Low Molecular Weight Esters
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Ester is obtained by an esterification reaction of an alcohol and a carboxylic acid. Esterification is the process of combining an organic acid (RCOOH) with an alcohol (ROH) to form an ester (RCOOR) and water; or a chemical reaction resulting in the formation of at least one ester product.
General methods of preparation of esters
➢ Fischer Esterification
Fischer esterification is an organic reaction which is employed to convert carboxylic acids in the presence of excess alcohol and a strong acid catalyst to give an ester as the final product. This ester is formed along with water.
The main disadvantage of direct acylation is the unfavorable chemical equilibrium that must be remedied (e.g., by a large excess of one of the reagents), or by the removal of water (e.g., by using Dean-Stark distillation, anhydrous salts, molecular sieves, or by using a stoichiometric quantity of acid catalyst).
Fig. 1 Fischer esterification
➢ Steglich Esterification
The Steglich esterification is a variation of an esterfication with dicyclohexylcarbodiimide as a coupling reagent and 4-dimethylaminopyridine as a catalyst. The reaction was first described by Wolfgang Steglich in 1978. It is an adaptation of an older method for the formation of amides by means of DCC (dicyclohexylcarbodiimide) and 1-hydroxybenzotriazole (HOBT).
Fig. 2 Steglich esterification
➢ Shiina Esterification
Shiina esterification is an organic chemical reaction that synthesizes carboxylic esters from nearly equal amounts of carboxylic acids and alcohols by using aromatic carboxylic acid anhydrides as dehydration condensation agents. In 1994, Prof. Isamu Shiina reported an acidic coupling method using Lewis acid, and in 2002, a basic esterification using nucleophilic catalyst.
Fig. 3 Shiina esterification using Lewis acid catalyst
➢ Corey-Nicolaou Macrolactonization
Corey-Nicolaou macrolactonization is a named reaction of organic chemistry, for the synthesis of lactones from hydroxy acids, found in 1974. The reaction should take place in a polar aprotic solvent with mild conditions, with the use of 2,2'-Dipyridyldisulfide and triphenylphosphine.
Fig. 4 Corey-Nicolaou macrolactonization
➢ Yamaguchi Esterification
The Yamaguchi esterification is an organic reaction used to convert a carboxylic acid and an alcohol to an ester using triethylamine, the Yamaguchi reagent, and DMAP. The mechanism begins with deprotonation of the carboxylic acid by Et3N to form a carboxylate anion which then attacks the Yamaguchi reagent. The resultant acid anhydride gets attacked by DMAP to form a better leaving group which is then displaced by the alcohol reagent. A final deprotonation step results in the ester product.
Fig. 5 Yamaguchi esterification
➢ Johnson-Claisen Rearrangement
The Johnson-Claisen rearrangement is an organic reaction where an allylic alcohol is heated with trialkyl orthoacetate under midly acidic conditions to produce a γ,δ-unsaturated ester. The reaction begins with protonation of one of the alkoxide groups of the orthoacetate. The protonated alkoxide is then released as a molecule of alcohol and forms an oxonium cation which is then attached by the alcohol. Proton transfer steps then follow, protonating another alkoxide group, that is again lost as alcohol and forms another oxonium cation. A deprotonation step then produces a 1,5 diene intermediate which undergoes a Claisen sigmatropic rearrangement to provide the final γ,δ-unsaturated ester product.
Fig. 6 Johnson-Claisen rearrangement
➢ Others
In addition to the synthesis methods listed above, esters can also be prepared in the following ways: Tishchenko reaction, Baeyer–Villiger oxidation, Pinner reaction, Roskamp reaction, Fétizon oxidation, transesterification, and from acid anhydrides, acyl chloride, etc.
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