Substitution with Elimination of Substituents

In the reactions of 2,5-disubstituted thiophenes elimination of an a-substituent occurs to a much greater extent than in the benzene series. The Friedel-Crafts acetylation of 5-bromo-2-ethylthiophene in the presence of SnCE gives 2-ethyl-5-acetyIthiophene. Elimination of an a-bromine occurs also in the chloromethylation of 2,5-di-bromothiophene, leading to a mixture of 2-bromo-5-chloromethyIthio-phene and 2,5-dibromo-3-chloromethyIthiophene. Bromine atoms at both the and -position are exchanged for chlorine in the 

POCls-catalyzed formylation, so POBrs must be used instead. Another example is the replacement of halogen by the nitro group in the nitration of halogenated 2-acetamidothiophenes.  

Such eliminations do not occur with chloro compounds. 2,5-Dichlorothiophene undergoes AlCU-catalyzed sulfonylation with ben-zenesulfonyl chloride. Attempts to extend this reaction to thiophene, 2,5-dimethylthiophene, and 2-chloro-5-iodothiophene resulted in the formation of intractable tars even with other catalysts. Sul-fones of this type are often prepared in much better yields by using thiophenesulfonyl chlorides and benzenes in the Friedel-Crafts reaction.  

Carboxyl groups are easily removed during nitration especially at higher temperatures. A detailed study on the nitration of 5-bromo-2-thiophenecarboxylic acid showed that the proportion of 2,4-dinitro-5-bromothiophene increased with the concentration of sulfuric and nitric acid. The mechanism of this decarboxylative nitration 

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Generally, oxepins have a tendency to contract to a six-membered carbocycle when treated with acid. The driving force is the aromaticity of the phenol formed. However, when the less stable cyclohexa-2,5-diene-1,4-diol with an appropriate substitution pattern is treated with acid, the oxepin system is obtained. The treatment of cyclohexadienediols that are substituted with tert-butyl groups in the 2- and 6-positions and aryl at Cl and C4 with trifluoroacetic acid produces oxepins 1 with elimination of water.186 187 This reaction, however, is restricted to certain aryl substituents with at least some electron-donating effect. Generally, cyclohexa-2,4-dienone derivatives 2 are formed.187,188... 

The thermolysis of (358) also leads to aromatisation, in this case in a process believed to involve an intermediate nitrile ylid. Evidence for this is obtained by thermolysis of a series of cyclopropenyl-substituted oxazolinones such as (359) for which cycloreversion with elimination of C02 is known to lead to a nitrile ylid. In some cases the ylid could be trapped by addition to methyl propiolate. Substituent effects suggest that the nitrile ylids undergo stepwise addition to produce a bicyclobutyl zwitterion which can either collapse to an azabenzvalene or rearrange to a cyclobutenyl cation 286>. 

The reactions of complex (25) with carbonyl compounds show a strong dependence on the substituents leading to the substitution or elimination of the silyl-substituted acetylene or the insertion into the Zr-C bond. Moreover, the reaction of methacrolein with (25) depends strongly of the solvent used in hexane elimination of bis(trimethylsilyl)acetylene leads to the formation of dimer (33) while in THF insertion with the formation of zirconacycle (34) is observed. 

With 2-substituted 1,4-naphtboquinones 32 the reaction proceeds cither with elimination of the substituent and formation of benzo[a]phenazin-5-ols 33 a-b or by addition and oxidation to give 6-substituted henzo[t/]phenazin-5-ol 33c. The reaction pathway is influenced by the electronic effects of the substituents. ... 

When, under the assumption of conformational homogeneity, looking at the set of reactions possible with educts containing an equatorially oriented LG, hydride shift (MS [H], route f. Scheme 4) and elimination (E[H], route h. Scheme 4) cannot be found. However, besides the direct nucleophilic substitution with inversion of configuration (by the external nucleophile or the solvent, Sn2), all other sorts of participation, from vicinal positions, by antiperiplanarly oriented substituents - including each single ring atom of the pyranose - appear. These are in particular ... 

When we discussed elimination reactions in Chapter 5, we learned that a Lewis base can react with an alkyl halide to form an alkene. In the present chapter, you will find that the same kinds of reactants can also undergo a different reaction, one in which the Lewis base acts as a nucleophile to substitute for the halogen substituent on carbon. 

It is believed (54IZV47 72JPR353) that in the first stage the intermediate 282 is formed due to the addition of the CH acid to the enamine moiety with subsequent elimination of amine. The enol form of the intermediate 282 undergoes cyclization in two fashions, depending on the nature of substituent X. In the case of the ester (X = OMe) the attack is directed to the cyano group to form substituted 3-methoxycarbonyl-I//-pyridin-2-one (283) or its tautomer (2-hydroxy-3-methoxycarbonylpyridine). With the amide (X = NH2) intramolecular condensation leads to 3-cyano-l//-pyridin-2-one and its hydroxy tautomer (284). 

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