M. B. Sridhara1, D. M. Suyoga Vardhan2, H. K. Kumara2, D. Channe Gowda2*
1Department of Chemistry, Rani Channamma University, Vidhya Sangama, Belagavi-591156, Karnataka, India.
2Department of Studies in Chemistry, University of Mysore, Manasagangotri, Mysore, Karnataka, India-570006.
*Corresponding Author E-mail: dchannegowda@yahoo.co.in
ABSTRACT:
A facile, efficient and cost-effective protocol for the synthesis of symmetrically substituted azoarenes from nitroarenes by using readily available ammonium iodide as hydrogen donor and magnesium powder as catalyst at room temperature in methanol media is described. The conversion of using this system is clean, rapid, chemo-selective and high yielding. The azoarenes containing some additional substituents such as halogen, methyl, hydroxy, methoxy, ethoxy, etc. functions have been achieved in a single step by use of this reagent.
KEYWORDS:Catalytic transfer hydrogenation, nitroarenes, azocompounds, magnesium, ammonium iodide.
Azoarenes are useful synthetic intermediates.1 Azo dyes are extensively used in textile, printing, leather, papermaking, drug, cosmetics and food industries.2 Various studies have shown that azo compounds possess excellent optic memory and photoelectric properties.3 They have long played a significant role in the development of mechanistic and synthetic organic chemistry.4 There are many methods available in the literature for the preparation of azo compounds.5-13 Most of these methods documented in the literature is associated with cyclization, rearrangement and isomerization in strong acid and alkaline medium. Recently, Wada et al.,14 reported the newborn surface of dull metals in organic synthesis using bismuth-mediated one-step conversion of nitroarenes to azoxy and azo compounds. But this method forms a mixture of both azoxy and azo compounds.
Now a day, heterogeneous catalytic transfer hydrogenation method has proved to be a potent choice for reduction of organic compounds.15-18 In comparison with catalytic hydrogenation or with other methods of reduction, catalytic transfer hydrogenation method of reduction is easy, safer, rapidity, milder reaction condition, simple operation and workup and the reactions proceeds through ambient temperature and pressure. Furthermore, catalytic transfer hydrogenation reactions do not requires any elaborate experimental setup or high pressure apparatus.
The utility of magnesium19-21 as a powerful reducing agent in organic synthesis and in the field of catalytic transfer hydrogenation for the removal of commonly used protecting groups in peptide synthesis22 and also for various functional group transformation in organic synthesis is reported.23,24 Recently, in our laboratory the substituted nitroarenes are directly converted into azocompounds by employing the systems like HCOONH4/Pb,25 HCOONHEt3/Pb,26 and HCOONHEt3/Mg.27 In this context, we have developed a facile, efficient, cost-effective and superior compared to our earlier methods for the synthesis of symmetrically substituted azoarenes from nitroarenes using readily available inexpensive ammonium iodide as hydrogen donor and magnesium powder as catalyst in methanol at room temperature as shown in Scheme 1.
R= -Cl, -OH -CH3, -OCH3, -OC2H5.
Scheme 1
2. EXPERIMENTAL SECTION:
The 1H–NMR spectra were recorded on an AMX-400 MHz spectrometer using CDCl3 as the solvent and TMS as internal standard. IR spectra were recorded on Shimadzu FTIR- 8300 spectrometer. Elemental analysis was performed on a model Vario EL III elemental analyzer. The melting points were determined by using Thomas–Hoover melting point apparatus and are uncorrected. Thin layer chromatography was carried out on silica gel plates obtained from Whatman Inc. The substrates were either commercial products and were used as purchased or were prepared according to literature procedures. Magnesium metal powder was purchased from Himedia Pvt. Ltd., Mumbai (India) and ammonium iodide was purchased from Ace Rasayana (India) Pvt. Ltd. All of the solvents used were analytical grade or were purified according to standard procedures.
General procedure for the synthesis of azoarenes:
A suspension of the nitroarene (1g) and ammonium iodide (2g) in methanol (15mL) taken in a round bottomed flask was stirred at room temperature. The reaction was initiated by the addition of magnesium power (1g). The progress of the reaction was monitored by TLC. After completion of the reaction, the reaction mixture was filtered through a celite pad and washed with methanol. The combined filtrate and washings were evaporated under reduced pressure and taken into chloroform or petroleum ether (2 × 25mL). The organic layer was washed with 50% saturated brine solution (2 × 15mL) and was dried over anhydrous sodium sulphate. The solvent was removed using rotary evaporator. Further the residue was purified either by preparative TLC or by column chromatography.
