Emerging approaches for the synthesis of triazoles: beyond metal-catalyzed and strain-promoted azide-alkyne cycloaddition

Metal-free 1,3-dipolar cycloaddition reactions have proven to be a powerful tool for the assembly of key heterocycles, in particular diversely functionalized 1,2,3-triazoles. A number of metal-free (3+2)-cycloaddition approaches have been developed up to date with the aim to circumvent the use of metal catalysts allowing these reactions to take place in biological systems without perturbation of the naturally occurring processes. This feature article specifically provides an overview of emerging metal-free synthetic routes, and their mechanistic features, in the formation of functionalized 1,2,3-triazoles. Organocatalytic click reactions to 1,4- and 1,5-disubstituted triazoles are rapidly emerging as most valuable alternatives to other triazole-forming protocols. Here the latest developments are discussed leading to intriguing synthetic possibilities.