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advances in heterogeneous catalytic transfer hydrogenation of
SHI Qi-xun et al. Advances in Heterogeneous Catalytic Transfer Hydrogenation of Aromatic Nitro Compounds Advances in Heterogeneous Catalytic Transfer Hydrogenation of Aromatic Nitro Compounds SHI Qi-xun, LV Rong-wen, ZHANG Zhu-xia, ZHAO De-feng (State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, P. R. China) Abstract: The literature related to the reduction of aromatic nitro compounds is reviewed. The reduction of nitroarenes to the corresponding amines is an important step in the industrial synthesis of dyes, pharmaceutical products, agricultural chemicals, surfactants and polymers. This review focuses mainly on the heterogeneous catalytic transfer hydrogenation of nitroarenes covering the literature through 2000. The characterization of these methods is summarized and discussed. Keywords: hydrogen donor; reduction of aromatic nitro compounds; catalyst Introduction In general, the aromatic amines are considered to be important intermediates in the synthesis of a number of nitrogen heterocycles. Some of them are used in dyes, drug, pharmaceuticals, pesticides and so on. Numerous methods containing homogeneous catalytic transfer hydrogenation and heterogeneous catalytic transfer hydrogenation have been reported in the literature for the reduction of nitroarenes to anilines. As is well known, these catalysts in the homogeneous catalytic transfer hydrogenation are not reusable due to various problems such as deactivation by metal precipitation or ligand degradation, separation difficulties, etc. On the other hand, the use of heterogeneous catalysts offers several advantages over homogeneous systems with respect to easy recovery and recycling of the catalysts as well as minimization of undesired toxic wastes. Therefore in this paper we review the heterogeneous catalytic transfer hydrogenation of aromatic nitro compounds covering the literature through 2000 categorized by different hydrogen donors. 1. NH4Cl as the hydrogen donor Rahat H. Khan treated a wide range of aromatic nitro compounds with tellurium powder in aqueous methanolic ammonium chloride and resulted in selective reduction of the nitro groups. The reactions were typically completed within 2-5 h, and gave the corresponding aniline in good to excellent yield. Ester, nitrile, amide and halide substituents were unaffected. Takehito Tsukinoki and Hirohisa Tsuzuki demonstrated an environmental friendly method for the synthesis of anilines by the chemoselective reduction of nitroarenes with zinc metal and NH4Cl in water. In their work, any organic solvent was avoided. So the reaction wasn’t hazardous to the environment, furthermore the procedure was versatile, quick and enabled easy separation of the products from the inorganic reagents. D. G. Desai et al and Yugang Liu et al also reported the reduction of nitroarenes with FeS-NH4Cl-CH3OH-H2O and Fe-NH4Cl system respectively, and had a satisfying yield. 2. Formate as the hydrogen donor 54 The Proceedings of the 3rd International Conference on Functional Molecules Ammonium formate was most convenient and efficient hydrogen source in catalytic hydrogen transfer reductions. D Channe Gowda et al[5-8] proceeded a series of research about the reduction of aromatic nitro compounds with ammonium formate as hydrogen donor in different catalytic system containing Pd/C, Ni, Zn and Mg. The result indicated that hydrogenolysis of halogens wasn’t observed and the reducible substituents, such as ethenes, ethynes, nitriles, carbonyl compounds, acids, alcohols and actones, remained unchanged under the reaction condition as well as at reflux temperature, that is, the selectivity of the method was very high. Based on the research of Ammonium formate, D Channe Gowda et al[9,10] further developed the hydrazinium monoformate as the hydrogen donor. The result indicated that reaction was complete within 2-10 minutes in room temperature and all the compounds reduced by Raney Ni/N2H4•HCOOH and Zn/ N2H4•HCOOH were obtained in good yields (90-95%). P. Haldar and V. V. Mahajani also reported the catalytic transfer hydrogenation of 2-nitroanisole to 2-anisidine in the temperature range 35-85 with ammonium formate as hydrogen donor and propan-2-ol as solvent using Pd/C as catalyst above agitation speed 1000 rpm. In their paper, the author detailedly discussed the effect of several chemical and physical parameters on reaction rate. result of the research, the author presented the reaction mechanism for the reduction of the nitro compounds (Fig. 1). He thought since the nitro group attached to ring pulled electrons more strongly from the benzene ring compared to other functional groups, it could easily be adsorbed on the catalyst surface, so that the electron-withdrawing/donating groups such as –F, -Cl, -Br, -OH, -CN, -CH3, -OCH3 and –NH2 could be tolerated during the reaction. 