Physical and analytical data of the synthesized compounds:
Azobenzene.
Orange colour solid; mp; 66-68°C (lit.2868°C); reaction time, 2.0 hr; yield, 95%.
1H NMR:d = 7.48-8.12 (m, 10H, ArH).
Anal. Calcd. For C12H10N2: C, 79.09; H, 5.53; N, 15.37.
Found C, 79.02; H, 5.52; N, 15.32.
2, 2′ Dimethyl azobenzene.
Yellow colour solid; mp; 53-55°C(lit.2855°C); reaction time, 2.0 hr; yield, 92%.
1H NMR:d = 7.32-7.90(m, 8H, ArH), 2.31(s, 6H, CH3)
Anal. Calcd. For C14H14N2: C, 79.96; H, 6.71; N, 13.32
Found C, 79.92; H, 6.69; N, 13.28.
3, 3′ Dimethyl azobenzene.
Yellow colour solid; mp; 55-57°C(lit.2855°C); reaction time, 2.1 hr; yield, 93%.
1H NMR:d = 7.29-7.87(m, 8H, ArH), 2.33(s, 6H, CH3)
Anal. Calcd. For C14H14N2: C, 79.96; H, 6.71; N, 13.32
Found C, 79.94; H, 6.67; N, 13.30.
4, 4′ Dimethyl azobenzene.
Yellow colour solid; mp; 144-146°C(lit.28144°C); reaction time, 2.2 hr; yield, 92%.
1H NMR:d = 7.32-7.90(m, 8H, ArH), 2.33(s, 6H, CH3)
Anal. Calcd. For C14H14N2: C, 79.96; H, 6.71; N, 13.32
Found C, 79.89; H, 6.63; N, 13.27.
2, 2′Dichloro azobenzene.
Orange needles solid; mp; 135-138°C(lit.28137°C); reaction time, 2.2 hr; yield, 90%.
1H NMR:d = 7.45-7.92(m, 8H, ArH).
Anal.Calcd. For C12H8N2Cl2: C, 57.39; H, 3.21; N, 11.12
Found C, 57.35; H, 3.19; N, 11.06.
3, 3′Dichloro azobenzene.
Yellow needles solid; mp; 101-103°C(lit.28101°C); reaction time, 2.0 hr; yield, 95%.
1H NMR:d = 7.49-8.02(m, 8H, ArH).
Anal. Calcd. For C12H8N2Cl2: C, 57.39; H, 3.21; N, 11.12
Found C, 57.36; H, 3.20; N, 11.05.
4, 4′Dichloro azobenzene.
Red colour solid; mp; 185-187°C(lit.28188°C); reaction time, 2.5 hr; yield, 91%.
1H NMR:d = 7.49-7.92(m, 8H, ArH).
Anal. Calcd. For C12H8N2Cl2: C, 57.39; H, 3.21; N, 11.12
Found C, 57.27; H, 3.23; N, 11.09.
2, 2′ Dimethoxy azobenzene.
Red colour solid; mp; 130-133°C(lit.28131°C); reaction time, 2.5 hr; yield, 93%.
1H NMR:d = 7.10-7.92(m, 8H, ArH); 3.62(s, 6H, CH3).
Anal. Calcd. For C14H14N2O2: C, 69.40; H, 5.82; N, 11.56
Found C, 69.35; H, 5.78; N, 11.48.
3, 3′ Dimethoxy azobenzene.
Yellow colour solid; mp; 93-93°C(lit.2891°C); reaction time, 2.5hr; yield,90%.
1H NMR:d = 7.10-7.52(m, 8H, ArH); 3.62(s, 6H, CH3).
Anal. Calcd. For C14H14N2O2: C, 69.40; H, 5.82; N, 11.56
Found C, 69.37; H, 5.76; N, 11.52.
4, 4′ Diethoxy azobenzene.
Red colour solid; mp; 159-161°C(lit.28160°C); reaction time, 2.5hr; yield, 92%.
1H NMR:d = 7.10-7.92(m, 8H, ArH); 4.12(q, 4H, CH2); 1, 38(s, 6H, CH3).
