Transition-metal-catalyzed π-bond-assisted C - H bond functionalization: an emerging trend in organic synthesis.

Transition-metal-catalyzed C - H activation is considered to be an important tool in organic synthesis and has been accepted and widely used by chemists because it is straightforward, cost-effective, and environmentally friendly. A variety of functional groups have been used to direct metal complexes and achieve regioselective C - H activation. Most directing is achieved through the σ-bond coordination of functional groups to the metal catalyst, followed by ortho-selective C - H bond cleavage. However, recent work has demonstrated that π-coordinating functional groups can also assist in guiding metal complexes for site-selective C - H bond activation. This emerging approach significantly expands the scope of C - H activation reactions in organic synthesis. Herein, recent developments in this field are summarized.