Homogeneous and Heterogeneous Approaches to 1,2,4-Triazine-Accelerated Copper-Catalyzed Azide-Alkyne Cycloadditions

Over the last decade, the domain of click chemistry has grown exponentially and has significantly impacted the fields of organic synthesis, medicinal chemistry, molecular biology, and materials science. The ideal model of a click reaction has become the copper-catalyzed azide-alkyne cycloaddition (CuAAC). Inherent limitations of CuAAC, including high temperatures, long reaction times, and difficult purifications, have been minimized by the development of nitrogen-based ligands. Herein, we present a novel application of 1,2,4-triazines by investigating their use as accelerants for CuAAC.

A diverse library of 1,2,4-triazines were synthesized in order to examine the molecular determinants of their catalytic activity. These ligands were found to be potent accelerants, at catalytic concentrations, in the presence of both copper(I) and copper(II) salts. Remarkably, these catalyzed reactions proceeded at room temperature, generating high isolated yields, in both polar and nonpolar solvents. 5,6-Diphenyl-3-(pyridin-2-yl)1,2,4-triazine was the most active ligand studied, producing an 89% yield in a model click reaction within one hour. Additional experiments with an array of azides and alkynes yielded similar results, defining a broad substrate scope for 1,2,4-triazines as catalysts for click chemistry.

Heterogeneous 1,2,4-triazines were designed using different solid supports and different sites of attachment with respect to the 1,2,4-triazine ligand. The primary advantages offered by these immobilized catalysts are the prevention of metal contamination in 1,2,3-triazole products and the recyclability of the catalyst. Results indicated that 1,2,4-triazine-functionalized silica was a more effective accelerant of CuAAC, whereas polystyrene-supported 1,2,4-triazines displayed modest activity. In coordination with copper(II), 1,2,4-triazines appended onto silica generated isolated yields greater than 90% after four consecutive reaction cycles with minimal copper leaching. Further research will utilize both homogeneous and heterogeneous 1,2,4-triazine-accelerated CuAAC in the derivatization of solid supports for energy-related chemical processes and in the synthesis of novel enzyme inhibitors.