Reactions of Nitro Compounds

Reduction of a nitro group to an amino group is one of the most important reactions in aromatic chemistry and there are many methods available to achieve this transformation. 

Catalytic hydrogenation is a heterogeneous reaction which occurs at the surface of the catalyst. The mechanism is complex and proceeds through the nitroso and hydroxylamine derivatives minor by-products such as the azo and azoxy compounds occasionally appear. 

Various combinations of metal and acid reduce nitro groups and this method lends itself to both small-scale laboratory work and industrial conditions. Tin and hydrochloric acid are used when small quantities are involved, but work-up requires basification to destroy the 

The metal catalysts are recovered and regenerated by the manufacturers. Care is required when handling finely divided metal catalysts since some of them react readily with atmospheric oxygen and burst into flames, 

Commercial use of this method is increasing and some companies have developed this technique to a high degree of sophistication. 

The method of choice in industry is catalytic hydrogenation under pressure. The nitro compound, dissolved in an alcohol such as propan- 

Introduction. Nitro compounds are insoluble in water. Primary nitro compounds, RCH2NO2, and secondary, RjCHNOj, are soluble in alkaline solutions, due to the formation of an oci-form by tautomerization, which reacts with bases to form salts. The most important reaction of nitro compounds is their reduction in acid media to amines. In neutral solutions the intermediate products, hydroxylamines, may be isolated. In alkaline media, azoxy, azo, and hydrazo compounds are formed. The object of this experiment is to illustrate some of these reactions. 

If it is desired to prepare the sodium salt of the aci-iorm, proceed thus Prepare an alcoholic solution of sodium methoxide by adding 0.2 g of sodium to 5 ml of methanol, cooling the tube shghtly. Vhen all the sodium has dissolved, add 0.5 g of 1-nitropropane, shake, and cool. The salt may be filtered, washed with methanol, and dried. 

Place in an eight-inch test tube arranged for reflux 1 g of nitrobenzene, 6 ml of methanol, and 1.4 g of solid sodium hydroxide. Heat for thirty minutes or until the odor of nitrobenzene is very faint. Add the reaction mixture to 30 ml of water, and cool. The oily azoxybenzene which separates out soon solidifies. Filter the crystals. It may be lecrystallized from methanol. 

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Cycloaddition reactions of nitro compounds, nitrones, and nitrile oxides 99JCS(P1)749. 

A new multicomponent reaction of nitro compounds with isocyanides gives ct-oxyimi-noamides, which are important for dnig synthesis such as cephiilosporin and fi-lactamase inhibitor fEq 6 65 Multicomponent reactions using isocyanides fUgi reacdoni is re-... 

Reactions of nitro compounds with chiral imines have only recently been described. Either chiral 1-phenylethylamine (auxiliary) or the glyceraldehyde acetonide aldehyde was used as the chiral precursors of the imines 66 and 68, which reacted with 3-mesyloxynitropropane to give the 3-nitropyrrolidines dl)-67 and 69, respectively, with good diastereoselectivity. In fact, both products were obtained (almost) exclusively as trans diastereomers with high level of asymmetric induction, but the configurations of the newly formed stereocenters were not determined [44] (Scheme 13). N-Boc imines can be formed... 

Similarly, the reaction of nitro compounds with the M-Boc aromatic imines 86 occurred in the presence of the enantiopure protic catalyst 87, which is a white, crystalline bench-stable salt [52] (Scheme 15). The reactions of ni-tromethane, very slow at - 20 °C, were accelerated in the presence of 10 mol % of 87, and the /3-amino compounds 88 were obtained with moderate yields and moderate to high enantioselectivities. Positive results were also obtained in the corresponding reactions of nitropropane to give the products 90. Hence, the primary diamines 89 and 91 are available by this route, which is advantageous for the significantly lower cost and toxicity of the catalyst and its easy removal from the reaction mixture simply by a basic wash. These results should stimulate further research on the development of new acid-catalyzed systems. 

Another application of diastereoselective nitro-aldol reactions catalyzed by Bu4NF-3H20 is demonstrated in a simple synthesis of l,4-dideoxy-l,4-imino-D-mannitol (DIM) and amino analogues (Eq. 3.85).134 The nitro-aldol reaction of nitro compounds bearing a-oxy or a-amino function with glyceraldehyde leads to nitrohexitols, which can be reduced to the corresponding amino compounds. Cyclization gives iminopolyols, as shown in Eq. 3.85. 

