Abstract

In last few decades, the transition metal-catalyzed C-H bond activation and alkyne annulation reactions have turned out to be effective methods for the construction of highly important heterocycles. In particular, the Ru(II) catalysts have been used for the oxidative coupling between an internal alkynes and readily available nitrogen directed compounds in a rapid and sustainable manner. The Ru(II) catalysts are very much beneficial due to their stability in both air and water, ease of preparation, inexpensive than those of Rh(III) and designer Co(III) catalysts usually used for alkyne annulation reactions, requirement of mild reaction conditions, and compatible with various oxidants. Owing to these advantages of Ru(II) catalysts herein, we attempt to highlight the recent development in C-H activation and annulation reactions, which lead to the formation of several important N-heterocycles.

Highlights

  • The development of highly efficient methods for the synthesis of N-heterocyclic skeletons is one of the important targets in organic synthesis

  • In 2014, Wang and co-workers reported an efficient access to various imidazo[2,1-a]isoquinolines via a ruthenium(II)-catalyzed oxidative alkyne annulation reaction of 2-phenylimidazole in the presence of parabenzoquinone as the oxidant (Figure 33) [52]

  • The development of methodologies introducing multiple C-H/N-H activation is an emerging area of research and the oxidative alkyne annulation reactions allowing for the formation of C-C and C-N bonds in a single step

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Summary

Introduction

The development of highly efficient methods for the synthesis of N-heterocyclic skeletons is one of the important targets in organic synthesis. 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. Cheng and co-workers developed a one pot three-component reaction for the synthesis of isoquinolinium salts from benzaldehydes, amines, and alkynes using Ru(II)-catalyst via C-H bond activation and annulation (Figure 9) [28] In this reaction, the active Ru(II) complex first coordinates with the nitrogen atom of the in situ generated imine followed by ortho-CH activation forming a five membered ruthenacycle, this is followed by an alkyne insertion, reductive elimination of the ruthenium to afford the isoquinolinium salts and reoxidation to the active Ru(II) by Cu (BF4) allows its further participation in the catalytic cycle. A subsequent alkyne insertion and reductive elimination of the ruthenium affords various N-hetreocyclic molecules pyrroles, 2-pyridones, indoles, isoquinolines/isoquinolones derivatives, and various others π-conjugated polycyclic N-heteroaromatic molecules in a step-economical fashion via C-C and C-N bond formation [35–58]

Synthesis of pyrroles via C-H/N-H alkyne annulation i
Synthesis of 2-pyridones via C-H/N-H alkyne annulation
Synthesis of Isoquinolines/Isoquinolones via C-H/N-H alkyne annulation i
Conclusion
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