Thiophen, 2- nitro derivatives

Nitration of benzo[6]thiophene (HNOs/AcOH) yields mainly the 3-nitro derivative. Under these conditions the /3 to a ratio of substitution is approximately 5 1, which is... 

Redox molecules are particularly interesting for an electrochemical approach, because they offer addressable (functional) energy states in an electrochemically accessible potential window, which can be tuned upon polarization between oxidized and reduced states. The difference in the junction conductance of the oxidized and the reduced forms of redox molecules may span several orders of magnitude. Examples of functional molecules used in these studies include porphyrins [31,153], viologens [33, 34,110,114,154,155], aniline and thiophene oligomers [113, 146, 156, 157], metal-organic terpyridine complexes [46, 158-163], carotenes [164], nitro derivatives of OPE (OPV) [165, 166], ferrocene [150, 167, 168], perylene tetracarboxylic bisimide [141, 169, 170], tetrathia-fulvalenes [155], fullerene derivatives [171], redox-active proteins [109, 172-174], and hydroxyquinones [175]. 

The reactions of 3-diazobenzo[6]thiophene-2-one (780) with enaminones leads to the formation of 1,2,3-triazoles (781) (19-77%) as the major products along with by-products (782) (Equation (74)). The nitro derivatives (783) react with various enaminones to give generally good yields of 1,2,3-triazoles (55-82%). The mechanism for the formation of the triazoles is described <93JOC7079>. 

A concerted elimination-cyclization mechansim, involving a sulfenyl halide in a 1,3-butadiene-1-thio system, is the most probable mechanism for the formation of benzo[6 Jthiophenes from cinnamic acids or 4-aryl-2-butanones by treatment with thionyl chloride. The reactions shown in Scheme 5 have been carefully worked out, and the intermediates isolated (75JOC3037). The unique aspect of this synthesis is the reduction of the sulfinyl chloride (a) by thionyl chloride to form the sulfenyl chloride (b). The intermediate (b) was isolated and converted in pyridine to the 3-chlorobenzo[6]thiophene-2-carbonyl chloride in 36% yield (73TL125). The reaction is probably initiated by a sulfenyl ion attack on the aromatic ring, since it is promoted by electron-releasing groups para to the site of ring closure. For example, when X in (36) was N02, a 23% yield of (37), a mixture of 5-and 7-nitro derivatives, was obtained, but when X in (36) was OMe, a 54% yield of (37) was obtained, contaminated with some 3,4-dichloro-5-methoxybenzo[6]thiophene-2-carboxylic acid. 

Thiophenes and Benzo[6 jthiophenes Having a Nitrogen-linked Substituent 3.15.9.3.1 Nitro derivatives of thiophenes and benzo [b] thiophenes... 

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). 

Benzo[6 jthiophenes with nitro groups at the 4-, 5-, 6- or 7-positions may be synthesized by ring closure reactions of appropriately substituted benzene derivatives. Nitration of 5-acetaminobenzo[6]thiophene gives the 4-nitro derivative, which can be hydrolyzed and deaminated to yield 4-nitrobenzo[6]thiophene (equation 44). 5-Nitrobenzo[6]thiophene is conveniently available by decarboxylation of 5-nitrobenzo[6]thiophene-2-carboxylic acid, which in turn is available by a Perkin reaction of 4-nitro-2-formylphenylthioglycoIic ester (equation 45 Section 3.15.2.3). The 7-nitro isomer may be obtained similarly. 

Aminobenzo[6]thiophenes may be prepared by reduction of the 2-nitro derivatives to form the hydrochloride salts, or by rearrangement of the oxime of 2-acetyIbenzo[6]thio-... 

These derivatives may be obtained by reduction of the appropriate nitro derivative catalytically or with a metal-acid system, or by Beckmann or Hofmann rearrangements of suitable acyl or carboxamido derivatives. 4-Aminobenzo[6]thiophene has also been prepared by means of a Bucherer reaction with 4-hydroxybenzo[6 Jthiophene. Several 5-aminobenzo[6]thiophenes have been prepared by cyclization reactions of p-acetamino-phenylthio derivatives. 6-Acetaminobenzo[6 Jthiophenes may be obtained from the corresponding 6-acetyl derivative by Schmidt or Beckmann rearrangements. 7-Aminobenzo[6 ]thiophene can also be prepared from 7-hydroxybenzo[6 ]thiophene by a Bucherer reaction (70AHC(ll)l77). 

Gronowitz found the Pictet-Spengler reaction a convenient route for the preparation of (259 equation 22) (70AK(32)217). Thiophene-3-carbaldehyde was condensed with nitromethane to give w-nitrovinylthiophene. Reduction of the nitro derivative with LAH afforded the /3-(thienyl)ethylamine. The condensation of this amine with formaldehyde to the corresponding imine followed by cyclization with hydrochloric acid proceeded very smoothly and in high yield. The aromatization was achieved with potassium ferricyanide in an alkaline dioxane-water mixture. 

This methodology was applied to annulation of the pyridine ring (see structure 260). Nitro derivatives of benzene, naphthalene, thiophene, pyridine and quinoline... 

Rate constants for the mercuration of selenophene and its 2-bromo, 2-carboethoxy, 2-acetyl, and 2-nitro derivatives under the conditions employed by Motoyama et al.,121 have been determined at various temperatures by Yur ev and his co-workers.304 The selenophene derivatives were mercurated from 1.5 to 3 times more rapidly than the corresponding thiophene derivatives. A straight line is obtained in the Hammett plot with ap+ constants, with a p value of — 5.7 at 25°. 

Nitration of l,3-dihydrobenzo[c]thiophene 2,2-dioxide with fuming nitric acid is reported 9 to give a good yield of the 5-nitro derivative (87), which may be reduced to the corresponding amine. This amine reacts normally with acetic anhydride, but attempted diazonium reactions have not led to any useful products.9... 

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