Nano Cu-catalyzed efficient and selective reduction of nitroarenes under combined microwave and ultrasound irradiation
© Feng et al.; licensee Chemistry Central Ltd. 2014
Received: 8 March 2014
Accepted: 2 June 2014
Published: 18 June 2014
In situ preparation of copper nanoparticles from a copper acetate precursor and its application as an efficient catalyst for the selective reduction of aromatic nitro compounds with hydrazine hydrate under combined microwave and ultrasound irradiation were described in detail. The results reveal the synergetic effect of microwave and ultrasound on the synthesis of copper nanoparticles, and formation of various amino derivatives.
From the viewpoint of environmentally benign and sustainable chemistry, there has been an increasing interest in the search for more sustainable chemical processes during the last decade [1, 2]. Based on this context, the use of efficient synthetic method, nontoxic chemicals, and benign solvents, is the most valuable feature for the design of a “ideal” chemical protocol. Recently, microwave (MW) and ultrasound (US) technologies have been widely adopted as important synthetic methods in modern organic chemistry, owing to the fact that these technologies can usually reduce the reaction time, minimize energy consumption and in certain cases, increase the yield and selectivity of product [3–6].
Catalytic reduction of aromatic nitro compounds remains appealing because it is widely employed in numerous syntheses of intermediates and final products throughout the dyes, agricultural chemicals, pharmaceuticals and materials both in the laboratory and in industry [7–9]. Although many synthetic routes have been reported for the preparation of anilines from the corresponding aromatic nitro compounds [10–14], there is still necessary for the development of cost-effective catalysts with high activity in particular Fe-  and Cu-catalyzed  protocol. Recently, much attention has been attracted to the metal nanoparticles catalysts which have revealed high catalytic activities that far exceed those of conventional homogeneous catalysts, owing to their extremely small dimensions and huge special surface [17, 18]. A few procedures involving hydrogenation of nitrobenzene into aniline catalyzed by metal Pt [19–22], Ru , Au [24–26] and Rh  nanoparticles have been demonstrated. Pal et al. obtained amino derivatives by the reduction of aromatic nitro compounds with NaBH4 catalyzed by coinage of metal nanoparticles (Cu/Ag/Au) . Wen et al. studied the catalytic transfer hydrogenation of aromatic nitro compounds in presence of polymer-supported nano-amorphous Ni-B catalyst . Among the pioneering works of Cu nanoparticles [30–34] emerged as a promising catalyst for organic synthesis, Saha and Ranu  reported the reduction of nitro-compounds catalyzed by nano copper particles, however, synthesis required high stoichiometric ratio of copper nanoparticles (3 equiv.) and excess reductant (5 equiv.) to nitro-compounds, long reaction time (8–12 hours) and argon protection.
As our continuing efforts on microwave and ultrasound-assisted reaction [36–40], we found that combined microwave and ultrasound irradiation (CMUI) could strongly promote the nano Cu-catalyzed reduction of aromatic nitro compounds due to its simultaneous enhancement on heat and mass transfer. High reaction rate, low dosage of catalyst and excellent yields were achieved via a chemoselective reduction of aromatic nitro compounds with hydrazine hydrate under CMUI.
Results and discussion
Reduction of nitrobenzene using different methods a
Con. b (%)
MW + US
metallic Cu (20)
MW + US
In conclusion, we have presented an efficient and convenient method for the chemoselective reduction of aromatic nitro compounds catalyzed by in situ prepared Cu nanoparticles under combined microwave and ultrasound irradiation. It allowed us to achieve wide range of anilines bearing both electron-donating and electron-withdrawing substituents in excellent yields. The intriguing results presented herein might open a promising new approach for the efficient preparation and application of nanoparticles.
All solvents and reagents were purchased from commercial sources and were used without prior purification. All combined microwave and ultrasound irradiation experiments were carried out in a apparatus (a professional TCMC–102 microwave apparatus (Nanjing Lingjiang Technological Development Company, China), operating at a frequency of 2.45 GHz with continuous irradiation power from 0 to 500 W, and a FS–250 professional ultrasound apparatus (Shanghai S. X. Ultrasonics, China), operating at a frequency of 20 KHz with controllable irradiation power from 10 to 100 W. The reactions were carried out in 15 mL two-necked Pyrex flask, placed in the microwave cavity and the tip of detachable horn should be immersed just under the liquid surface. TLC analysis was performed on aluminum backed plates SIL G/UV254. The products were purified by column chromatography and were identified by 1H NMR, 13C NMR spectra recorded on 400 MHz Bruker NMR instrument and GC–MS.
General experimental procedure for the peduction of nitroarenes
A mixture of nitro compounds (1 mmol), copper acetate (0.2 mmol), hydrazine hydrate (3 mmol) and ethylene glycol (4 mL) was subjected to microwave-ultrasound activation condition. Then hydrazine hydrate (3 mmol) in ethylene glycol (2 mL) was added and the ultrasound and microwave source are switched on successively (power level: US 50 W, MW 100 W maximum power). The mixture was irradiated simultaneously by microwaves and ultrasound until nearly complete conversion of aromatic nitro compounds. The progress of the reactions was monitored by TLC and GC–MS. The reaction mixture was then subjected to centrifugation. After decanting the liquid, the Cu nanoparticles were washed with ethanol (5 mL, three times), which was combined with the decantate. Water (10 mL) was added to the centrifugal liquid, and the product was extracted into ethyl acetate. Evaporation of solvent and the crude product was purified by column chromatography over silica gel (ethyl acetate/petroleum ether = 1:9–1:6) to afford the products.
