Nitro-debromination thiophenes

The activated halogenothiophene is more reactive than a similarly activated halogeno-benzene. A quantitative study of the rate of piperidino-debromination of six isomeric bromonitrothiophenes reveals that these substrates react considerably faster than the corresponding bromonitrobenzenes. As in electrophilic substitution, the 2,3-, 2,4- and 2,5-relationships have been equated to ortho, meta and para substitution in benzene derivatives. Although there is some validity in this, a few inexplicable results have been encountered in the above study thus 2-bromo-4-nitrothiophene reacts much faster than either 2-bromo-3-nitro- or 2-bromo-5-nitro-thiophene (64AHC(3)285>. 

Mononitration of benzo[6]thiophene gives a complex mixture in which the 3-isomer predominates (73% in acetic anhydride at 25 °C) and can be separated from other isomers, which may include 4-, 5-, 6- and 7-nitro derivatives. However, benzo[6]thiophenes with electron-donating groups at the 3-position, e.g. 3-methyl-, 3-acetamido- or 3-bromo-benzo[6]thiophene, are nitrated almost exclusively in the 2-position. 2-Nitro-benzo[6] thiophene may be prepared by debromination of 3-bromo-2-nitro-benzo[6 ]thiophene, or by decarboxylation of 2-nitrobenzo[6]thiophene-3-carboxylic acid (70AHC(11)177). 

Selenophene compounds are also more reactive in nucleophilic substitutions. Thus, 3-nitro- and 5-nitro-2-bromoselenophenes90 react with piperidine faster than do the corresponding thiophenes.91 The effect of electron-accepting substituents on the nucleophilic substitution of bromine located at the 2-position of 3,5-disubstituted selenophenes has been studied, as also has the applicability of the Hammett equation to this piperidino-debromination.92... 

Spinelli and co-workers have shown that linear free-energy or/Ao-correlations can be successfully established for the nucleophilic displacement reactions on activated halogenothiophenes. This has been validated on the benzenethiolate debromination of 2-bromo-3-A -5-nitrothiophenes (557) <91JCR(S)270>, the chlorine-isotopic exchange between ClLi, and some 2-chloro-3-3T-5-nitro-thiophenes (558) <88JCR(S)198>, and the piperidino-debromination of some 2-bromo-3-A -4-methyl-5-nitrothiophenes (559) <85JCS(P2)519>. 

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