Mechanism of Click chemistry reveals its secrets

Click chemistry is not a specific reaction but rather chemistry tailored to generate substances quickly and reliably by joining small units together. It is meant to mimic nature which also generates substances by joining small modular units (eg, amino acids into proteins). The reactions are robust and easy to perform, and are widely employed to synthesise new pharmaceuticals, biological probes, new materials and other products. But precisely how it works had been unclear since its invention at The Scripps Research Institute (TSRI) more than a decade ago. Now, TSRI scientists have illuminated the mechanism at the heart of this useful process.

“These new findings allow us to exert greater control of the reaction and make it faster and more efficient under the most challenging conditions,” said chemist Valery Fokin, an associate professor at TSRI, who was principal investigator for the new study. “The reaction-tracking techniques we developed here also can be applied to the study of other complex processes, both chemical and biological.”

The report, which sheds light on the reaction known as copper-catalysed azide–alkyne cycloaddition (CuAAC), appears on 4 April 2013 in Science Express, the advance online edition of the journal Science, and in the 18 April 2013 issue of the journal.

Classic Click reaction

Fokin and his laboratory, and the laboratory of K. Barry Sharpless, a Nobel laureate and the W.M. Keck Professor of Chemistry at TSRI, reported the discovery of the CuAAC reaction in 2002. Danish researchers independently reported a similar reaction in the same year. The reaction involves the use of copper compounds to catalyse the linkage of two functional groups, a nitrogen-containing azide and a hydrocarbon alkyne, to make a stable five-membered heterocycle, 1,2,3-triazole. Azides and alkynes are small functional groups that can be easily introduced into a wide variety of structures using chemical or biological methods without interfering with normal biological processes.

The experimental simplicity and reliable performance of CuAAC under virtually all conditions, including in water and in the presence of oxygen, has made it a ‘go-to’ method whenever covalent stitching of small man-made molecules or large biopolymers is needed, exemplified by protein and nucleic acid labelling, in vitro and in vivo imaging, drug synthesis and the forging of complex molecular architectures with surgical precision.

“Despite its many uses, the nature of the copper-containing reactive intermediates that are involved in the catalysis had not been well understood, in large part due to the promiscuous nature of copper, which rapidly engages in dynamic interactions with other molecules,” said Fokin.

Previous studies had hinted that in the swirl of short-lived bondings and partings that occur during a given CuAAC reaction, not one but two copper-containing catalytic units - ‘copper centres’ - are needed to help build the new triazole structure. To confirm this, Fokin and two of his graduate students, Brady Worrell and Jamal Malik, tried to reproduce key steps of the CuAAC catalytic cycle with either one or two copper atoms available. Analysis of the reaction course by tracking the heat given off by each reaction as well as product yield indicated whether it worked efficiently. “By monitoring the reaction in real time, we showed that both copper atoms are needed and established the involvement of copper-containing intermediates that could not be isolated or directly observed,” said Worrell, who was the paper’s first author.

In a second set of experiments, Worrell, Malik and Fokin introduced a pure isotope of copper - which differs slightly in mass from the isotope blend found in natural copper - as one of the two copper centres so that they could track their respective fates during the reaction. “We hypothesised that the two copper centres would have distinct roles, but found unexpectedly that their functions during key steps in the reaction are effectively interchangeable,” said Malik.

New linkages

The research reveals the popular CuAAC reaction in unprecedented detail. In addition to the fundamental insights into the chemistry of copper and its interactions with organic molecules, the techniques will lead to better understanding of many chemical and biological processes involving copper. The current study also enables development of new reactions that exploit weak interactions of copper catalysts with carbon-carbon triple bonds. In fact, based on the new findings, Fokin and his team have begun to devise new reactions in which one copper centre can be replaced with a different element, to pursue complementary biocompatible and efficient techniques.

Funding for the study, Direct Evidence of a Dinuclear Copper Intermediate in Cu(I)-Catalysed Azide–Alkyne Cycloadditions, was provided by grants from the National Science Foundation (CHE-0848982) and the National Institute of General Medical Sciences at the National Institutes of Health (GM-087620).