 However, with complicated systems, electronic difference between different C–H bonds can be subtle and electronic directing effect alone could become less synthetically useful.The fast development of C–H activation in the past few decades provides synthetic chemists with the powerful tools to synthesize functionalized aromatic compounds with high selectivity.The widely used approach to achieve "ortho"-selectivity involves metal-chelating directing groups, which forms a relatively stable 6- or 7-membered cyclic pre-transition state to bring the metal catalyst to the proximity of the "ortho-"hydrogen.