Selective Reduction of Nitroarenes

Tafesh et al. [26] first began using water-soluble Pd complexes with TPPTS or BINAS as ligands in the CO selective reduction of nitroarenes, obtaining a yield less than 65%. 

Recently, the CO selective reduction of nitroarenes catalyzed by Ru3(CO)9(PETPP)j [27] and Ru3(CO)9(PEO-DPPSA) [28] has been reported by Jin et al. The results are listed in Table 2. After being used four times, the activity of the catalyst is not evidently decreased. 

Wiebus, B. Cornils, Abstr. 1st lUPAC Symp. Org. Chemistry, Jerusalem 1986, p. 295  

ScHONFELDT, Suifuce-Active Ethylene Oxide Jidducts, Wissenschaftliche Verlagsges. mbH, Stuttgart 1976. 

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The selective reduction of nitroarenes containing benzyl-protected phenolic groups without concomitant hydrogenolysis has also been achieved using hydrazine hydrate and Raney nickel. This procedure avoids strongly acidic conditions. The reaction is selective and a variety of (V-benzyl- and chloro-sub-stituted nitroarenes are reduced to the corresponding anilines without dehalogenation or debenzylation. 

BINAS, 38) catalyzed the selective reduction of nitroarenes to anilines at 100°C under 120 bar CO [149]. 

A similarly selective reduction of nitroarenes was achieved under WGS conditions by using [ Ru3(CO)12] in the presence of certain amines, such as diisopropylamine, piperidine, dibutylamine, and triethylamine [146]. Importantly, the latter reaction yielded no unwanted H2 as by-product from a concomitant WGS reaction. 

Associated with some metals (Al, Fe, Zn), BiCl3 gives Bi(0) which is a catalyst for the allylation of aldehydes and amines (ref. 35), and for the reduction of aromatic nitro compounds to azoxy compounds (ref. 36). When associated with sodium borohydride, BiCl3 gives an efficient system for the selective reduction of nitroarenes and azomethines (ref. 37). 

Reduction. A selective reduction of nitroarenes to arylamines occurs with Zn in near-critical water (at 250°). Cyclization that follows reduction is expected in the case of 2-nitroaryl compounds containing a leaving group at proper distance, ArN02 is converted to ArNR2 by a reductive alkylation process in protic solvents (promoted by Zn or Sn). ... 

TRPTC has already been applied successfully in the hydroformylation of higher olefins and the CO selective reduction of nitroarenes, both on a laboratory scale. 

Pasha, M. A. Jayashankara, V. P. Sn/NH4Br assisted selective reduction of nitroarenes into anilines under neutral conditions. Indian J. Chem., Sect. B 2004, 43B, 2464-2466. 

Selected examples of the ruthenium promoted reduction of nitroarenes... 

Selective reduction to hydroxylamine can be achieved in a variety of ways the most widely applicable systems utilize zinc and ammonium chloride in an aqueous or alcoholic medium. The overreduction to amines can be prevented by using a two-phase solvent system. Hydroxylamines have also been obtained from nitro compounds using molecular hydrogen and iridium catalysts. A rapid metal-catalyzed transfer reduction of aromatic nitroarenes to N-substituted hydroxylamines has also been developed the method employs palladium and rhodium on charcoal as catalyst and a variety of hydrogen donors such as cyclohexene, hydrazine, formic acid and phosphinic acid. The reduction of nitroarenes to arylhydroxyl-amines can also be achieved using hydrazine in the presence of Raney nickel or iron(III) oxide. ... 

Sodium borohydride reduces disulfides to thiols, which can then be used to reduce nitro groups. Based on the redox properties of 1,2-dithiolane, lipoamide (17) was used for the selective reduction of mono-substituted nitrobenzenes to the corresponding anilines. Lipoamide (17) can also be immobilized on hydrophilic polymers such as polyvinylamine, polyethyleneimine and chitosan. These polymeric reducing catalysts can be recycled and are easy to separate from the reaction mixture. The system has been used to reduce nitroarenes to anilines. ... 

The iron cluster [Fe4S4(SPh)4] catalyzes the reduction of nitroarenes to arylamines. " A less hydridic nucleophile [HFe(CO)4] has also found application as a selective reducing agent for nitroarenes." Although [HFe(CO)4]"] is known to reduce aldehydes, ketones and acid halides," in THF solvent with tri-fluoroacetic acid, it selectively reduces nitrobenzenes to anilines in the presence of aldehyde and acid halide groups. 

Magnesium oxide exhibited high activity and high selectivity in the hydrogen transfer from alcohols to studied nitroarenes. Because of the limited space of the paper the complete amine yield - temperature dependence was shown only for nitrobenzene reduction (Table 1). However, also for other reactants the yield of the aminic product increased continously between the values obtained at the lowest (350°C) and the highest (450°C) reaction temperatures. Below 350°C the complete lack of activity of MgO in the studied transformation was noted. The same was observed by us earlier (ref. 2) in the case the catalytic transfer reduction of other functional groups. 

Future trends in reduction of substituted nitrobenzenes will probably be based on novel catalysts. Homogenous transition metal (ruthenium and rhodium) catalysts offer routes to chemospecific reduction of aromatic nitro groups16. Novel catalytic methods involving combinatorial chemistry may offer pathways to new industrial hydrogenation processes, where selective reduction is desired. A number of solution- and solid-phase C /Mo0 redox couple reductions of substituted nitroarenes to the corresponding anilines have been proposed17. 

Selective hydrogenation of enones," particularly assisted by ultrasonic irradiation," is a synthetically valuable process. Ultrasonically activated nickel reduces nitroarenes to the azoxyarene stage. On the other hand, with formic acid or ammonium formate as the hydrogen source, the reduction results in arylamines."... 

Ruthenium catalysts was explored and applied in the reduction of nitro compounds as well. But because of its high price, their large scale applications are limited. Cenini and colleagues reported a Ru3(CO)i2 and Ru(CO)3(PPh3>2 catalyzed carbonylation of nitroarenes [44, 45]. The corresponding carbamates are produced in high selectivity in the presence of NEt4Cl under 82 bar of CO. 

Trisulphonated triphenylphosphine (TPPTS) and a mixture of tetra-, penta-and hexasulphonated 2,2 -bis(diphenylphosphinomethylene)binaphthyl (BINAS) have also been used in conjunction with PdCb for the reduction of several nitroarenes in a biphasie xylene/H20 medium [82], but the experimental conditions (especially the pressure) are very foreing (120 bar, 100 °C), the catalytic ratio is very poor (40, for a reaction run for 20 h), and the selectivity (best 85 %, but around 50 % in other cases) is also much lower than for most of the catalytic systems discussed in this chapter, making the present catalytic system unattractive (Note that the claims in the paper that other systems are unselective for the nitro group and that this is the first system in which a water soluble catalyst was used are unbased. Apart from the selectivity problem, which is dealt in detail in the introduction, several catalytic systems have been previously mentioned which work under phase-transfer-catalysed conditions, under which anionic water-soluble complexes are surely formed). 

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