Transition-Metal-Catalyzed Annulations Involving the Activation of C(sp )-H Bonds.

The selective functionalization of C(sp )-H bonds using transition-metal catalysis is among the more attractive transformations of modern synthetic chemistry. In addition to its inherent atom economy, such reactions open unconventional retrosynthetic pathways that can streamline synthetic processes. However, the activation of intrinsically inert C(sp )-H bonds, and the selection among very similar C-H bonds, represent highly challenging goals. In recent years there has been notable progress tackling these issues, especially with regard to the development of intermolecular reactions entailing the formation of C-C and C-heteroatom bonds. Conversely, the assembly of cyclic products from simple acyclic precursors using metal-catalyzed C(sp )-H bond activations has been less explored. Only recently has the number of reports on such annulations started to grow. Herein we give an overview of some of the more relevant advances in this exciting topic.