An Update of N-Tosylhydrazones: Versatile Reagents for Metal-Catalyzed and Metal-Free Coupling Reactions

N-Tosylhydrazones have had widespread application in organic synthesis for more than a half century. In most of cases, N-tosylhydrazones, as masked diazo compounds, have been generally used in a series of important carbon–carbon and carbon–heteroatom bond-forming reactions. This review provides an update on progress in diverse coupling reactions of N-tosylhydrazones since 2012. The examples selected are mainly categorized by metal-catalyzed and metal-free systems, wherein four main types of transformations including insertion, olefination, alkynylation, and cyclization are discussed for each system. 1 Introduction 2 Transition-Metal-Catalyzed Coupling Reactions 3 Metal-Free Coupling Reactions 4 Conclusion and Outlook