A layered double hydroxide, a synthetically useful heterogeneous catalyst for azide−alkyne cycloadditions in a continuous-flow reactor

• Cu(II)Fe(III)-based-layered double hydroxide was prepared and fully characterized.
• The as-prepared material was highly active as catalyst in alkyne–azide cycloadditions.
• The catalytic capabilities were investigated in continuo under high-p/T conditions.
• No decrease of activity or destruction of the catalyst structure arose upon scaling-up.
• Formation of reversible lattice defects was responsible for the observed activity.

A Cu(II)Fe(III)-layered double hydroxide is shown to efficiently catalyze 1,3-dipolar cycloadditions of organic azides to alkynes leading to valuable 1,2,3-triazoles. The benefits of continuous processing were exploited for reaction optimization and synthesis, and the production of triazoles was realized in an inherently safe and scalable manner. It was established that the catalytic activity of the as-prepared material can be derived from in situ reduction of Cu(II), generating lattice defects containing the catalytically active Cu(I) species. It was verified that oxidative homocoupling of the alkyne component was responsible for the conversion of Cu(II) to Cu(I), but a cooperation of certain structural items of the hydrotalcite-like material could also contribute to the outstanding catalytic efficiency. After a successful gram-scale triazole synthesis under high-pressure/high-temperature conditions, no destruction of the catalyst structure was found, suggesting that the layered double hydroxide acted as a highly robust catalytic system.

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