text: Copper-free click chemistry is a bioorthogonal reaction as a variant of an azide-alkyne Huisgen cycloaddition. By eliminating cytotoxic copper catalysts, the reaction proceeds without live-cell toxicity. It was developed as a faster alternative to the Staudinger ligation with the first generation of Cu-free click chemistry, producing rate constants over 63 times faster.
  Although the reaction produces a regioisomeric mixture of triazoles, the lack of regioselectivity in the reaction is not a major concern for its applications in bioorthogonal chemistry. More regiospecific and less bioorthogonal requirements are best served by the traditional Huisgen cycloaddition, especially given the low yield and synthetic difficulty of synthesizing a strained cyclooctyne (compared to the addition of a terminal alkyne).
  The bioorthogonality of the reaction has allowed the Cu-free click reaction to be applied within cultured cells, live zebrafish, and mice. 
  The absence of exogenous metal catalysts makes the Cu-free chemical reactions suitable for the in vivo applications of bioorthogonal chemistry or bioorthogonal click chemistry.