**4. Ru(II)-catalyzed oxidative alkyne annulation reactions**

The chelating group assisted ruthenium(II)-catalyzed insertion of an internal alkynes into the *ortho*-CSp2-H bond, followed by an intramolecular cyclization with the directing heteroatom (particularly O and N atom) or insertion of the alkynes into the CSp2-H/heteroatom-H bonds is commonly known as oxidative alkyne annulation reaction. These oxidative alkyne annulation reactions provide an environmentally friendly approach and are popular for the synthesis of a verity of useful heterocyclic compounds via the formation of C-C and C-heteroatom bonds in a step economical fashion (**Figure 3**) [21]. The ruthenium(II)-catalyzed alkyne annulation reaction proceeds mainly via insertion of the *in situ generated* active ruthenium(II) complex into the *ortho*-CSp2-H bond thereby forming a ruthenacycle complex,

**Figure 3.** *Ru(II)-catalyzed oxidative alkyne annulation reactions.*

insertion of an internal alkyne partner and finally reductive elimination of the active ruthenium complex. Herein, the applications of the ruthenium(II)-catalyzed CSp2-H/N and CSp2-H/N-H oxidative alkyne annulation reactions leading to the construction of various *N*-heterocyclic molecules is highlighted.
