Nitro groups elimination

Interest in an old reducing agent, sodium sulfide, was renewed when it was discovered that its reactivity toward nitro groups was sensitive to water. In the absence of water, the aromatic nitro group in (28) can be reduced selectively to (29), without reduction of the aliphatic nitro group, but compounds such as (30), which contain a tertiary aliphatic nitro group eliminate the nitro moiety to yield styrene derivatives (31 Scheme 5). A unique application of the sulfide reduction involves the preparation of isomerically pure substituted aromatic nitro compounds and anilines. ... 

For 2-amino-4- m-nitrophenyl) seienazole, the yield is particularly high. This has been explained by the oxidizing effect of the nitro group, which liberates iodine from the hydrogen iodide eliminated in the condensation reaction. 

A variety of ring syntheses have been devized which depend on carbanion addition to an activated double bond. The examples depicted in Scheme 74 illustrate the use of inter alia cyano and nitro groups which are subsequently eliminated. In appropriate instances the inclusion of additional eliminable groups ensures the formation of fully aromatized products. 

A variation of the general method for the synthesis of 2-amino-selenazoles is to avoid the use of the free a-halogenocarbonyl compound and in its place react the corresponding ketone and iodine with selenourea.This procedure is also taken from thiazole chemistry. By contrast with thiourea, the reaction with selenourea needs a longer reaction time and the work up of the reaction mixture is somewhat more difficult. Usually an excess of the ketone is used. In the preparation of 2-amino-4-( n-nitrophenyl)selenazole, a very high yield, calculated on the amount of iodine used, was obtained. To explain this peculiar result, the oxidative action of the nitro group was invoked. This liberates free iodine from some of the hydrogen iodide eliminated in the condensation reaction, and the free iodine then re-enters into the reaction. 

The majority of analgesics can be classified as either central or peripheral on the basis of their mode of action. Structural characteristics usually follow the same divisions the former show some relation to the opioids while the latter can be recognized as NSAlD s. The triamino pyridine 17 is an analgesic which does not seem to belong stmcturally to either class. Reaction of substituted pyridine 13 (obtainable from 12 by nitration ) with benzylamine 14 leads to the product from replacement of the methoxyl group (15). The reaction probably proceeds by the addition elimination sequence characteristic of heterocyclic nucleophilic displacements. Reduction of the nitro group with Raney nickel gives triamine 16. Acylation of the product with ethyl chlorofor-mate produces flupirtine (17) [4]. 

The nitro group of aromatic nitro compounds has been removed with sodium borohydride. This reaction involves an addition-elimination mechanism. 

The condensation of nitro compounds and imines, the so-called aza-Henry or nitro-Mannich reaction, has recently emerged as a powerful tool for the enantioselective synthesis of 1,2-diamines through the intermediate /3-amino nitro compounds. The method is based on the addition of a nitronate ion (a-nitro carbanion), generated from nitroalkanes, to an imine. The addition of a nitronate ion to an imine is thermodynamically disfavored, so that the presence of a protic species or a Lewis acid is required, to activate the imine and/or to quench the adduct. The acidic medium is compatible with the existence of the nitronate anion, as acetic acid and nitromethane have comparable acidities. Moreover, the products are often unstable, either for the reversibility of the addition or for the possible /3-elimination of the nitro group, and the crude products are generally reduced, avoiding purification to give the desired 1,2-diamines. Hence, the nitronate ion is an equivalent of an a-amino carbanion. 

SRN I reactions using related p-nitrophenyl or p-nitrocumyl systems41 as reductive alkylating agents have been studied by Komblum and co-workers these are well summarized in the reviews.39 At the same time, Russell discovered the S l reaction of geminal halonitroalkanes with stabilized carbanions (see Eq. 5.25).42 The products are readily converted into alkenes via elimination of nitro groups (see Section 7.3). 

The nitro groups in Eqs. 7.88-7.90 are readily replaced by hydrogen with tin hydride under radical conditions as discussed already. However, the nitro groups in the a-nitrosulfides or (3-nitrosulfides are not replaced by hydrogen on treatment with tin hydride but the reaction affords desulfonated products (Eq. 7.51) and alkenes (Eq. 7.97) such radical elimination reactions are discussed in Section 7.3.1. (see Eqs. 7.91 and 7.92).138... 

Base-promoted fragmentation of products resulting from SRN I reactions between gem-halonitroalkanes and cyclic [1-keto-esters as nucleophiles give rise to di- or trifunctionalized olefins (Eq. 7.141).187 If the product is treated with NaCl in DMSO at 120 °C, the ester and nitro groups are eliminated. 

Functionalized nitroalkenes are important dienophiles in the Diels-Alder reaction. For example, ( )-methyl P-nitroacrylate is an important reagent in organic synthesis. The nitro group can be readily eliminated the Diels-Alder reaction of P-nitroacrylate is equivalent to that of ethyl propiolate with an inverse regiochemistry (Eq. 8.4).11... 

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... 

Adjacent 2-azido and 3-nitro groups facilitate the elimination of nitrogen followed by ring closure to form the corresponding furazan iV-oxide compound 342 (Equation 77) <1996JOC5801, 2005AGE7089>. 

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