Advances in the Synthesis of Amides via Alpha Oxygenation of Amines

Abstract

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, amide is an important functionality in chemistry and biology and consequently synthesis of it 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 in to 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 which include Beckmann rearrangement, Schmidt reaction, Willgerodt-Kindler reaction, Passerini reaction and so on. Recently there is surge of revelations regarding alternate pathways to synthesize amides focussing into green or catalytic approaches. In this review we have covered several such alternate methods which use amines as the precursors. Oxidation and reduction are the back bone 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 potential to oxidize amines to amides but they were not studied earlier as green chemistry was not in much focus then. With present focus on sustainability and green chemistry, scientists have embarked to synthesise amides in greener way. One such way is 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 which have been used to make amides or have the potential for such transformation. Other than use of several oxidizing reagents, tandem oxidation amidation and other miscellaneous methods are included in this review. The reactions which give amides as by-products are also included as such reactions are potential methods to synthesize amides. Mechanisms are also included at relevant places. The review is classified within the report available for the Oxidations of amines using transition metals, transition metal salts and transition metal oxides; Oxidations of amines using non-metals; Photocatalytic oxidations of amines; Air oxidation of amines; Electrochemical oxidations; Enzymatic conversions; Oxidative coupling of Aldehydes; Oxidative coupling of Alcohols; Oxidative amidation of Methylbenzenes; and Oxidation of aromatic nitrogen heterocycles.