**6. Ru(II)-catalyzed C-H activation and oxidative alkyne annulation reactions for the synthesis of** *N***-heterocycles**

In the chelation-assisted Ru(II)-catalyzed C-H bond activation the nitrogencontaining directing groups have been consistently used for the reaction with internal alkynes to access *N*-heterocycles through the formation of C-C and C-N bonds respectively [26–34]. In this annulation processes the lone pair of nitrogen atom directs the active ruthenium complex to get inserted into the *ortho*-C-H bond, thereby forming a cyclic ruthenium complex. This cyclic ruthenium complex on subsequent alkyne insertion and finally reductive elimination of the active Ru(II) catalyst left the nitrogen atom becomes part of the final cyclic product.


*Access to* N*-Heterocyclic Molecules* via *Ru(II)-Catalyzed Oxidative Alkyne… DOI: http://dx.doi.org/10.5772/intechopen.95987*

#### **Figure 8.**

*Ruthenium(II)-catalyzed synthesis of substituted Isoquinolines.*

(**Figure 10**) [29]. A possible mechanism involving pyridine assisted vinylic *ortho*-C-H activation of 2-vinylpyridines, followed by an alkyne insertion and reductive elimination is proposed.

v. In 2014, Ackermann *et al.* effectively used carboxylate-assisted cationic Ru (II) complex for the synthesis of exo-methylene-1,2-dihydroisoquinolines via imine-assisted C-H bond activation and oxidative alkyne annulation reaction of ketimines with alkynes (**Figure 11**) [30]. This C-H bond functionalization proceeded with excellent chemo-, site-, and regioselectivity under an ambient atmosphere of air. The mechanistic studies were indicative of a reversible C-H bond ruthenation step followed by tautomeraization and migratory insertion of the alkyne.

**Figure 9.** *Ruthenium(II)-catalyzed synthesis of Isoquinolinium salt.*

**Figure 10.** *Ruthenium(II)-catalyzed synthesis of Quinazoline salt.*


*Access to* N*-Heterocyclic Molecules* via *Ru(II)-Catalyzed Oxidative Alkyne… DOI: http://dx.doi.org/10.5772/intechopen.95987*

**Figure 11.** *Ruthenium(II)-catalyzed synthesis of Exo-methylene-1,2-dihydroisoquinolines.*

#### **Figure 12.**

*Ruthenium(II)-catalyzed synthesis of Isoquinoline-2(1*H*)-ones.*

syntheses of ferrocenated isoquinolones in water as a sustainable reaction medium (**Figure 13b**).

viii. In 2015 wang *et al.* Reported a ruthenium(II)-catalyzed dehydrative [4 + 2] cycloaddition between enamides and alkynes for the construction of a highly substituted pyridines (**Figure 14**) [33]. Herein, instead of the N atom, the carbonyl group of the enamide coordinated to the Ru center to

**Figure 13.** *Ruthenium(II)-catalyzed synthesis Isoquinolines/Isoquinolones.*

#### **Figure 14.**

*Ruthenium(II)-catalyzed substituted pyridine synthesis.*

direct the C-H activation to generates a six-membered ruthenacycle intermediate. Then the alkyne is inserted into the Ru-C bond giving rise to an eight-membered ruthenacycle intermediate and finally afforded the pyridine analogue through a dehydration path with excellent regioselectivities.

*Access to* N*-Heterocyclic Molecules* via *Ru(II)-Catalyzed Oxidative Alkyne… DOI: http://dx.doi.org/10.5772/intechopen.95987*

**Figure 15.** *Ruthenium(II)-catalyzed synthesis of Quinoxalinium salts.*

ix. In 2019 our group reported a Ru(II)-catalyzed synthesis of highly luminescent quinoxalinium salt *via* quinoxaline *N*-directed oxidative annulation of 2-arylquinoxalines with an internal alkyne in the presence of a Cu(OAc)2H2O via the formation of C-C and C-N bonds (**Figure 15**) [34].
