Nitroso compounds hydrogenation, nitro compound reduction

Reductive alkylations have been carried out successfully with compounds that are not carbonyls or amines, but which are transformed during the hydrogenation to suitable functions. Azides, azo, hydrazo, nitro and nitroso compounds, oximes, pyridines, and hydroxylamines serve as amines phenols, acetals, ketals, or hydrazones serve as carbonyls 6,7,8,9,12,17,24,41,42,58). Alkylations using masked functions have been successful at times when use of unmasked functions have failed (2). In a synthesis leading to methoxatin, a key... 

Aromatic nitro compounds are hydrogenated very easily aliphatic nitro compounds considerably more slowly. Hydrogenations have been carried out successfully under a wide range of conditions including vapor phase (S9). Usually the goal of reduction is the amine, but at times the reduction is arrested at the intermediate hydroxylamine or oxime stage nitroso compounds never accumulate, although their transient presence may appreciably influence the course of reaction. In practice, nitro compounds often contain other reducible functions that are to be either maintained or reduced as well. 

Aromatic nitroso compounds usually are considered to be intermediates in the hydrogenation of a nitroaromatic compound to the aromatic hydroxyl-amine or amine. However, nitroso compounds do not accumulate in these reductions, suggesting that they are reduced more easily than are nitro compounds. Catalysts effective for the nitro group should also be effective for nitroso. 

Formation of azo-type products might be troublesome. These by-products, arising from reduction of aromatic nitro compounds, usually are assumed to be derived from the coupling of intermediate nitroso and hydroxylamine compounds. The coupling problem is accentuated in reduction of nitroso compounds because of much higher concentrations. It can be alleviated by dropwise addition of the substrate to the hydrogenation and use of acidic media. 

The main field of applications of hydrogen and alkali sulfides is reduction of nitrogen functions in nitro compounds [236, 240], nitroso compounds... 

With the exception of the nitroso stage, all the intermediate stages of the reduction of nitro compounds can be obtained by controlled catalytic hydrogenation [572], and all reduction intermediates were prepared by reduction with appropriate hydrides or complex hydrides. However, the outcome of many hydride reductions is difficult to predict. Therefore more specific reagents are preferred for partial reductions of nitro compounds. 

Nitro compounds in presence of carbonyl group are selectively reduced to amines in the presence of Raney nickel catalyst. Hydrazine reduces nitrdes yielding hydrazones. Under controlled reaction conditions other functional groups, including nitroso and oxime, may be reduced. Many partially hydrogenated derivatives, such as azo-, hydrazo-, and azoxy compounds may be obtained by partial reduction with hydrazine. Reaction with chlorobenzene yields benzene. 

In this paper the differences between the behaviour of aliphatic and aromatic nitro compounds adsorbed on a-Mn304 are discussed. The presence of a hydrogen atom on the a-carbon of aliphatic nitro compounds prevents their selective reduction to the nitroso analogues. Suggestions are made concerning the mechanisms of the reduction of nitrobenzene to nitrosobenzene and of the formation of some side products of the reduction (azobenzene and azoxybenzene). 

Catalytic reduction of aromatic nitro compounds to the amines is highly exothermic (AH = —548 12 kJ/mol) and has high potential for hazard in the event of cooling- or other process-failure. The total reaction proceeds via nitroso and hydroxylamino intermediates, both of which are reactive and may undergo undesired condensation or disproportionation reactions, and the thermochemistry of all these possibilities was investigated. The reduction or disproportionation of the hydroxylamino intermediate (which is of low thermal stability) is identified as the fastest and most exothermic step (despite which it can frequently be concentrated or trapped) implications for process safety are considered in detail and verified by experiment with typical compounds and intermediates [1]. A calorimetric study of the hazards inherent in hydrogenation of nitroaromatics was made, using nitrobenzene as model compound [2]. Individual incidents of this type are ... 

Cr(II) may be used to carry out all the reactions of Ti(III), but usually under milder conditions. Applications of Cr(II) as a reductant have been reviewed. The applications include Sn(IV) chloride in the presence of catalysts such as Sb(V) or Bi(III), Sb(V) in 20% HCl at elevated temperatures, Cu(II), silver, gold, mercury, bismuth, iron, cobalt, molybdenum, tungsten, uranium, dichromate, vanadate, titanium, thallium, hydrogen peroxide, oxygen in water and gases, as well as organic compounds such as azo, nitro, and nitroso compounds and quinones. Excess Cr(II) in sulfuric acid solution reduces nitrate to ammonium ion. The reduction is catalyzed by Ti(IV), which is rapidly reduced to Ti(III). 

Nitroso compounds, like the nitro compounds, are converted into amines by reduction with hydrogen. 