1) H transfer H O N O M O H O 2) (CH3)2CO H transfer N O O H O N H OH H2O 1) + (CH3)2CHOH 2) NH2 3) (CH3)2CO 1) + (CH3)2CHOH 2) 3) (CH3)2CO H2O NHOH H2O N O Fig. 1 reduction of aromatic nitro compounds on FeHMA. 3. Propan-2-ol as the hydrogen donor P.Selvam et al[12,13,14,15] reported the chemoand regioselective reduction of nitroarenes over NiHMA (nickel-incorporated hexagonal mesoporous aluminophosphate molecular sieves), FeHMA, CoHMA, NiMCM-41 using the propan-2-ol as the hydrogen donor. The results of the experiment indicated that all the compounds reduced were obtained in excellent yields in these catalytic systems and the catalyst could very well be reused without affecting either the activity or the catalyst characteristics. According to the 55 Recently Radha V. Jayaram et al[16,17] firstly reported the catalytic transfer hydrogenation of aromatic nitro compounds on LaMO3 (M=Mn, Fe, Co, Cr, Al) perovskites prepared by microwave irradiation using propan-2-ol as hydrogen donor and KOH as promoter. In their work, a series of LaMO3 (ABO3-type perovskites ) oxides were prepared to study the influence of A-site substitution on the reduction, and kinetic studies had also been performed for nitrobenzene reduction. Mixed metal oxides were also effective catalysts for catalytic transfer hydrogenation. Sachin U. Sonavane and Radha V. Jayaram reported catalytic transfer hydrogenation of nitroarenes with propan-2-ol and KOH/NaOH over mixed metal oxides, such as ZrO2-NiO, ZrO2-CoO and ZrO2-Fe2O3. The studies they had performed showed that ZrO2-NiO was the most effective catalyst in their research system, however they found that this catalyst didn’t have any activity with 4-nitrotoluene, 4-nitrobenzoic acid and 4-nitrophenol. The reason for this SHI Qi-xun et al. Advances in Heterogeneous Catalytic Transfer Hydrogenation of Aromatic Nitro Compounds phenomenon was attributed to that the rate of reduction was more decided by the subsequent hydride transfer rather than the initial adsorbtion of the substrates. 4. Hydrazine hydrate as the hydrogen donor Hydrazine hydrate was also an attractive alternative reducing agent for nitro compounds due to its stereo and functional selectivity, furthermore the reduction with hydrazine hydrate produced harmless by-products such as nitrogen gas and water. In recent years, the paper about hydrazine reduction of nitrocompounds could often be seen. D Channe Gowda et al[19,20] researched the reduction of nitrocompounds by hydrazine hydrate over zinc and magnesium. The results of the experiment were the same as that of the hydrazinium monoformate[9,10] as the hydrogen donor. The reduction of nitro compounds can be accomplished with commercial zinc dust or magnesium dust within three to ten minutes, most of them were completed in less than five minutes. All the compounds reduced by this system were obtained in 90-95% yields. Pramod S Kumbhar et al and S. M. Auer et al also researched the reduction of aromatic nitro compounds with hydrazine hydrate over Mg-Fe hydrotalcite and Fe-Mg-Al hydrotalcite respectively, furthermore through the research of mössbauer spectroscopy, Pramod S Kumbhar et al gave the reaction mechanism in their reaction system (Fig. 2) N2 H4 + 2Fe 2Fe 3+ 3+ With the development of the molecular sieve, M. Kumarraja and K. Pitchumani found a simple and efficient reduction of nitroarenes by hydrazine in faujasite zeolites. The system had several advantages over other catalysts employed for the same reaction such as: (1) the reduction was carried out in a nonpolar solvent where most of the aromatic compounds were soluble; (2) the isolation of products and regeneration of the catalyst could be carried out more readily; (3) it was clean and simple. 5. Other hydrogen donor Nowadays Microwave irradiation in organic synthesis is a useful technique. Recently S. Ravi Kanth et al reported a convenient method for the reduction of nitroarenes on Al2O3 support in presence of sodium hydrogen sulphide under microwave condition. The result indicated that under microwave condition the reaction could be speeded up dramatically and reaction time could be shortened from several hours in thermal conditions to several minutes in microwave conditions, furthermore the yields could be improved greatly. Wang Lei et al reported the reduction of aromatic nitro compounds with metallic tellurium in near-critical water. Their studies indicated that the reduction could be smoothly proceeded when the reaction temperature was 275 and the amount of tellurium was three times as much as that of water. However during the reaction the bromo or iodo group on the aromatic ring underwent reductive elimination in a competitive process, and carboxylic group on the aromatic ring also underwent the decarboxylation process. 2Fe 2Fe 2+ 2+ + + 2H+ + 2H + + N 2H 2 N2 N2 H2 + R O 2+ N + Fe O +·O +H + N R O N O + Fe R + N OH +H +·OH+ Fe 2+ N R O 6. Summary · N + O +H OH R N O 2+ R 2+ N OH + Fe R H 2+ N OH + Fe R R R · H N +H + R NH2 Fig. 2 reduction of aromatic nitro compounds on activated MgFe hydrotalcite. 