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DOI: 10.1055/s-2006-951481
An Easy Access to Aromatic Azo Compounds under Ultrasound/Microwave Irradiation
Publication History
Publication Date:
22 September 2006 (online)
Abstract
Chemoselective reduction of nitroarenes to azo and azoxy compounds was easily achieved using zinc powder and ammonium chloride in DMF or DMF-water (95:5) under high intensity ultrasound (US) or microwave (MW) irradiation, separately or combined. When carried out under conventional heating the reaction required much higher temperatures and gave lower yields. The addition of a small amount of water caused a dramatic increase in the reactivity, permitting the reduction of hindered nitroarenes at the expense of selectivity. A novel reactor for combined US/MW irradiation was employed which demonstrated additional beneficial effects.
Key words
nitroarenes - azo compounds - reduction - ultrasound - microwave
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References and Notes
The disappearance of asymmetric and symmetric stretching bands due to the N-O of NO2 (near 1520 cm-1 and 1345 cm-1 and the appearance of a strong band around 1460 cm-1 due to N=N stretching was diagnostic. All the azo and azoxy compounds were fully characterised by IR, mass and NMR spectroscopy and when available compared with published data.
18General Procedure: To a suspension of the nitro compound (1 mmol) and Zn powder (4 mmol) in DMF (5 mL) or DMF-H2O (95:5, 5 mL, substrates 4, 5, 7, 8) NH4Cl (2 mmol) was added under stirring and the mixture was heated or irradiated by US or/and MW as indicated before. After the reaction (monitored by TLC or GC) was complete, the mixture was filtered through a pad of celite and washed with H2O and EtOAc. The organic layer was washed with brine, dried with anhyd Na2SO4, concentrated under vacuum and purified either by preparative TLC or by flash chromatography. (For substrate 1, reaction at 80 °C: the product was precipitated with 2 N HCl and the mixture was stirred until the Zn powder had completely dissolved. The crude product was collected on a Hirsch funnel and dried under vacuum.)
19GC analyses were carried out on a Shimadzu-14B gas chromatograph, using SE52 capillary column (length 25 m; internal diameter 0.25 mm; film thickness 0.25 µm). GC conditions: injection split, 1:20; injector temperature, 250 °C; detector temperature, 300 °C; temperature program: from 50 °C (1 min) to 300 °C at 10 °C/min (30 min); H2 as carrier gas at 25 kPa. The analysed products had the following retention time: 6: t R 16.2 min; azo-6: t R 27.3 min; azoxy-6: t R 29.4 min; 7: t R 14.9 min; azoxy-7: t R 25.3 min; 8: t R 14.4 min; azoxy-8: t R 24.2 min.
201,2-Bis{4-[2-(2,2-dichloroacetamido)-1,3-dihydroxy-propyl]phenyl}diazene Oxide: Orange powder; mp 248.7 °C (dec.); R f 0.52 (CHCl3-MeOH, 8:2). IR (KBr): 3400 (OH, NH), 2926, 1674 (C=O), 1541 (NCO), 1464 (N=NO), 1196, 1074, 810, 774 cm-1. 1H NMR (300 MHz, acetone-d 6): δ = 8.24 (d, J = 8.7 Hz, 2 H, ArH-2,6), 8.15 (d, J = 8.7 Hz, 2 H, ArH-2′,6′), 7.65 (d, J = 8.7 Hz, 2 H, ArH-3,5), 7.59 (d, J = 8.7 Hz, 2 H, ArH-3′,5′), 7.35 (br m, 2 H, CONH), 6.42 (s, 1 H, COCHCl2), 6.41 (s, 1 H, COCH′Cl2), 5.28 (d, J = 2.4 Hz, 1 H, PhCHOH), 5.23 (d, J = 2.4 Hz, 1 H, PhCH′OH), 5.19 (br, 1 H, PhCHOH), 5.08 (br, 1 H, PhCHOH′), 4.23 (br, 2 H, CH2OH), 4.17 (m, 2 H, NHCH), 3.83 (m, 2 H, CH 2OH), 3.74 (m, 2 H, CH′2OH). 13C NMR (75 MHz, acetone-d 6): δ = 164.27, 164.25 (CONH), 147.6 (ArC-1), 147.5 (ArC-1′), 145.1 (ArC-4′), 143.4 (C-4), 127.2 (ArC-3,5), 126.9 (ArC-3′,5′), 125.5 (ArC-2′,6′), 122.1 (ArC-2,6), 70.6 (PhCHOH), 70.3 (PhC′HOH), 67.1 (COCHCl2), 67.0 (COC′HCl2), 61.3 (CH2OH), 57.7 (NHCH), 57.6 (NHC′H). MS (ESI): m/z = 621 [M+ + Na].
