Cu on porous glass: An easily recyclable catalyst for the microwave-assisted azide–alkyne cycloaddition in water

Authors: Katharina Jacob, Achim Stolle, Bernd Ondruschka, Klaus D. Jandt, Thomas F. Keller

Abstract:
The propagation of the Click-reaction concept in synthetic chemistry, particularly the Cu-catalyzed cycloaddition between azides and alkynes, led to numerous applications in materials science, bio-, medicinal, and polymer chemistry. Often copper salts or complexes are applied as catalysts. In the present case those have been replaced by heterogeneous catalysts using porous glasses as support materials. Loading the supports with Cu using wet-impregnation technique led to Cu-loadings of 80% on theoretical basis, whereas other support materials revealed lower metal loads. Characterization ofthe catalyst morphology by SEMidentifiedCu-agglomerates atthe surface.A2 eV-shift ofthebinding energy oftheCu2pcore levels in the catalysts’ XPS spectra and the disappearance of satellite peaks led to the assumption that reduction of Cu(II) to Cu(I) or Cu(0) occurred during catalyst application. Indeed, working without a reducing agent (sodium ascorbate) resulted in decreased catalyst activity regarding the model reaction. The microwaveassisted cycloaddition of benzyl azide with phenylacetylene in water led to full conversion after 10 or 20 min at 120 or 100 ^◦C, respectively. Reaction is characterized by excellent regioselectivity forming the 1,4-triazole almost exclusively. Optimization of the reaction conditions with respect to time and catalyst loading affords maximal TOF >635 h⁻¹. Recycling studies revealed that up to four reapplications of the catalyst are possible without lost of activity.

Keywords:
Azide
Alkyne
Porous glass
Microwave
XPS

Published in: Applied catalysis A: General  (Volume 451, Pages 1-288, 31 January 2013)

Publisher: Elsevier

ISSN Information: 0926-860X

Cu on porous glass: An easily recyclable catalyst for the microwave-assisted azide–alkyne cycloaddition in water

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