Reduction of Nitro and Nitroso Compounds

The deoxygenation of nitro and nitroso groups may generate nitrenes, which can be carried out with various reagents although trivalent phosphorus compounds, particularly triethyl phosphite, are most commonly used. The deoxygenation may 

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The following paragraphs describe the methods of forming azo compounds in which an N-N bond is created (for preparation by coupling see the Section on Preparation of the C-N bond by replacement .) These methods can be classified as (a) oxidative junction of amines (b) oxidation of hydrazo compounds (c) disproportionation of hydrazo compounds (d) action of hypochlorite on iV,jV -diarylsulfamides (e) condensation of nitroso compounds with amines (f) reaction of nitro compounds with amines (g) reduction of nitro and nitroso compounds and (h) reaction of JV-arylhydroxylamines with sulfinylimines. 

Most of the work in this area was done by Kokkinidis and coworkers [2] who demonstrated catalytic effects of metal adlayers for the reduction of nitro and nitroso compounds [131-134], and for the catalytic... 

The influence of ad-atoms on the reduction of nitro and nitroso compounds on... 

The possible role of nitrene intermediates in the carbonylation and reduction of nitro- and nitroso compounds has been a matter of speculation since the very beginning. However, most, if not all, of the experimental data on this topic is not univocal in giving an answer to this problem and the common believing has evolved during the years mainly because of a general feeling, rather than because of new clear-cut data. 

Cadogan JIG, Marshall R, Smith DM, Todd MJ (1970) Reduction of nitro- and nitroso-compounds by tervalent phosphorus reagents. Part Vin. Syntheses of benzimidazoles and anthranils. J Chem Soc C 18 2441-2443... 

Reduction of nitrocompounds involves a series of one-electron additions and chemical steps and an important reaction intermediate is the nitrosocompound. The redox properties of nitro- and nitroso-compounds will be discussed first. Some values of reduction potentials for nitrocompounds are collected in Table 11.1. 

Oxidation of azo compounds 9-36 Oxidation of amines 9-66 Reduction of nitro or nitroso compounds reaction between nitroso compounds and hydroxylamines... 

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 - . ... 

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 and nitroso compounds are easily reduced at carbon and mercury electrodes. Other nitro compounds such as nitrate esters, nitramines, and nitrosamines are also typically easily reduced. The complete reduction of a nitro compound consists of three two-electron steps (nitro-nitroso-hydroxylamine-amine). Since most organic oxidations are only two-electron processes, higher sensitivity is typically found for nitro compounds. Several LCEC based determination of nitro compounds have been reported... 

NITRO- AND NITROSO-COMPOUNDS. Nitro-compounds contain the nitro group (-NOil attached directly to a carbon atom nitroso-compounds contain the nitroso-group (—NO) similarly attached. A very important member of this group is nitrobenzene, which upon reduction yields a variety of products, important in the synthesis of drugs and dyes. See Table I. 

In this section the electrode reactions of amines and nitro and nitroso compounds will be discussed. The presence of an amino group usually makes the reduction of a molecule more difficult, but the amino group is, in most cases, not directly involved in the reduction. Three types of exception have been found. 

REDUCTION OF AROMATIC NITRO AND NITROSO COMPOUNDS TO AZO AND AZOXY COMPOUNDS... 

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). 

The reduction of heterocyclic C- and N-nitro and nitroso compounds exhibits generally no special features and is not discussed here compare Chapter 9. 

Less known is the reduction of nitro to nitroso group. It was reviewed by Boyer in a chapter Reduction and Deoxygenation of Nitro Compounds [147]. The following are methods giving good yields of nitroso derivatives ... 

It is intriguing to consider the mode of reduction that elemental phosphorus might take if, in the absence or presence of iodine, it is effective in reducing nitro and nitroso compounds (Eq. 3). What type of reduced nitrogen compounds would form Also, this might be an elegant method for reducing the N-oxldes of heterocyclic compounds. These N-oxides are often formed in heterocylic compounds, such as pyridine, to alter the orientation of substitution. 

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