56 Hydrogenation of nitro aromatics with heterogeneous catalysts is often the method of choice for the production of corresponding anilines. With more attention paid to environmental protection, heterogeneous catalytic transfer hydrogenation of aromatic nitro The Proceedings of the 3rd International Conference on Functional Molecules compounds will develop towards simple, rapid, chemoselective and environmental friendly direction.  Shankare Gowda, B. K. Kempe Gowda, D. Channe Gowda. Hydrazinium monoformate: a new hydrogen donor. Selective reduction of nitrocompounds catalyzed by commercial zinc dust[J]. Synthetic Communications, 2003, 33(2): 281-289.  Shankare Gowda, D. Channe Gowda. Application of Hydrazinium monoformate as new hydrogen donor with Raney nickel: a facile reduction of nitro and nitrile moieties[J]. Tetrahedron, 2002, 58:2211-2213.  P. Haldar, V. V. Mahajani. Catalytic transfer hydrogenation: o-nitro anisole to o-anisidine, some process development aspects[J]. Chemical Engineering Journal, 2004,104:27-33.  Parasuraman Selvam, Susanta K. Mohapatra, Sachin U. Sonavane, Radha V. Jayaram. Chemoand regioselective reduction of nitroarenes, carbonyls and azo dyes over nickel-incorporated hexagonal 45:2003-2007.  S. K. Mohapatra, S. U. Sonavane, R. V. Jayaram, P. Selvam. Reductive cleavage of azo dyes and reduction of nitroarenes over trivalent iron incorporated hexagonal mesoporous aluminophosphate molecular sieves[J]. Applied Catalysis B: Environmental, 2003, 46:155-163.  Susanta K. Mohapatra, Sachin U. Sonavane, Radha V. Jayaram, Parasuraman Selvam. Heterogeneous catalytic transfer hydrogenation of aromatic nitro and carbonyl compounds over cobalt ( ) substituted hexagonal mesoporous molecular sieves[J]. aluminophosphate mesoporous aluminophosphate molecular sieves[J]. Tetrahedron Letters, 2004, References  Rahat H. Khan. Tellurium mediated reduction of aromatic nitro groups[J]. J. Chem. Research (S), 2000, 290-291.  Takehito Tsukinoki, Hirohisa Tsuzuki. Organic reaction in water. Part 5.1 novel synthesis of anilines by zinc metal-mediated chemoselective reduction of nitroarenes[J]. Green Chemistry, 2001, 3:37-38.  D. G. Desai, S. S. Swami, S. K. Dabhade, M. G. Ghagare. FeS-NH4Cl-CH3OH-H2O: an efficient and inexpensive to system for reduction of nitroarenes anilines[J]. Synthetic Communications, 2001, 31(8): 1249-1251.  Yugang Liu, Yansong Lu, Mahavir Prashad, Oljan Repic, Thomas J. Blacklock. A practical and chemoselective reduction of nitroarenes to anilines using activated iron[J]. Adv. Synth. Catal., 2005, 347:217-219.  D. Channe Gowda, B. Mahesh. Catalytic transfer hydrogenation of aromatic nitro compounds by employing ammonium formate and 5% platinum on carbon[J]. Synthetic Communications, 2000, 30(20): 3639-3644.  Keelara Abiraj, Gejjalagere R. Srinivasa, D. Channe Gowda. Simple and efficient reduction of aromatic nitro compounds using recyclable polyer-supported formate and magnesium[J]. Aust. J. Chem., 2005,58:149-151.  D. Channe Gowda, A. S. Prakasha Gowda, A. Ramesha Baha, Shankare Gowda. Nickel-catalyzed formic acid reductions. A selective method for the reduction of nitro compounds[J]. Synthetic Communications, 2000, 30(16): 2889-2895.  D. Channe Gowda, B Mahesh, Shankare Gowda. Zinc-catalyzed ammonium formate reductions: rapid and selective reduction of aliphatic and aromatic nitro compounds[J]. Indian Journal of Chemistry, 2001, 40B:75-77. 57 Tetrahedron Letters, 2002, 43:8527-8529.  Susanta K. Mohapatra, Sachin U. Sonavane, Radha V. Jayaram, Parasuraman Selvam. Regioand chemoselective catalytic transfer hydrogenation of aromatic nitro and carbonyl as well as reductive cleavage of azo compounds over novel mesoporous NiMCM-41 molecular sieves[J]. 4297-4300.  Amrita S. Kulkarni, Radha V. Jayaram. Liquid Organic Letters, 2002, 4(24): SHI Qi-xun et al. 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Preparation of aromatic amines by reduction of aromatic nitro compounds with metallic tellurium in near-critical water[J]. Chinese Journal of Chemistry, 2003, 21:222-224. Catalysis Chemical, 2000, nitro compounds of with hydrazine 2000, Journal Catalysis, aromatic perovskites prepared by microwave irradiation[J]. Applied Catalysis A: General, 2003, 252:225-230.  Amrita S. Kulkarni, Radha V. Jayaram. Liquid phase catalytic nitro transfer compounds hydrogenation on of aromatic La1-xSrFeO3 perovskites prepared by microwave irradiation[J]. Journal of Molecular Catalysis A: Chemical, 2004, 223:107-110.  Sachin U. Sonavane and Radha V. Jayaram. Catalytic transfer hydrogenation of nitro arenes, aldehydes, and ketones with propan-2-ol and KOH/NaOH Synthetic 843-849.  Shankare Gowda, D Channe Gowda. Zinc/hydrazine: a low cost-facile system for the reduction of nitro compounds[J]. Indian Journal of Chemistry, 2003, 42B:180-183.  G R Srinivasa, K Abiraj, D Channe Gowda. 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