Azide-alkyne cycloaddition by recyclable heterogeneous Cu catalysts on reverse phase silica gel

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Previously, we developed heterogeneous Pd catalysts immobilized onto functionalized reverse phase silica gel. These catalysts were applied to Suzuki cross coupling reactions and the hydrogenation reactions in water with good results. In this study, Cu metal was supported on reverse phase silica gel. Azide-alkyne cycloaddition was performed by using Cu(I) and Cu(II) catalysts immobilized onto reverse phase iminopropyl-functionalized silica gel (IPSi). The reverse phase silica gel possessed an end capped alkyl group yielding hydrophobicity. We anticipated that transport of the hydrophobic substrate towards the hydrophobic surface would enhance reactivity by bringing the substrate and catalyst in close proximity. This property would also enable the azide-alkyne cycloaddition reaction in water. Cu(I)@IPSi and Cu(II)@IPSi catalysts were synthesized; First, the 2-pyridinecarboxaldehyde ligand was anchored to a commercially available reverse phase 3-aminopropyl-functionalized silica gel (APSi). The complexation of [Cu(CH3CN)4]PF6 and CuSO4 with the resulting iminopropyl-functionalized silica gel (IPSi) subsequently followed, yielding Cu(I)@IPSi and Cu(II)@IPSi. Introduction of the 2-pyridinecarboxaldehyde ligand to the starting silica gel was confirmed via solid-state 13C NMR. The copper catalysts possessed an irregular shape based on scanning electron microscopy (SEM) images acquired primarily from the shape of the starting silica gel. Energy dispersive X-ray analysis (EDXA) further confirmed that Cu was indeed present. Copper loading values on the catalysts were determined via inductively coupled plasma (ICP); the results showed the loading values to be 0.498 mmol/g of Cu(I)@IPSi and 0.319 mmol/g of Cu(II)@IPSi. As a result of the Brunnauer-Emmet-Teller (BET) method, the catalysts possessed pore sizes of 6-8 nm similar to the starting silica gel and pore volumes of 0.332 cm3/g and 0.416 cm3/g for Cu(I)@IPSi and Cu(II)@IPSi, respectively. We optimized the reaction temperature, time and amount of copper catalysts on the basis of the reaction of benzyl bromide, sodium azide and phenyl acetylene. Both catalysts were effective and Cu(I)@IPSi is more reactive than Cu(II)@IPSi. Through the optimization of reaction, we considered various substrates for further investigation. These catalysts are air stable and reusable several times showing little or no change in the product yield. As another application, we are interested in the synthesis of polymeric binder in solid propellant, such as glycidyl azide polymer (GAP). Among various polymeric binders, GAP was regarded as a promising material in energetic thermoplastic elastomer (ETPE) because of a good compatibility with high energy oxidizers. In order to improve the chemical stability of GAP, the azide-alkyne cycloaddition reaction was conducted in the presence of Cu catalyst. However, quantitative functionalization of GAP, which means all azide groups convert to the triazole groups, makes it hard to handle in manufacturing process due to the higher glass transition temperature (Tg). So, we performed partial cycloaddition with azide groups in GAP and long chain alkyne using Cu(I)@IPSi. Consequently, we synthesized GAP-triazole copolymer which was characterized by 13C NMR.

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Journal: TechConnect Briefs
Volume: 2, Materials for Energy, Efficiency and Sustainability: TechConnect Briefs 2018
Published: May 13, 2018
Pages: 9 - 12
Industry sectors: Advanced Materials & Manufacturing | Energy & Sustainability
Topics: Catalysis
ISBN: 978-0-9975117-9-6