Copper-Nanoparticle-Catalyzed A³ Coupling via C-H Activation

Abstract

Recyclable metal nanoparticles provide an efficient, economic, and novel route for the synthesis of propargylamines via three-component A³ coupling reaction of aromatic aldehyde, amine and alkyne. This method provides a wide range of substrate applicability. This protocol avoids the use of heavy metal, co-catalyst and gives propargylamines in quantitative yield.

metal nanoparticles - propargylamines - heterogeneous catalysis - cyclic secondary amines - one-pot multicomponent reaction

References and Notes

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Preparation of Cu Nanoparticles: A chemical method involving the reduction of Cu²⁺ ions to Cu(0) in a reverse micellar system was employed to prepare the Cu nanoparticles. Poly(oxyethylene)(tetramethyl)phenyl ether, commercially known as Triton X-100 (TX-100) was used as surfactant in the process. To a reverse micellar solution of CuSO_{4 (aq)}, another reverse micellar solution of N₂H_{2 (aq)} was added with constant stirring. In the presence of N₂ atmosphere the resulting solution was further stirred for 3 h to allow the complete Oswald ripening (particle growth). The Cu nanoparticles were extracted using anhyd EtOH followed by centrifugation. By varying the H₂O content parameter W_o (defined as the molar ratio of water to surfactant concentration, W_o = [H₂O]/[surfactant]) the size of nanoparticles could be controlled. The nanoparticles prepared were spherical in shape, with an average size of 18 ± 2 nm as confirmed by TEM photograph and QELS data. The metallic nature of the Cu(0) nanoparticles was confirmed by a characteristic UV absorption of the particles dispersed in cyclohexane (580 nm).
Typical Procedure A 50-mL round-bottomed flask was charged with aromatic or heterocylic aldehydes 1a-h (1 mmol), secondary amine (1 mmol) and phenylacetylene (1.5 mmol) in MeCN (5 mL) and the contents of the flask were stirred under a nitrogen atmosphere followed by the addition of Cu nanoparticles (15 mol%, 18 ± 2 nm). The resulting solution was refluxed at 100-110 °C for the appropriate time mentioned in Table [4] . The extent of reaction was monitored by TLC. After completion of the reaction, the reaction mixture was centrifuged at 2000-3000 rpm, at 10 °C for 5 min. The organic layer was decanted and the remaining Cu nanoparticles were reused for further reactions. The organic layer was dried over anhyd Na₂SO₄ and the solvent was removed in vacuo. The crude product was subjected to purification by silica gel column chromatography using a mixture of 15% EtOAc, 5% MeOH and 80% PE as eluent to yield the propargylamine 3a-h. The structures of all the products were unambiguously established on the basis of their spectral analysis (IR, ¹H NMR, ¹³C NMR and GC-MS data). All the products are known compounds.

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