Figure 5. In a metal-free, strain-promoted click reaction, an azide-containing moiety (red) is reacted with a cyclooctyne-containing moiety (blue)

Figure 5. In a metal-free, strain-promoted click reaction, an azide-containing moiety (red) is reacted with a cyclooctyne-containing moiety (blue) to produce a triazole ring. No catalyst is required and release of ring-strain from the cyclooctyne provides the driving force for the reaction.