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Published online: 10 January 2008 | doi:10.1038/nchina.2008.4

Bond activation: A hazard-free approach

Felix Cheung

Introduction

© (2007) Wiley Interscience

Organic chemists are constantly searching for ways to transform C–H bonds, especially aromatic ones, into other functional groups for building complicated compounds. Zhangjie Shi and co-workers at Peking University in Beijing¹ have come up with an environmentally friendly approach to selectively activate aromatic C–H bonds. This technique allows them to build carbazoles, the key structural units in many biologically active molecules and synthetic optical materials.

Carbazoles are biaryl molecules, which consist of a pair of aromatic rings (aryls) — one ring is the mirror (chiral) image of the other — linked on either side of a five-membered nitrogen-containing ring. Biaryls are typically constructed through transition-metal-catalysed coupling reactions, in which aromatic C–H bonds are activated by transition metal or halide-based catalysts. Palladium-catalyst reactions provide an alternative approach that avoids the use of these harmful metals and halides, but controlling which C–H bonds of the chiral ring to activate is difficult.

The researchers overcame this problem by firstly building a library of aryl intermediates, the C–H bonds of which had been functionalized with acetanilides through palladium-catalysed coupling reactions. The acetanilides act as directing groups for any two types of aryl intermediates to make biaryl C–C bonds through further palladium-catalysed coupling reactions (see image).

The approach will assist the development of a diverse library of carbazoles with different functionalities from commercially available acetanilides and common aromatic compounds.

The authors of this work are from:
Beijing National Laboratory of Molecular Sciences (BNLMS), PKU Green Chemistry Centre and Key Laboratory of Bioorganic Chemistry and Molecular Engineering, Ministry of Education, Peking University, Beijing, China; College of Chemistry, Peking University, Beijing, China; State Key Laboratory of Organometallic Chemistry, Chinese Academy of Sciences, Shanghai, China.

Reference

1. Li, B. J., Tian, S. L., Fang, Z. & Shi, Z. J. Multiple C–H activations to construct biologically active molecules in a process completely free of organohalogen and organometallic components. Angew. Chem. Int. Edn doi: 10.1002/anie.200704092 (2007). | Article |