Triazoles heterogeneous copper catalysts

Other types of heterogeneous copper catalysts have also been developed for the azide-alkyne cycloaddition. Zhang et al. (2013) utilized porous copper(0) to facilitate the three-component reactions in water, and the desired 1,2,3-triazoles could be assembled in good to excellent yields, though long reaction times were usually required (at 55°C for 20-54 h). 

After the optimization of these conditions, by adding an azide to the input stream it was possible to synthesize a range of substituted triazoles in a heterogeneously catalysed three-component reaction (Scheme 18). After the CFC, the stream was passed through a column containing a resin-immobilized copper-based catalyst, which was used in a previous work by the same authors to successfully catalyze the formation of triazoles from alkynes and azides [44]. An immobilized thiourea-containing cartridge was subsequently used to remove any leached Cu catalyst. In a similar way as for the alkynes production, the series of resins was used to purify the product. 

Several heterogeneous catalysts have been shown to effect related multicomponent couplings. These include cross-linked polymeric ionic liquid material-supported copper (Cu-CPSIL), silica-dispersed CuO (CuO/Si02), and imidazolium-loaded Merrifield resin-supported copper (Cu-PSIL), all of which can be used in water at room temperature to arrive at 1,4-disubstituted-1,2,3-triazoles from alkyl halides, NaNs, and terminal alkynes. Each can be filtered and reused several times with minimal loss of efficacy. Multistep flow synthesis, specifically including generation of underused vinyl azides and their subsequent click conversions to vinyl triazoles, has also been reported. ... 

Insofar as heterogeneous Cu(I)-catalyzed click chemistry is concerned, new catalysts continue to appear. Nanoparticles of copper can be generated and used in aqueous solution, from which crystalline products often precipitate. Inclusion of copper (as CuCl) into Lewis acidic zeolites, using in particular Cu(I)-USY (pore size 6-8 A), is one such catalyst employed in toluene. Other solvents such as DCM, CH3OH, CH3CN, and benzene were not recommended, and yields of 1,4-disubstituted triazoles can be highly variable. 

Kale et al. (2012) reported a rapid and green approach to achieve 1,4-disubsti-tuted triazoles via three component reactions of alkynes and in situ generated azides in the presence of copper apatite (25% weight) as a heterogeneous and recyclable catalyst in aqueous media without using any base. The yield was same (99%) in both the methods, but it needs 1.5-6 h under conventional heating while it requires only 5-20 min in presence of microwave radiations. 

Cu nanoparticulates in Guar gum, Cu(0) nanoparticles stabilized on activated montmorillonite, copper supported on Si02 nanoparticles, PEI/Cu (polyethyleneimine/Cu) nanoparticles supported onto the Si02 surface, act as heterogeneous catalysts for the click reaction at room temperature in ethanol, water, H20 EtOH (v v=l l), DMSO, respectively, in the presence of tertiary amines or carboxylic acids in some cases. Triazoles are synthesized in good to excellent yields in rather short times. The catalysts may be easily recovered and reused up to the third or fourth run [51-53]. 

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