Ligand-Promoted, Copper Nanoparticles Catalyzed Oxidation of Propargylic Alcohols with TBHP or Air as Oxidant

Abstract

A highly efficient oxidation of propargylic alcohols to ynones was catalyzed by copper nanoparticles (Cu Nps) with TBHP as an oxidant at room temperature. With bipyridine as the ligand, the reaction was accelerated significantly and led in good to excellent yields to a variety of propargylic alcohols. Furthermore, with Cu Nps as the catalyst, molecular oxygen in air could be utilized as oxidant effectively in the presence of bpy ligand.

Key words

propargylic alcohols - oxidation - ynones - copper nanoparticles - ligand effect

References and Notes

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13

The diameter of Cu Nps is ca. 20-30 nm. The detailed procedure about the synthesis of Cu Nps is described in the Supporting Information.

14

The in situ formed Cu₂O on the surface of Cu Nps was proposed as the catalytic active species in the reaction. However, when pure Cu₂O Nps were utilized as catalyst, similar catalytic activity but lower selectivity was observed.

15

Typical Procedure for the Cu Nps Catalyzed Oxidation of Propargylic Alcohols with TBHP as Oxidant (Entry 1, Table 2): 1,3-Diphenyl-2-propyn-1-ol (1a, 0.2 mmol), Cu NPs (1.3 mg, 10 mol%), bipyridine (3.2 mg, 10 mol%), TBHP (55 µL, 2.0 equiv, 70% in H₂O), and CH₂Cl₂ (1.5 mL) were added into a 20-mL Schlenk tube under air. The mixture was stirred at r.t. for 2 h. Then, the reaction was stopped, and the reaction mixture was purified by flash column chromatography on silica gel (hexanes-EtOAc, 30:1). Compound 2a was obtained in >98% yield.

16

From crude ^¹H NMR and TLC, there are no overoxidized by-products observed.

18

Typical Procedure for the Cu Nps Catalyzed Aerobic Oxidation of Propargylic Alcohols (Entry 1, Table 5): 1,3-Diphenyl-2-propyn-1-ol (1a, 0.2 mmol), Cu Nps (1.3 mg, 10 mol%), bipyridine (3.2 mg, 10 mol%) were mixed with toluene (1.5 mL) in a 20-mL Schlenk tube under air. The mixture was stirred at 80 ˚C for 8 h. Then, the reaction was stopped, and the reaction mixture was purified by flash column chromatography on silica gel (hexanes-EtOAc, 30:1). Compound 2a was obtained in 80% yield.

20

This result also indicated the possibility of recycling the catalyst. In fact, the Cu Nps catalyst could be separated and recovered conveniently by centrifugation, and then, be reused directly without additional bpy ligand. With 1d as the substrate, ca. 80% yield was still obtained in the third reaction with prolonged reaction time. To improve recycle capability, the effects of particle size and particle support are currently under investigations.