Recent developments in aryl-aryl bond formation by transition metal-catalysed C-H activation

The formation of aryl-aryl bonds is an important transformation in organic chemistry. The moiety is commonly found in pharmaceutically relevant compounds as well as in advanced materials. The apparent structural simplicity of biaryls belies their preparative difficulty however, and it is usually necessary to prefunctionalise both aryl rings prior to the coupling event. The process of prefunctionalisation itself is not trivial. Moreover, after the coupling event, stoichiometric amounts of side products from both coupling partners must be disposed of. This inherently wasteful process has, up until very recently, been the most commonly used approach to the formation of biaryls. Of late a new methodological approach has been used whereby only one aryl ring is preactivated or indeed coupling occurs where neither of the coupling partners are preactivated. This is most commonly achieved by judicial choice of directing groups or regioselective coupling guided by the inherent electronics or sterics of the rings. Several transition metals can be used to mediate this process, with palladium the forerunner. This report will review the area of aryl-aryl bond formation (Jan 2006 to Feb 2008) orchestrated by C-H activation. In some cases discussion of the C-H activation event in isolation, is also warranted.