Isomerization thiophene ring

Intramolecular diazo coupling can be also achieved <85JCS(Pl)l3l). Starting from suitable (o-aminophenyl)thiophenes various isomeric thieno[c]cinnolines have been obtained (Scheme 24). The reaction can proceed either at an a- or at (3-position of the thiophene ring. 

The initially formed singlet excited state 1 can convert either to the corresponding triplet state 2 by intersystem crossing, or to the Dewar isomer 4. In the former case, homolytic cleavage of the S-C(5) bond in 2 can lead to the biradical 3 and ultimately result in ring-opened or ring-contracted products. The Dewar isomer 4 is responsible for the formation of the isomeric thiophene 6 via 5 obtained by a sulfur walk . 

Direct substitution in the 3-position of the thiophene ring is difficult and can be achieved only by activation of this reaction site. Thus the isomeric 3-thiophenethiol may be prepared by this general method starting from a 3-haIogenothiophene. For example, this thiol may be obtained from 3-bromothiophene in 63% yield.5... 

The isotropic ESR spectra of 101 at temperatures accessible to the radical show signs neither of the existence of separate rotamers nor of line-broadening effects suggestive of their rapid interconversion. An INDO MO study has shown that the radical probably exists in two preferred propeller-like conformations (that of C3 symmetry is shown in 101 that of Cj symmetry is obtained by a rotation of any single thiophene ring in 101 through 180°). The calculations indicate the C3 conformer to be more stable by 0.75 kcal mol" and the barrier to interconversion of the two forms, by a preferred two-ring-flip mechanism, to be about 6 kcal mol . This low barrier to interconversion is consistent with the failure to observe effects of the isomerism in the ESR spectra at temperatures above those at which the radical dimerizes. 

A careful analysis based on these experimental results excluded a chain propagation process [33a]. On account of the 3-position of the methylthio substituent in the thiophene ring, three isomeric dimers may be formed. The main reaction path can be deduced from the mesomeric forms of the radical cation 1. The two most important mesomeric structures are those with the unpaired electron in an a-position (II, II). Structure I, with the positive charge next to the methylthio sulfur, is preferred, because of the stabilizing -l- M effect of the methylthio substituent. Therefore, the 3,3 -connected dimer 2a is the essential... 

Reactions at Benzylic Positions. - The catalytic oxidation, using a cobalt bromide catalyst, of 2,5-dimethylthiophen to the dialdehydeof 3-methyl-2-ethylthiophen to 2-acetyl-3-methylthiophen and l-(3-methyl-2-thienyl)-ethyl acetate, of the four isomeric methyl acetothienones to the corresponding acetylthiophencarboxylic acids, and of 2-acetoxymethylthio-phen to thiophen-2-carboxylic acid has been reported. The Wittig reaction between 2- and 3-thenyltriphenylphosphonium salts and crocetin dialdehyde has been used for the synthesis of carotenoid analogues with terminal thiophen rings. ... 

Direct attack of RpCu on C(3) carbon leads to the most favorable product, probably by a concerted mechanism. The ratio of two isomers is defined by the nature of the halogen in thiophene ring. For example, in the case of the reaction of 3-bromothio-phene and K-perfluorooctyl iodide, 18% of a rearranged product (2-isomer) was formed, whereas in the reaction of 3-iodothiophene, <4% of 2-perfluoro-K-octylthio-phene was formed. The possible reason for 2-isomer formation may be addition of perfluoroalkyl anion to C(2)-C(3) double bond with formation of carbene, which undergoes isomerization into 2-fluoroalkylthiophene. ... 

The GC/MS pyrogram of PST (Figure 5.10) obtained by pyrolysis at 400°C shows a limited number of pyrolysis compounds with respect to DPMS. The most abundant pyrolysis compound (peak 1) is due to styrene, while peak 2 corresponds to styrene sulfide and peak 3 is due to Sg (octasulfur ring). The broad and flat shape of the latter peak is most likely due to the condensation of elemental sulfur on cold spots and successive slow evaporation as Sg. The peaks 4 and 5 are due to two isomeric thiophenes 2,4-diphenylthiophene and 2,5-diphenylthiophene. ... 

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