Nitroarenes, hydrogenation

The vicarious nucleophilic substimtion of carbo- and hetero-cyclic nitroarene hydrogen by a hydroxyl group, on reaction with silylhydroperoxide anions, has been shown to proceed via nucleophilic addition of ROO followed by base induced elimination of ROH by an ii2-type mechanism the required orientation of the hydroxylation can be controlled by the conditions selected. ... 

From nitroarenes hydrogenation in the presence of aldehydes leads to A-monoalkyl arylamines. Hydrogenation of 2-(o-nitroaryl)-2-cycloalkenones gives annulated indoles (e.g., tetrahydrocarbazoles). ... 

Nakamula 1, Yamanoi Y, Yonezawa T, Imaoka T, Yamamoto K, Nishihara H (2008). Nanocage catalysts-rhodium nanoclusters encapsulated with dendrimers as accessible and stable catalysts for olefin and nitroarene hydrogenations. Chem Comm 44 5716... 

The carbanion of 2,3-dimethylthiazolidine-4-one reacted with nitroarenes to give either a ting opened product (50) via a VNS (vicarious nucleophilic substitution) reaction or a product resulting from oxidative nucleophilic substitution of hydrogen (51). Ring opening VNS reactions with 5-membered 5-heterocycles are limited to those heterocycles which show some conformational flexibility <96TL983>. 

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...

Nitrosoarenes are readily formed by the oxidation of primary N-hydroxy arylamines and several mechanisms appear to be involved. These include 1) the metal-catalyzed oxidation/reduction to nitrosoarenes, azoxyarenes and arylamines (144) 2) the 02-dependent, metal-catalyzed oxidation to nitrosoarenes (145) 3) the 02-dependent, hemoglobin-mediated co-oxidation to nitrosoarenes and methe-moglobin (146) and 4) the 0 2-dependent conversion of N-hydroxy arylamines to nitrosoarenes, nitrosophenols and nitroarenes (147,148). Each of these processes can involve intermediate nitroxide radicals, superoxide anion radicals, hydrogen peroxide and hydroxyl radicals, all of which have been observed in model systems (149,151). Although these radicals are electrophilic and have been suggested to result in DNA damage (151,152), a causal relationship has not yet been established. Nitrosoarenes, on the other hand, are readily formed in in vitro metabolic incubations (2,153) and have been shown to react covalently with lipids (154), proteins (28,155) and GSH (17,156-159). Nitrosoarenes are also readily reduced to N-hydroxy arylamines by ascorbic acid (17,160) and by reduced pyridine nucleotides (9,161). 

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. 

Ort/io-substituted nitroarenes can be distinguished from meta- or para-substituted isomers due to a characteristic change in their mass spectra. In precence of a hydrogen-donating ortho subtituent, [M-OH] replaces the [M-O] the fragment ion, e.g., in case of the above nitrophenols, the ion at m/z 123 is replaced by one at m/z 122 (Fig. 6.49c). The mechanism of this OH loss is not quite clear, [209] because different proposals [210-212] are in good accordance with the experimental data. Here, the mechanism which resembles the previously discussed ortho eliminations is shown [212]... 

The catalytic reduction of nitro groups is usually achieved using heterogeneous catalysts, although the iridium complex 28 has been shown to be effective for the reduction of p-nitroanisole 29 to the corresponding aniline 30 using isopropanol as the hydrogen donor (Scheme 8) [30]. In the reduction of some nitroarenes, azo compounds (Ar-N=N-Ar) could be formed as by-products or as the major product by variation of the reaction conditions. 

Hydrogen telluride and sodium telluride in a protic solvent easily reduce nitroarenes to anilines. In contrast, sodium telluride in an aprotic solvent exhibits a milder reducing capacity and reduces the nitro compounds only to azo compounds. ... 

Catalytic hydrogenation of the nitroarene 601 afforded the corresponding arylamine 598. An electrophilic aromatic substitution of the arylamine 598 by... 

Quinoline synthesis. In the presence of RuC13 3H20 and Bu3P, 1,3-propanediol and an aniline condense to form a quinoline. Yields are low in the absence of a hydrogen acceptor. Yields are improved by use of the nitroarene which is reduced to the aniline undergoing reaction.1... 

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. 

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. 

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. 

There has been a short review of the oxidative nucleophilic substitution of hydrogen in nitroarenes in which recent results with carbon, nitrogen, and oxygen nucleophiles are summarized and the preferred oxidants are discussed.11 The oxidative substitution of nitroarenes with carbanions of isopropyl phenylacetate in liquid ammonia-KMn04 initially yields products (4) which may suffer hydroxylation at the o -position, and dimeric and trimeric products may be formed by couplings of nitrobenzylic radicals formed during the reaction.12... 

---