Non-Classical Amide Bond Formation: Transamidation and Amidation of Activated Amides and Esters by Selective N-C/O-C Cleavage

In the past several years, tremendous advances have been made in non-classical routes for amide bond formation that involve transamidation and amidation reactions of activated amides and esters. These new methods enable the formation of extremely valuable amide bonds via transition-metal-catalyzed, transition-metal-free, or metal-free pathways by exploiting chemoselective acyl C-X (X = N, O) cleavage under mild conditions. In a broadest sense, these reactions overcome the formidable challenge of activating C-N/C-O bonds of amides or esters by rationally tackling n(N)-> pi*(C=O)delocalization in amides and n(O)-> pi*(C=O)donation in esters. In this account, we summarize the recent remarkable advances in the development of new methods for the synthesis of amides with a focus on (1) transition-metal/NHC-catalyzed C-N/C-O bond activation, (2) transition-metal-free highly selective cleavage of C-N/C-O bonds, (3) the development of new acyl-transfer reagents, and (4) other emerging methods.