Thiophene-2-carbaldehyde, bromination

Thiophene-2-carbaldehyde, 3-bromo-synthesis, 4, 81 Thiophenecarbaldehydes benzothiophene synthesis from, 4, 906 reactions, 4, 807 synthesis, 4, 148 Wittig reactions, 4, 807 Thiophene-2-carbaldehydes bromination, 4, 753 conformation, 4, 33 halogenation, 4, 753 reactions, 4, 72-73 reactivity, 4, 72-73 reduction, 4, 776 Thiophene-3-carbaldehydes conformation, 4, 33 reactivity, 4, 72... 

The synthesis of 4-substituted products from 2-acylfurans and 2-acylselenophenes is difficult. Their predominant formation was observed on bromination of aluminum chloride complexes of furfural and to a lesser extent of 2-acetylfuran (731ZV2733). Bromination of complexes of selenophene-2-carbaldehyde and 2-acetylselenophene with AICI3 leads mainly to 4-substituted derivative (exceeded 70%) (95JHC53). In all cases a significant amount of 4,5-dibromo-substituted products was obtained (Scheme 21). Competing reactions show that the complex of selenophene-2-carbaldehyde with aluminum chloride is brominated more rapidly than its thiophene analogue (95JHC53). 

A very convenient method for the synthesis of (158) consists in the side-chain bromination of 2-thienyl ethyl ketone followed by reaction with sodium methoxide in methanol and with methanesulphonyl chloride in pyridine, which gave (157) in 88% yield. Treatment of (157) with calcium carbonate led to 1,2-rearrangement of the thienyl group to give (158). Metal complexes of s> -thiophen-2-aldoxime, thiophen-2-carbaldehyde 2-benzothiazolylhydrazone, and thiophen-2-carbaldehyde thiosemicarba-zone have been studied. 

Thiophene is chlorinated by CI2 or SO2CI2. Bromination occurs by Br2 in acetic acid or with N-bromosuccinimide. Nitration is effected by concentrated nitric acid in acetic acid at 10°C. Further substitution predominantly yields 2,4-dinitrothiophene. Sulfonation with 96% H2SO4 occurs at 30°C within minutes. Benzene reacts extremely slowly under these conditions. This provides the basis for a method to remove thiophene from coal tar benzene (see p 77). Alkylation of thiophenes often gives only poor yields. However, more efficient procedures are the Vilsmeier-Haack formylation, which yields thiophene-2-carbaldehyde, and acylation with acyl chlorides in the presence of tin tetrachloride, to give 2-acylthiophenes. Like furan, thiophene is mercurated with mercury(II) chloride. 

The copolymer 174 was prepared from diaUcyl thienylated benzodithiophene (173) and perfluororalkyl-carbonyl substituted thieno[3,4-h]thiophene (172) monomers (Scheme 41). Thus, reaction of thieno[3,4-6]thiophene-2-carbaldehyde (150) with heptadecafluorooctyl iodide in the presence of methyllithium to form 170 was followed by oxidation with Mn02 giving ketone 171. Bromination with two equivalents of NBS furnished dibromide 172, and this was Stille cross-coupled with 173 in the presence of Pd2(dba)3 to obtain the copolymer 174 [63]. 

The report in 1971 by MacDowell and Wisowaty on the S5mthesis of BDT unit started from thiophene derivatives [11,16], which was further improved by Beimling et al. in 1986 (Scheme 3.2) [12, 17]. Instead of using 3-thiophenecarboxaldehyde and 2,3-dibromothio-phene as starting materials in the original report, the improved method started with only 3-bromothiophene. It was first converted to 3-bromothiophene-2-carbaldehyde, which was then treated with 3-thienyllithium to afford compound 1. The newly formed hydroxyl group was reduced off to afford compound 2. The bromine on the 3 position of compound 2 was lithiated and quenched by N,N-dimethylformamide (DMF) to yield compound 3, which imder-went a cyclization in 48% hydrobromic acid solution to complete the synthesis of BDT monomer 4 with high yield. 

Photochemistry.—Benzo[b]thiophen-3-carbaldehydes and -carboxylic acid have been obtained uia light-catalysed N-bromosuccinimide bromination of 3-methylbenzo[b]thiophen. The photochemical behaviour of benzo[b]thiophen was found to differ from that of thiophen. It does not undergo photoisomerization and is not converted into an indole when irradiated in the presence of a primary amine. Its photochemical behaviour resembles that of naphthalene. Photolysis of various... 

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