Transfer hydrogenation nitroarenes

The iridium complex 35 has been also used as catalyst for the transfer hydrogenation of substituted nitroarenes [34]. Good to very good conversions were observed (2.5 mol%, in refluxing isopropanol, 12 h). A mixture of two products was obtained, the relative ratio of which depends on the concentration of added base (KOH) and catalyst. (Scheme 2.5)... 

Scheme 2.5 Products formed in the transfer hydrogenation reduction of nitroarenes by complex 35...

In our previous paper (ref. 2) we demonstrated the particular role played by one-electron donor centres on magnesia surface in catalytic transfer hydrogenation. Moreover, nitroarenes exhibit high tendency to convert themselves into corresponding anion radicals during adsorption on MgO. Thus, it was expected that esr spectroscopy would reveal new data concerning the reactants activation. 

Transfer hydrogenation of nitroarenes. Transfer hydrogenation of the ni-troarene 2 with Raney nickel and isopropanol as the hydride donor results in the... 

Iron-substituted mesoporous aluminophosphate was used as a novel, efficient and ecofriendly catalyst for reductive cleavage of azo dyes and reduction in nitroarenes and catalytic transfer hydrogenation of nitro and carbonyl compounds. Unlike most of the iron-containing molecular sieves, dislodgement of tetrahedral Fe(III) was not observed upon various process treatments such... 

Sabater S, MataJA, Peris E. Dual catalysis with an Ir —Au heterodimetaUic complex reduction of nitroarenes by transfer hydrogenation using primary alcohols. Chem EurJ. 2012 18 6380-6385. 

Quaternary ammonium salts aid the transfer of the hypophosphite anion in the palladium-catalysed reduction of, for example, alkynes to alkenes, nitroarenes to aminoarenes, and in the hydrogenolysis of tetrazolyl aryl ethers to phenols [12-14], It has been demonstrated that the hydrogenolysis is ineffective when preformed tetra-n-butylammonium hypophosphite is employed in a dry homogenous organic solvent [13, 14], For optimum hydrogen transfer, the concentration of hypophosphite relative to the substrate must be controlled at a low level and this is most effectively accomplished with a two-phase system. 

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. 

Strong centres, forming anion radical even from nitrobenzene molecule are poisoned irreversibly, however, their presence is not necessity for the preservation of catalytic activity. Taking into consideration that regenerated MgO which is not able to ionize nitrobenzene molecule is still active in its reduction by hydrogen transfer and that only a few from reduced nitro compounds form ion radicals on catalyst surface one can ascertain that ion radicals formation is not necessary step in nitroarenes (or nitroparaffins) activation. Probably, one-electron donor sites take part only in activation of alcohol what was demonstrated by us earlier. 

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. ... 

For the reduction of nitroarenes to aminoarenes by the catalytic hydrazine H-transfer reduction method, the classical hydrogenation catalysts Ni, Pd and Pt are most commonly used [1] [2]. In a more extended study [3] we were able to confirm previously reported observations [4] that these reductions can also be catalysed by modified iron oxides hydroxides. This method for the production of many aromatic amines offers several advantages compared to the conventional processes still employed in industry, such as the environmentally imfavourable Bechamp [5] and Zinin reductions [6]. It is an outstanding feature of the novel reduction method presented here that further reducible substituents in nitroazo compounds, such as... 

Finally the formation of the aldehyde 58 during the catalytic reaction could be due to an internal hydrogen transfer from the alcoholic group to nitrogen. Catalytic hydrogen transfer reduction of nitroarenes by alcohols is a known reaction. 

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