Abstract
A set of Cu(I) complexes with 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo-[3.3.1]nonane (DAPTA) phosphine ligands viz. [CuX(κP-DAPTA)3] (1: X = Br; 2: X = I) and [Cu(μ-X)(κP-DAPTA)2]2 (3: X = Br; 4: X = I) were immobilized on activated carbon (AC) and multi-walled carbon nanotubes (CNT), as well as on these materials after surface functionalization. The immobilized copper(I) complexes have shown favorable catalytic activity for the one-pot, microwave-assisted synthesis of 1,2,3-triazoles via the azide-alkyne cycloaddition reaction (CuAAC). The heterogenized systems with a copper loading of only 1.5–1.6% (w/w relative to carbon), established quantitative conversions after 15 min, at 80 °C, using 0.5 mol% of catalyst loading (relative to benzyl bromide). The most efficient supports concerning heterogenization were CNT treated with nitric acid and NaOH, and involving complexes 2 and 4 (in the same order, 2_CNT-ox-Na and 4_CNT-ox-Na). The immobilized catalysts can be recovered and recycled by simple workup and reused up to four consecutive cycles although with loss of activity.
Highlights
The interesting properties of 1,2,3-triazole based heterocycles have been a focus of scientific interest due to their manifold potential to interact with diverse systems, from a synthetic point of view and in the context of biological and pharmacological applications [1]
To investigate the generality and versatility of 2_CNT-ox-Na and 4_CNT-ox-Na as catalysts for the CuAAC reaction to furnish the triazole derivatives, we explored the effect of substituted benzyl bromides using the aforementioned optimized experimental conditions (Table 3, entries 6 and 23)
A simple and reproducible synthetic method for recyclable heterogeneous catalysts based on Cu(I)-DAPTA complexes (1–4) on carbon materials (AC and carbon nanotubes (CNT)) was developed
Summary
The interesting properties of 1,2,3-triazole based heterocycles have been a focus of scientific interest due to their manifold potential to interact with diverse systems, from a synthetic point of view and in the context of biological and pharmacological applications [1]. The triazole ring system has been shown to be compatible with many functional groups and to exhibit good stability under several reaction conditions [2]. The copper catalyzed azide−alkyne cycloaddition approach (CuAAC) established by Meldal [3] and Sharpless [4] constitutes the most important protocol to obtain 1,4disubstituted triazoles in a completely regioselective manner. The presence of a significant amount of expensive copper complexes in the end products remains a main challenge hampering the utilization of the CuAAC reaction for large scale applications [7,8].
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