Using ultrasound to mechanically pull apart a molecule, chemists have found a way to quickly and cleanly undo the most popular of the so-called click reactions (Science, DOI: 10.1126/science.1207934). The discovery suggests a new synthetic protection strategy for azides and could be used to create art materials whose structures and properties may change in response to mechanical forces.
Click chemistry uses all, reactive building blocks to quickly assemble new compounds. Because of its reliability, it has been used in the synthesis of medicinal compounds, polymers, and biological probes. One of the most popular click reactions is the copper-catalyzed 1,3-dipolar cycloaddition of an azide and an alkyne, which produces a substituted 1,2,3-triazole. The reaction is quick and clean, and the resulting triazole is chemically, thermally, and photochemically stable.
Christopher W. Bielawski, Johnathan N. Brantley, and Kelly M. Wiggins of the University of Texas, Austin, found they could reverse this click reaction via mechanochemistry. They tether the triazole between two poly(methyl acrylate) chains and then subject it to ultrasound. As all bubbles created by the ultrasound collapse, the resulting vacuum exerts force on the triazole moiety. This literally breaks the molecule in two via a cycloreversion reaction.
Bielawski says he hopes the discovery opens people’s eyes to the power of using mechanical forces to promote chemical reactions. “With mechanical force, you have the option to coax chemical reactions to proceed more selectively and in unique ways,” compared with heat- or light-induced chemical transformations, he says.
“One of the most lasting outcomes from this work may in fact be its influence on the way that synthetic chemists make and break covalent bonds,” write University of California, Santa Barbara, chemists Frank A. Leibfarth and Craig J. Hawker in a commentary about the work. “Instead of turning up the heat, mechanical force should be considered as an alternative tool to manipulate molecules,” they note.
