Metal-Catalyzed Annulations through Activation and Cleavage of C-H Bonds.

The exponential increase in the number of catalytic transformations that involve a metal-promoted activation of hitherto considered inert C-H bonds is promoting a fundamental change in the field of synthetic chemistry. Although most reactions involving C-H activations consist of simple functionalizations or additions, recent years have witnessed an upsurge in related transformations that can be formally considered as cycloaddition processes. These transformations are particularly appealing from a synthetic perspective because they allow the conversion of readily available substrates into highly valuable cyclic products in a rapid and sustainable manner. In many cases, these annulations involve the formation of metallacyclic intermediates that resemble those proposed for standard metal-catalyzed cycloadditions of unsaturated precursors.