The carbon-carbon bond forming reactions of nitro compounds by alkylation with alkyl halides or acylation with acyl halides have been encountered with difficulties of the competing O-alkylation or O-acylation, respectively. In this chapter, the recent developments of C-alkylations and C-acylations of nitro compounds are summarized. The O-alkylated compounds undergo cycloaddition reactions, which are discussed in the chapter of cycloaddition (Chapter 8). 

The methods considered in this section concern mainly reactions of nitro compounds. 

Although many light-induced reactions of nitro compounds are known, attempts to elucidate the multiplicity of the reacting excited state have been made in only a fraction of cases. Conclusive information is not easily obtained since in general nitroaromatics suffer from the disadvantage that they show very weak or even no fluorescence and have very short triplet lifetimes. 

In combustion reactions of nitro-compounds and nitro-ethers it is possible that the deoxidation of NO, i.e., the reactions 2NO+2H2 — N2+H2O and 2NO+2CO = N2+2CO2, is such a stage. 

Undyed pegamoid fabrics are treated with a few drops of concentrated sulphuric add containing a small quantity of diphenylamine in solution a blue spot is formed (reaction of nitro-compounds). 

The reaction of nitro compound 337 with /-butyloxycarbonic anhydride in the presence of DMAP resulted in 2-nitro-benzenesulfonamide 338, which upon reduction with iron in AcOH gave 339 (Scheme 70). The latter when refluxed with 2,5-dimethoxytetrahydrofuran in AcOH afforded pyrrole sulfonamide 340 in good yield along with the fused 1,2,5-thiadiazepines 103 and 104, as a side product. Subsequently, 340 upon reaction with triphosgene afforded 103 and 104. 

Today, many stable radicals are known, as shown in Figures 1.11 and 1.12. However, most of them are nitroxyl radicals like NO or N02. Standard generation methods of nitroxyl radicals are as follows. One is the oxidation of amines or hydroxyamines by Pb02, or by less toxic oxidants such as oxone, Cu(OAc)2, mCPBA (eqs. 1.13 and 1.14). Another one is the reaction of nitro compounds with Grignard reagents (eq. 1.15) [9-14]. 

Reaction of nitro compounds such as nitro-tert-butane with Bu3SnH or (Me3Si)3SiH produces a nitroxyl radical (27). This is just an addition product of Bu3Sn or (Me3Si)3Si onto the nitro group [15, 16]. This radical is also a tt radical (eq. 1.16). 

A nitro group attached to an aromatic ring may activate the ring. Owing to this the nitro-substituted aromatic ring can take part in reactions in which it usually remains more or less inactive. Nucleophilic reactions of nitro compounds should be mentioned here first. 

Another example of the homolytic reaction of nitro compounds with free radicals is their inhibiting effect on addition polymerization of vinyl compounds, especially by the higher nitrated compounds which have the most pronounced inhibiting properties. 

Complex metal hydroborate-d., and hydroaluminate-d reductions are used in syntheses of deuterioamines. Reactions of nitro compounds ... 

Action of bases in nucleophilic reactions of nitro compounds Action of Grignard reagent on nitro aimpounds Reaction of aromatic nitro compounds with diazomethane... 

A few more free radical reactions of nitro compounds should be mentioned. Some of these reactions are photo-induced and are described in the chapter on photochemistry of nitro compounds. 

The reactions of nitro compounds with bases are nucleophilic reactions and recently have received much attention. Numerous monographs include chapters on nucleophilic substitution of nitro compounds and review articles have been published. Such are the excellent reviews by de Boer and Dirkx [9], Hall and Poranski [10]. References in [9] are brought up to 1966, and it seems appropriate to complete the list of review articles by mentioning the later review papers Chains [11], Buncel and co-workers [12], Miller (13),Pietra [14], Buck [IS], Crampton [16], Strauss [17], Bemasconi [18], Sekiguchi and Yuki [19],... 

E.s.r. observations of the reactions of nitro compounds with reducing agents have given evidence about a number of the individual steps in the overall processes. The first step in the reaction with a one-electron reducing agent is the formation of the corresponding radical-anion. Subsequent steps depend on the structiu e of the nitro compound. 

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