Selected applications to organic synthesis of intramolecular C-H activation reactions by transition metals

Cyclometallated compounds obtained via the well-known intramolecular C-H activation by transition metals of Nor Scontaining ligands may lead to organic heterocycles through reaction with alkynes. The mechanistic implications of the formation of both C-C and C-Y bonds are discussed. A reaction pathway involving insertion of the alkyne into the metalcarbon bond of the starting material followed by intramolecular addition of the Y atom to the resulting metallated-vinyl unit is proposed based on both the regioselectivity of the insertion and the existence of genuine isolated intermediates for the formation of the heterocyclic compounds. INTRODUCTION Transition metal complexes have been known to activate hydrocarbon C-H bonds for many decades and this aspect of their chemistry, whereby metal-carbon 0 bonds can be formed from a C-H bond, has been thoroughly investigated. The C-H activation reaction between an organic substrate and a transition metal complex can occur either inter(ref. 1) or intra-molecularly (ref. 2) as represented in the following scheme: The intramolecular process, also known as the cyclometallation reaction, would appear to be attracting less attention nowadays despite the fact that it was initially recognised as a major advance for the functionnalisation of unactivated C-H bonds that might be useful for organic synthesis. However, until recently no clearcut breakthrough has been achieved in this direction. One may however mention several interesting stoicheiometric reactions (carbonylation, vinylation, ...) of such compounds derived from the palladation of nitrogen containing ligands which have been reviewed recently (ref. 3a). Our contribution to this field concerns the reactions of cyclopalladated compounds derived mainly from the palladation of nitrogen containing ligands. Since the latter are by far the most common ligands, any new reaction that is discovered can be tested on a large selection of starting materials. 335