Transition-Metal-Catalyzed Cross-Coupling with Ketones or Aldehydes via N-Tosylhydrazones

Ketones and aldehydes play central roles in organic synthesis. There are numerous broadly used reactions that are related to the carbonyl reactivity, such as Grignard reactions, Wittig reactions, aldol reactions, and so on. In addition, the formation of enol triflates is a classic protocol that enables the ketones to be applied in transition-metal-catalyzed cross-coupling reactions, in which case the ketones are considered as the precursors of alkenyl electrophiles in the C-C bond-forming transformations. In the past decade, a new type of ketone- or aldehyde-based C-C bond-forming transformations has emerged. In this type of reactions, the ketones or aldehydes are first converted to their corresponding N-tosylhydrazones, which are employed as reaction partners in various transition-metal-catalyzed carbene-based cross-coupling reactions. The N-tosylhydrazone-based carbene couplings significantly enhance the potential of ketones and aldehydes in modern organic synthesis. This Perspective aims to give an overview of carbene coupling reactions with N-tosylhydrazones from the viewpoint of exploring new potentials of ketones and aldehydes in organic synthesis.