211,2-Bis[3,4-di(pentan-3-yl)phenyl]diazene: Orange powder; mp 97.2 °C; R f 0.71 (hexane-EtOAc, 99:1). IR (KBr): 2963, 2872, 1458 (N=N), 1375, 901, 839, 812 cm-1. 1H NMR (300 MHz, CDCl3): δ = 7.76 (s, 2 H, H-2,2′), 7.65 (d, 2 H, H-6,6′), 7.34 (d, 2 H, H-5,5′), 2.91 [m, 4 H, CH(CH2CH3)2], 1.92-1.55 [m, 16 H, CH(CH 2CH3)2], 0.88-0.79 [m, 24 H, CH(CH2CH 3)2]. 13C NMR (75 MHz, CDCl3): δ = 151.1 (C-1,1′), 147.8 (C-4,4′), 145.6 (C-3,3′), 127.1 (C-5,5′), 122.9 (C-2,2′), 118.2 (C-6,6′), 42.2 [CH(CH2CH3)2], 29.5, 29.2, 29.1 [CH(CH2CH3)2], 12.5, 12.4, 12.3 [CH(CH2 CH3)2]. MS (CI, isobutane): m/z (%) = 463 (100) [MH]+.
221,2-Bis[3,4-di(pentan-3-yl)phenyl]diazene Oxide: Yellow powder; mp 114.2 °C; R f 0.64 (hexane-EtOAc, 99:1). IR (KBr): 2961, 2872, 1483, 1460 (N=NO), 1397, 928, 826 cm-1. 1H NMR (300 MHz, CDCl3): δ = 8.20-8.06 (overlap, 3 H, H-2,6,6′), 7.99 (s, 1 H, H-2′), 7.31 (d, 2 H, H-5,5′), 2.94 [br m, 4 H, CH(CH2CH3)2], 1.91-1.52 [m, 16 H, CH(CH 2CH3)2], 0.92-0.78 [m, 24 H, CH(CH2CH 3)2]. 13C NMR (75 MHz, CDCl3): δ = 148.5 (C-1), 146.8 (C-1′), 145.7 (C-3), 145.1 (C-3′), 142.4 (C-4,4′), 126.7 (C-2′), 126.4 (C-5′), 124.2 (C-5), 122.8 (C-2), 120.1 (C-6′), 119.7 (C-6), 42.6 [CH(CH2CH3)2], 29.5, 29.4, 29.3 [CH(CH2CH3)2], 12.6, 12.5, 12.4 [CH(CH2 CH3)2]. MS (CI, isobutane): m/z (%) = 479 (53) [MH]+, 463 (100) [MH - O]+.
231,2-Bis[2,4-di(pentan-3-yl)phenyl]diazene: Orange oil; R f 0.57 (hexane-EtOAc, 95:5). IR (film): 2963, 2874, 1603, 1456 (N=NO), 1379, 1173, 907, 831, 818 cm-1. 1H NMR (300 MHz, CDCl3): δ = 8.10-8.07 (d, 1 H, H-6), 7.42-7.40 (d, 1 H, H-6′), 7.07-7.05 (overlap, 4 H, H-3,5,3′,5′), 3.10-3.05 [m, 1 H, 2-CH(CH2CH3)2], 2.95-2.90 [m, 1 H, 2′-CH(CH2CH3)2], 2.50-2.25 [m, 2 H, 4,4′-CH(CH2CH3)2], 1.81-1.49 [m, 16 H, CH(CH 2CH3)2], 0.91-0.73 [m, 24 H, CH(CH2CH 3)2]. 13C NMR (75 MHz, CDCl3): δ = 147.6 (C-1), 146.6 (C-1′), 141.9 (C-2), 141.2 (C-4,4′), 138.4 (C-2′), 126.9 (C-3′), 126.8 (C-3), 125.5 (C-5′), 124.8 (C-5), 123.2 (C-6′), 121.1 (C-6), 49.8, 49.7 [4,4′-CH(CH2CH3)2], 42.3, 42.2 [2,2′-CH(CH2CH3)2], 29.3, 29.2, 29.0 [CH(CH2CH3)2], 12.4, 12.3, 12.2 [CH(CH2 CH3)2]. MS (CI, isobutane): m/z (%) = 479 (65) [MH]+, 463 (100) [MH - O]+.
241,2-Bis[2,5-di(pentan-3-yl)phenyl]diazene Oxide: Orange oil; R f 0.59 (hexane-EtOAc, 99:1). IR (film): 2963, 2930, 2874, 1462, 1456 (N=NO), 1379, 1073, 882, 830 cm-1. 1H NMR (300 MHz, CDCl3): δ = 7.83 (s, 1 H, H-6), 7.31-7.20 (overlap, 4 H, H-3,3′,4′,6′), 7.13 (d, 1 H, H-4), 2.95 [br m, 2 H, 2,2′-CH(CH2CH3)2], 2.42 [br m, 2 H, 5,5′-CH(CH2CH3)2], 1.84-1.59 [m, 16 H, CH(CH 2CH3)2], 0.90-0.75 [m, 24 H, CH(CH2CH 3)2]. 13C NMR (75 MHz, CDCl3): δ = 144.3 (C-1), 143.4 (C-1′), 143.1 (C-2), 139.2 (C-5,5′), 136.1 (C-2′), 129.3 (C-4′), 127.9 (C-3′), 127.1 (C-3), 126.8 (C-4), 122.6 (C-6′), 120.5 (C-6), 49.3, 49.1 [2,2′-CH(CH2CH3)2], 42.5, 42.0 [5,5′-CH(CH2CH3)2], 29.2, 29.1, 28.8 [CH(CH2CH3)2], 12.4, 12.3, 12.2 [CH(CH2 CH3)2]. MS (CI, isobutane): m/z (%) = 479 (100) [MH]+, 463 (27) [MH - O]+.