Copper nanoparticles on controlled pore glass (CPG) as highly efficient heterogeneous catalysts for “click reactions”

We herein report that supported copper nanoparticles (CuNPs) on commercially available controlled pore glass (CPG), which exhibit high mechanical, thermal and chemical stability as compared to other silica-based materials, serve as a useful heterogeneous catalyst system for 1,3-dipolar cycloadditions (“click” reactions) between terminal alkynes and organic azides under green chemistry conditions. The supported CuNPs-CPG catalyst exhibited a broad substrate scope and gave the corresponding triazole products in high yields. The CuNPs-CPG catalyst exhibit recyclability and could be reuced multiple times without contaminating the products with Cu.

starting materials and Dimethyl propargylmalonate, were purchased from Aldrich and used as received. For TEM characterization, the Cu nanocatalyst was crushed and diluted with ethanol, and a drop from this suspension was placed on a formvar/carbon coated copper grid and dried under argon. The sample was examined on JEOL JEM 2100F microscope. SEM was performed on JEOL JSM 7401F equipped with cold FEG.
Elemental analysis was performed with was performed with inductively coupled plasmaoptical emission spectrometry (ICP-OES) by Medac Ltd. (UK) on a Varian Vista MPZ for the quantification of copper. The XPS spectra were collected on a Kratos Axis Ultra DLD electron spectrometer using monochromated Al Ka source operated at 150 W. Analyser pass energy of 160 eV for acquiring wide spectra and a pass energy of 20 eV for individual photoelectron lines were used. The surface potential was stabilized by the spectrometer charge neutralization system. The binding energy scale was referenced to the Si 2p line of silica, set at 103.3 eV. Powder sample for the analysis was gently handpressed into a pellet directly on a sample holder using clean Ni spatula. The spectra was processed using the Kratos software.

Experimental Procedures
Preparation of Cu(I/II)-Amp-CPG nanoparticles: To a suspension of amine functionalized CPG (1.0 g, 1 equiv. amine content) in deionized water (25 mL, pH 9), was added a suspension of copper(II) trifluoromethanesulfonate (Cu(OTf) 2 , 0.3 g, 2 equiv.) in deionized water (20 mL, pH 9) at room temperature. After stirring for 24 h, the formed Cu(II)-AmP-CPG was transferred to a centrifuge vial (50 mL) and was washed with In the next step, the dry Cu(II)-AmP-CPG was suspended in deionized water (35 mL) and NaBH 4 (20 equiv.) in deionized water (15 mL) was added slowly at room temperature.
After stirring for 45 min, the resulting Cu(I/II)-AmP-CPG nanocatalyst was transferred to a centrifuge vial (50 mL) and was washed with deionized H 2 O (3 × 35 mL) and acetone (3 × 35 mL), using centrifuge technique. The washed Cu(I/II)-AmP-CPG 1 was collected by decantation and dried for 48 h under reduced pressure. The total Cu-content was 3.3 wt% as determined by elemental analysis.

Procedure for the preparation of starting materials
In a round bottom flask, 3.5 mmol (0.443 g, 3.5 mmol) of p-methoxyphenol and anhydrous potassium carbonate (1.5 g, 10.8 mmol) were dissolved in 8.0 mL anhydrous DMF. The mixture was heated at 60 o C and stirred for 2 hours under N 2 . Next, the mixtures was cooled down to room temperature, and a solution of propargyl bromide (0.650 g, 4.3 mmol, 80% in toluene) was added portion-wise to the solution. The mixture was kept stirring at room temperature for 18 h. Next, the mixture was poured into an ice water and extracted with EtOAc. The organic phase was washed with NaOH 5%, water, brine and dried over Na 2 SO 4 . Evaporating toluene afforded yellowish oil (0.400 g, 2,47 mmol, 70 %).   5 2-propynyl-2,3,4,6-tetra-O-acetyl-β-D-glucopyranoside (3g) was synthesized through the method described in the literature. 5 First, D-glucose-penta-acetate was synthesized via following procedure: in a 150 ml flask containing 35 mL of acetic anhydride, dextrose (5 g) was added and the reaction mixture was heated to 50 o C. Anhydrous sodium acetate (2.5 g) was added and the mixture was stirred for 2 h. The temperature was increased to 90 o C and the reaction was continued for 3 h. Then, it was cooled and poured with stirring onto 200 mL of ice water. After 3 h, the crystalline material was filtered and crystallized from hot methanol. The product is further purified by recrystallization. A suspension of D-glucose-penta-acetate (1.5 g, 3,85 mmol) in dry dichloromethane (30 mL) at 0 o C was treated with propargyl alcohol (0.27 mL, 4.6 mmol) and BF3·Et2O (0.72 ml, 5.7 mmol). Next, the reaction temperature was increased to room temperature and the reaction mixture were stirred for additional 3.5 h. Next, anhydrous K 2 CO 3 (0.75 g) was added to the mixture and stirred for 30 min at room temperature. The solid were filtered off and washed with dichloromethane. The filtrate (organic phase) was washed with H 2 O (2 x 15 mL). The water phase was extracted with dichloromethane. Next, combined organic phase (dichloromethane) washed with brine and dried over Na 2 SO 4 . Next, the solvent was evaporated under reduced pressure to yield a solid which was crystallized in dichloromethane-petroleum ether affording a white solid (1.19 g, 3.1 mmol, 80 % yield).  6 4-(propargyloxy)-coumarin (3h) was synthesized through the method described in the literature.

Typical procedure for 1,3-dipolar cycloaddition
A microvavevial (6-mL) with a magnetic stir bar was charged with the azide compound