Metallacycles, direct arylation

In side-bound ruthenacyclobutane 7a, the metallacycle is positioned directly underneath the N-aryl substituent of the NHC. It is this close proximity to the N-aryl... 

More commonly, the transition metal-catalyzed coupling of aryl halides with nonfunctionalized aromatic substrates has been used for the preparation of unsymmetrical biaryl systems. Different strategies have been developed over the last few years. One very elegant approach utilizes directing groups on aromatic substrates. The lone pair of the requisite heteroatom coordinates to the transition metal and promotes the arylation reaction via intermediary formation of a five-or six-membered metallacycle. 

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