Aromatic displacement, nitro group

I itro-DisplacementPolymerization. The facile nucleophilic displacement of a nitro group on a phthalimide by an oxyanion has been used to prepare polyetherimides by heating bisphenoxides with bisnitrophthalimides (91). For example with 4,4 -dinitro monomers, a polymer with the Ultem backbone is prepared as follows (92). Because of the high reactivity of the nitro phthalimides, the polymerkation can be carried out at temperatures below 75°C. Relative reactivities are nitro compounds over halogens, Ai-aryl imides over A/-alkyl imides, and 3-substituents over 4-substituents. Solvents are usually dipolar aprotic Hquids such as dimethyl sulfoxide, and sometimes an aromatic Hquid is used, in addition. 

Solvent for Displacement Reactions. As the most polar of the common aprotic solvents, DMSO is a favored solvent for displacement reactions because of its high dielectric constant and because anions are less solvated in it (87). Rates for these reactions are sometimes a thousand times faster in DMSO than in alcohols. Suitable nucleophiles include acetyUde ion, alkoxide ion, hydroxide ion, azide ion, carbanions, carboxylate ions, cyanide ion, hahde ions, mercaptide ions, phenoxide ions, nitrite ions, and thiocyanate ions (31). Rates of displacement by amides or amines are also greater in DMSO than in alcohol or aqueous solutions. Dimethyl sulfoxide is used as the reaction solvent in the manufacture of high performance, polyaryl ether polymers by reaction of bis(4,4 -chlorophenyl) sulfone with the disodium salts of dihydroxyphenols, eg, bisphenol A or 4,4 -sulfonylbisphenol (88). These and related reactions are made more economical by efficient recycling of DMSO (89). Nucleophilic displacement of activated aromatic nitro groups with aryloxy anion in DMSO is a versatile and useful reaction for the synthesis of aromatic ethers and polyethers (90). 

Intramolecular nucleophilic displacement reactions of aromatic nitro group by various nucleophiles include cydization reactions, which provide practical methods for the synthesis of a variety of heterocycles. 1 hope that the text of this review suggests a wide range of potential of this reaction in organic synthesis of various heterocycles. However, it is necessary to stress that some structural types described in this review could be prepared with similar, or even better yields by other methods. In spite of this, there are many heterocyclic systems for the synthesis of which the denitrocyclization strategy is a method of choice. 

F. Terrier, Nucleophilic Aromatic Displacement The Influence of the Nitro Group", VCH Publishers, New York (1991). 

In a similar vein, the amino group in sulfide 14 (obtained presumably by an aromatic displacement reaction) is first converted to the bromide by Sandmeyer reaction to give 25. Reduction of the nitro group (16) followed by cyclization gives the substituted phenothiazine. Alkylation with the familiar halide (3) affords dimethothiazine (18). ... 

When aromatic nitro compounds are treated with cyanide ion, the nitro group is displaced and a carboxyl group enters with cine substitution (p. 854), always ortho to the displaced group, never meta or para. The scope of this reaction, called the von Richter rearrangement, is variable. As with other nucleophilic aromatic substitutions, the reaction gives best results when electron-withdrawing groups are in ortho and para positions, but yields are low, usually < 20% and never > 50%. 

A kinetic study of the previously reported substitution of aromatic nitro groups by tervalent phosphorus has established an aromatic 5n2 mechanism. Similarities in values of activation energies, and in relative reactivities of phosphite and phosphonite esters, between this displacement and the Arbusov reaction suggest a related mechanism (31), while the lack of reactivity of p-dinitrobenzene is attributed to the need for intramolecular solvation (32). The exclusive formation of ethyl nitrite, rather than other isomers, is confirmed from the decomposition of triethoxy-(ethyl)phosphonium fluoroborate (33) in the presence of silver nitrite. A mechanism involving quinquevalent phosphorus (34) still seems applicable, particularly in view of the recent mechanistic work on the Arbusov reaction. ... 

Arylations of nitro compounds can be achieved by aromatic nucleophilic substitution using aromatic nitro compounds, as discussed in Chapter 9.100 Komblum and coworkers reported displacement of the nitro group of nitrobenzenes by the anion of nitroalkanes. The reactions are usually carried out in dipolar aprotic solvents such as DMSO or HMPA, and nitroaromatic rings are substituted by a variety of electron-withdrawing groups (see Eq. 5.63).101... 

Such nucleophilic displacements are likely to be addition-elimination reactions, whether or not radical anions are also interposed as intermediates. The addition of methoxide ion to 2-nitrofuran in methanol or dimethyl sulfoxide affords a deep red salt of the anion 69 PMR shows the 5-proton has the greatest upfield shift, the 3- and 4-protons remaining vinylic in type.18 7 The similar additions in the thiophene series are less complete, presumably because oxygen is relatively electronegative and the furan aromaticity relatively low. Additional electronegative substituents increase the rate of addition and a second nitro group makes it necessary to use stopped flow techniques of rate measurement.141 In contrast, one acyl group (benzoyl or carboxy) does not stabilize an addition product and seldom promotes nucleophilic substitution by weaker nucleophiles such as ammonia. Whereas... 

F. Terrier, in Nucleophilic Aromatic Displacement The Influence of the Nitro Group (H. Feuer, ed.), p. 257. Org. Nitro Chem. Ser., VCH Publishers, New York, 1991. 

Diazophenol formation is most competitive when a nitramine substrate contains an electron-withdrawing nitro group ortho to the nitro group being displaced and hence meta to the nitramine functionality, assumedly because that site is then activated towards nucleophilic aromatic substitution. Heating nitramines in inert chlorinated solvents also favours diazophenol formation but this is suppressed by using urea or sulfamic acid as additives. 

Sandmeyer-type reactions are a useful route to polynitroarylenes with unusual substitution patterns. In these reactions an arylamine is treated with a source of nitrous acid to form an intermediate diazonium salt which is readily displaced on reaction with a suitable nucleophile. Many substituents can be incorporated into the aromatic ring via this method, including the nitro group. 

In general, the more nitro groups present on the aromatic ring the easier the leaving group displacement. Nucleophilic aromatic substitution is therefore a very important reaction in the chemistry of polynitroarylenes. While the use of such reactions has been extensive in the synthesis of explosives, the reaction also has important implications for the chemical stability of many polynitroarylenes (discussed in Section 4.8.2). 

In a similar manner, of the isomeric trinitrobenzenes, only the symmetrical 1,3,5-isomer shows sufficient chemical stability for use as an explosive. Even so, the aromatic ring of 1,3,5-trinitrobenzene is highly electron deficient and reaction with alkali metal carbonates or bicarbonates in aqueous boiling methanol yields 3,5-dinitroanisole. Unsymmetrical isomers of trinitrobenzene are much more reactive than the 1,3,5-isomer, with only relatively mild conditions needed to effect the displacement of their nitro groups. ... 

A still different scheme is used for the preparation of the benzimidazole buterizine (74). Alkylation of benzhydrylpiperazine with substituted benzyl chloride 70 gives the intermediate 7U Nucleophilic aromatic displacement on this compound by means of ethyl amine leads to reduction of the nitro group then gives the diamine T. Treatment of that with the orthoformate ester of pentanoic acid serves to form the imidazole ring. There is thus obtained the peripheral vasodilating agent buteri zi ne (74). ... 

J.R. Beck, Nucleophilic Displacement of Aromatic Nitro Groups, Tetrahedron 34, 2057-2068 (1978). 

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