Alane ester acids

Decomposition of the reaction mixtures with water followed by dilute acids applies also to the reductions with boranes and alanes. Modifications are occasionally needed, for example hydrolysis of esters of boric acid and the alcohols formed in the reduction. Heating of the mixture with dilute mineral acid or dilute alkali is sometimes necessary. 

In dicarboxylic acids with one free and one esterified carboxyl group the free carboxyl only may be reduced with hydrides (alanes, boranes). The monoethyl ester or adipic acid was converted by an equimolar amount of borane in tetrahydrofuran at —18° to 25° to ethyl 6-hydroxyhexanoate in 88% yield [971] Procedure 19, p. 209). 

Alane (AIH3) and its derivatives have also been utilized in the reduction of carboxylic acids to primary alcohols. It rapidly reduces aldehydes, ketones, acid chlorides, lactones, esters, carboxylic acids and salts, tertiary amides, nitriles and epoxides. In contrast, nitro compounds and alkenes are slow to react. AIH3 is particularly useful for the chemoselective reduction of carboxylic acids containing halogen or nitro substituents, to produce the corresponding primary alcohols. DIBAL-H reduces aliphatic or aromatic carboxylic acids to produce either aldehydes (-75 °C) or primary alcohols (25 C) Aminoalu-minum hydrides are less reactive reagents and are superior for aldehyde synthesis. ... 

AIH3 and alkyl derivatives are powerful Lewis acidic reducing agents that rapidly convert carboxylic esters into primary alcohols. In many cases, the reduction may be stopped at the intermediate aldehyde stage, providing that the stoichiometry and temperature are carefully controlled. However, complete reduction is experimentally easier to carry out. DIBAL-H is the most useful and widely used alane. 

Aromatic and aliphatic A,iV-dialkyl carboxylic acid amides, carboxylic esters, and free carboxylic acids are converted into aldehydes by diisobutyl-alane if reduction is carried out at low temperatures (0°C or — 70°C) 269, 279, 280, 286) ... 

DIBAL is an alane its structure is shown in the margin. Its chemistry is in many ways like borane—it exists as a bridged dimer, and it becomes a reducing agent only after it has formed a Lewis acid-base complex, so like borane it too reduces electron-rich carbonyl groups most rapidly. DIBAL will reduce esters even at -70 °C, and at this temperature the tetrahedral intermediate, formed by the transfer of hydride from aluminium to carbon (and shown below), may be stable. Only in the aqueous work-up does it collapse to the aldehyde. This step also destroys any excess DIBAL so no further reduction is possible. 

Esterification s. Carboxylic acid esters from carboxylic acids, Redoxesterification Esters, active s. Carboxylic acid esters, active Etherates s. Alane etherates Ethers (s. a. Alkoxy...,... 

An ester is the most common protecting group for the COOH unit of an amino acid (see reactions that form amides in Chapter 20, Section 20.6). Methyl or ethyl esters are prepared most of the time, but benzyl, p-nitrobenzyl, and er -butyl esters are also used. In Section 27.3.3, an ester was prepared by adjusting the pH of an amino acid followed by reaction with HCl and an alcohol. In acidic solution, alanine (85) reacts with ethanol (HCl is the acid) to give ethyl alanate, 86. Methyl, ethyl, and benzyl esters are prepared this way. 

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