboxylic acids (acetate, butyrate) are the best substrates. Esters with substituents at the a-carbon (relative to the carboxylic add) and esters of... 

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