Nitroso, azoxy and azo groups are also reduced under the conditions used to effect the hydrogenation of nitro compounds. Nitroso compounds form the amine with about the same ease as is observed with the corresponding nitro compound. a-Nitroso-chlorides are hydrogenated to oximes over platinum if the reaction is stopped when the color disappears (Eqn. 19.30). No other catalyst is effective for this reaction, which is also superior to all chemical reduction methods. The use of other catalysts, as well as further hydrogenation over platinum, results in amine formation. ... 

Selective catalytic hydrogenation of aromatic nitro compounds finds many applications in fine and specialty chemical industries (1). This class of hydrogenation reactions has been studied extensively using various solvents, catalysts and under various reaction conditions (1). The hydrogenation reaction has been found to follow mainly a mechanism that was delineated by Haber in 1898 from his study of electrochemical reduction of nitrobenzene (2). The mechanism, consisting of two types of reaction pathways, is schematically described in Fig. 1. The first pathway is a monomeric one that proceeds in three consecutive steps (a) hydrogenolysis of one of the N-O bonds in the nitro group to produce the nitroso intermediate ... 

Only activated monoenes are hydrogenated . These include carvene, mesityl oxide, 2-cyclohexenone, and benzalacetone . Some styrenes are hydrogenated a-functionalized styrenes react, but )S-functionalized styrenes do not - " . Similarly, only activated ketones such as benzil, diacetyl and p-benzoquinone are hydrogenated to alcohols " . Often catalytic reduction of a ketone is observed only in the presence of added OH . The base is believed to react with an intermediate to give [Co(CN)j(OH)] and the reduced substrate . Aryl ketones such as acetophenone and benzophenone are not reduced . Several examples of nitro and nitroso compound reductions have been reported - . ... 

Catalytic hydrogenations of aromatic nitro compounds with a stable hydroxylamine intermediate often have two different kinetic phases hydrogen uptake is rapid up to ca 60 %, then distinctly slower in the second phase. This means that reduction of the hydroxylamine to the aniline, formally a hydrogenolysis, is difficult in these cases. In the presence of the promoters discussed in Section 8.5.4.3, the second phase is less pronounced or disappears. This suggests a mechanism which could be called catalytic by-pass (see Figure 4). Experiments in the absence of hydrogen indicated that the vanadium promoters catalyze the disproportionation to give aniline and the nitroso intermediates that re-enter the catalytic cycle. As a consequence, the hydroxylamine does not accumulate and aniline formation is accelerated. 

Sulfonation of nitroso compounds by sodium hydrogen sulfite leads to amino sulfonic acids by simultaneous reduction.1240 194 When aromatic nitro compounds are subjected to this reaction, sulfonation in the ring is accompanied by formation of arylamine-JV-sulfonic acids (arylsulfamic acids), which on acid fission also give amino sulfonic acids this reaction, known in the literature as the Piria reaction, has been reviewed in papers by Hunter and his co-workers.195... 

According to this mechanism, nitro compounds are reduced to the corresponding hydroxylamines or amines by an irreversible four- or six-electron process that proceeds through nitroso intermediates. Nitroso intermediates are not stable on the electrode surface because their reduction potential is more positive than that of the initial nitro compounds. Azoxy, azo, and hydrazo derivatives can also be obtained by electrolysis, particularly in alkaline solutions, as a result of chemical follow-up reactions rather than electrochemical reactions. The electrochemistry of all these compounds has been discussed in reviews [52-54]. Among the different nitro compounds, nitrobenzene has played a central role and its reduction is one of the most thoroughly studied electrochemical reaction. The reduction involves adsorption of reactants and intermediates and follows direct electron-exchange mechanism on most cathodes except of platinum, in which the reduction proceeds by an elec-trocatalytic hydrogenation mechanism. 

A general method has been developed for the rapid, metal-catalysed, transfer reduction of nitro-compounds to N-substituted hydroxylamines. High yields of hydroxylamines are obtained by reduction with hydrazine as hydrogen donor and a rhodium-charcoal catalyst, or alternatively using phosphinic acid as hydrogen donor and a two-phase solvent system, with a palladium-charcoal catalyst. Some hydroxylamines were too labile for isolation in a pure state and were further characterized by oxidation to the corresponding nitroso-compounds with ferric chloride. 

The N,]S -dialkyl-/)-PDAs are manufactured by reductively alkylating -PDA with ketones. Alternatively, these compounds can be prepared from the ketone and -lutroaruline with catalytic hydrogenation. The /V-alkyl-/V-aryl- -PDAs are made by reductively alkylating -nitro-, -nitroso-, or /)-aminodipheny1 amine with ketones. The AijAT-dialkyl- PDAs are made by condensing various anilines with hydroquinone in the presence of an acid catalyst (see Amines-aromatic,phenylenediamines). 

Hydrogen reduction of organic nitrogen compounds occurs readily in the presence of homogeneous and heterogeneous catalysts, " , e.g., nitro, nitroso, oxime, nitrile, imine, hydrazone and azide compounds are reduced ... 

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