Abstract
Amides are universal in nature. Proteins are polymers (polyamides) whose units are connected by amide (peptide) linkages. Proteins perform innumerable functions in the body. Important synthetic polymers (technology products) like nylon are also polyamides. Hence, the amide is an important element in chemistry and biology, and consequently, its synthesis has remained a focused research area. Many methods are available for the synthesis of amides. The classical methods involve making amides from carboxylic acids and amines. The energy unfavourable direct reaction between an acid and an amine is turned into a favourable pathway using coupling reagents. Coupling agents like DCC, HOBt, PyBOP, etc., are used. However, these reagents generate lots of waste. There are also other selective methods, including Beckmann rearrangement, Schmidt reaction, Willgerodt-Kindler reaction, Passerini reaction, and so on. There has been a recent flurry of revelations about alternative strategies to synthesize amides, with a focus on green or catalytic approaches. In this review, we have covered several alternate methods that use amines as the precursors. Oxidation and reduction are the backbone of synthetic organic transformations. Several elegant oxidizing agents have been developed for the oxidation of alcohols and olefins with selectivity in mind. However, many of these oxidizing agents have the potential to oxidize amines to amides, but they have not been studied earlier as green chemistry was not in much focus then. With the present focus on sustainability and green chemistry, scientists have embarked on synthesizing amides in a greener way. One such way to get amides in a cleaner way is to oxidize amines to amides. Hence, in this review, we have endeavoured to compile all such methods used to make amides or have the potential for such transformation. Other than the use of several oxidizing reagents, tandem oxidation amidation and other miscellaneous methods are included in this review. The reactions that give amides as by-products are also included as such reactions are potential methods to synthesize amides. Mechanisms are also included in relevant places. The review is classified as follows: Oxidation of amines using transition metals, transition metal salts, and transition metal oxides; Oxidation of amines using non-metals; Photocatalytic oxidation of amines; Air oxidation of amines; Electrochemical oxidation; Enzymatic conversion; Oxidative coupling of Aldehydes; Oxidative coupling of Alcohols; Oxidative amidation of Methylbenzenes; and Oxidation of aromatic nitrogen heterocycles.
Keywords: amide, synthesis, non-conventional methods, synthetic organic chemistry, MEDICINAL CHEMISTRY, ORGANIC TRANSFORMATION
Graphical Abstract
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