1H NMR (400 MHz, CDCl3): δ 7.34 (t, J = 7.5 Hz, 2H), 6.95 (d, J = 7.4 Hz, 1H), 6.80 (dd, J = 8.4, 0.8 Hz, 2H), 3.69 (s, 2H). 13C NMR (101 MHz, CDCl3): δ 146.78, 129.49, 118.60, 115.33. MS (GC-MS): 93 (M+)
1H NMR (400 MHz, CDCl3): δ 7.25 (t, J = 7.0 Hz, 2H), 7.25 (t, J = 7.0 Hz, 2H), 6.93 (t, J = 7.4 Hz, 1H), 6.93 (t, J = 7.4 Hz, 1H), 6.87–6.77 (m, 1H), 6.86–6.78 (m, 1H), 3.67 (s, 2H), 3.67 (s, 2H), 2.33 (s, 3H), 2.33 (s, 3H). 13C NMR (101 MHz, CDCl3): δ 144.89, 130.64, 127.16, 122.48, 118.73, 115.13, 17.50. MS (GC-MS): 107 (M+)
1H NMR (400 MHz, CDCl3): δ 7.36–7.18 (m, 1H), 6.78 (dd, J = 16.7, 7.8 Hz, 1H), 6.64 (ddd, J = 21.0, 11.0, 10.1 Hz, 2H), 3.73 (s, 2H), 2.43 (d, J = 34.7 Hz, 3H). 13C NMR (101 MHz, CDCl3): δ146.77, 139.21, 129.36, 119.52, 116.16, 112.50, 21.64. MS (GC-MS): 107 (M+)
1H NMR (400 MHz, CDCl3): δ 7.01 (d, J = 8.0 Hz, 2H), 6.69–6.62 (m, 2H), 3.49 (s, 1H), 2.29 (s, 2H). 13C NMR (101 MHz, CDCl3): δ 143.82, 129.77, 127.80, 115.29, 20.47. MS (GC-MS): 107 (M+)
1H NMR (400 MHz, DMSO-d6): δ 8.93 (s, 1H), 6.63 (dd, J = 7.7, 1.3 Hz, 1H), 6.55 (dtd, J = 9.1, 7.7, 1.6 Hz, 2H), 6.45–6.32 (m, 1H), 4.46 (s, 2H). 13C NMR (101 MHz, DMSO-d6): δ 143.93, 136.49, 119.46, 116.38, 114.39, 114.31. MS (GC-MS): 109 (M+)
1H NMR (400 MHz, CDCl3): δ 7.11 (t, J = 8.0 Hz, 1H), 6.86–6.74 (m, 1H), 6.69 (t, J = 2.1 Hz, 1H), 6.56 (ddd, J = 8.1, 2.2, 0.8 Hz, 1H), 3.71 (s, 2H). 13C NMR (101 MHz, CDCl3): δ 147.86, 134.79, 130.46, 118.41, 114.98, 113.39. MS (GC-MS): 127 (M+)
1H NMR (400 MHz, CDCl3): δ 7.17–7.08 (m, 2H), 6.68–6.57 (m, 2H), 3.57 (s, 2H).
13C NMR (101 MHz, CDCl3): δ 145.00, 129.12, 123.11, 116.26. MS (GC-MS): 127 (M+)
1H NMR (400 MHz, CDCl3): δ 7.05 (t, J = 8.0 Hz, 1H), 6.93 (ddd, J = 7.9, 1.7, 0.9 Hz, 1H), 6.85 (t, J = 2.0 Hz, 1H), 6.60 (ddd, J = 8.0, 2.2, 0.9 Hz, 1H), 3.73 (s, 2H). 13C NMR (101 MHz, CDCl3): δ 148.06, 130.80, 123.08, 121.31, 117.86, 113.84. MS (GC-MS): 170 (M+)
1H NMR (400MHz, CDCl3): δ 6.99–6.80 (m, 2H), 6.69–6.55 (m, 2H), 3.56 (s, 2H). 13C NMR (101MHz, CDCl3): δ 157.54, 155.21, 142.70, 142.68, 116.15, 116.08, 115.77, 115.55. MS (GC-MS): 111(M+)
1H NMR (400 MHz, CDCl3): δ 7.17 (d, J = 8.5Hz, 1H), 6.77 (d, J = 2.3 Hz, 1H), 6.68 (dd, J = 8.5, 2.4 Hz, 1H), 4.08 (s, 2H). 13C NMR (101 MHz, CDCl3): δ 143.82, 133.12, 130.19, 118.84, 117.44, 115.37. MS (GC-MS): 162(M+)
1H NMR (400 MHz, CDCl3): δ 7.64–7.53 (m, 1H), 7.53–7.46 (m, 1H), 7.36–7.20 (m, 1H), 6.96 (ddd, J = 8.0, 2.3, 0.8 Hz, 1H), 4.03 (s, 2H). 13C NMR (101 MHz, CDCl3): δ 149.24, 147.48, 129.91, 120.64, 113.11, 109.01. MS (GC-MS): 138(M+)
Financial support for this work from the National Basic Research Program of China (973 Program) (Grant 2010CB126101) and the Shanghai Leading Academic Discipline Project (Project Number: B507) are gratefully acknowledged.
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