Anal. Calcd. For C16H18N2O2: C, 71.08; H, 6.71; N, 10.36
Found C, 70.95; H, 6.75; N, 10.28.
4, 4′ Hydroxy azobenzene.
Dark Yellow colour solid; mp; 174-175°C(lit.28173-175°C); reaction time, 2.0hr; yield, 90%.
1H NMR:d = 7.12-7.82(m, 8H, ArH); 4.98(s, 2H, OH).
Anal. Calcd. For C12H10N2O2: C, 67.28; H, 4.70; N, 13.07
Found C, 67.22; H, 4.72; N, 12.97.
1, 1′ azonaphthalene.
Orange colour solid; mp;188-191°C(lit.28190°C); reaction time, 2.2hr; yield, 92%.
1H NMR:d = 7.40-7.92(m, 14H, ArH).
Anal. Calcd. For C20H14N2: C, 85.08; H, 4.99; N, 9.92
Found C, 84.95; H, 5.02; N, 9.85.
2, 2′ azonaphthalene.
Orange colour solid; mp; 207-209°C(lit.28 208°C); reaction time, 2.0hr; yield, 90%.
1H NMR:d = 7.40-8.42(m, 14H, ArH).
Anal. Calcd. For C20H14N2: C, 85.08; H, 4.99; N, 9.92
Found C, 84.92; H, 4.95; N, 9.85.
Azo Biphenyl.
Pale yellow colour solid; mp; 248-250°C(lit.28 250°C); reaction time, 2.5hr; yield, 90%.
1H NMR:d = 7.28-8.12(m, 18H, ArH).
Anal. Calcd. For C24H18N2: C, 86.19; H, 5.42; N, 8.37
Found C, 86.12; H, 5.35; N, 8.29.
3. RESULTS AND DISCUSSION:
As shown in scheme 1, the system can be conveniently applied for the synthesis of several structurally different symmetrically substituted azoarenes. Synthesis of unsymmetrically substituted azo compounds leads to the formation of a mixture, which needs extensive purification and yields are low (less than 30%).The conversion of substituted nitroarenes to the corresponding symmetrically substituted azoarenes was completed within 2-2.5 hrs. Sensitive substituents like Cl, CH3, OCH3, OH, and OC2H5 functions have been synthesized in a single step. Also these functionalities were tolerated by this novel reagent system. The course of reaction was monitored by TLC and IR spectra. The disappearance of asymmetric and symmetric stretching bands near 1520 cm-1 and 1345 cm-1 due to N=O of NO2 and the appearance of a strong band between 1630-1575cm-1 due to N=N stretching in the IR spectra clearly indicated the conversion. All the azoarenes were characterized by comparison of their TLC, IR spectra, 1H NMR spectra and melting point with authentic samples. The yields of the products were virtually quantitative and analytically pure. Control experiment was carried out by using nitroarene with ammonium iodide but without magnesium powder which did not yield the desired product. Furthermore, another attempted reduction was carried out by using nitroarenes with magnesium powder but without ammonium iodide which also does not yield the desired product. The mechanism for the formation of azo compound probably involves initial reduction of nitro compound to nitroso and hydroxylamine, which then condense to form azo compounds. This is evidenced by the isolation and characterization of the intermediate hydroxylamine compounds.
4. CONCLUSION:
In conclusion, we have developed a facile, neat and cost-effective method for the synthesis of azoarenes from nitroarenes using readily available magnesium powder and ammonium iodide at room temperature in methanol. Both magnesium metal powder and ammonium iodide are inexpensive, commercially available and can be handled easily. Moreover, the ease of product separation and high selectivity are the added advantages of this method. This system is also superior to our own earlier methods of synthesis of azoarenes in purity and yield wise. Employing the present methodology for the conversion of other functional group transformations is under progress.
5. ACKNOWLEDGMENT:
The authors gratefully acknowledge University Grants Commission (UGC), New Delhi for awarding UGC-Post Doctoral Fellowship (PDFSS), BSR faculty fellowship and DST Inspire fellowship.
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Received on 19.07.2017 Modified on 20.09.2017
Accepted on 08.10.2017 © AJRC All right reserved
Asian J. Research Chem. 2018; 11(1):15-18.
DOI:10.5958/0974-4150.2018.00004.4