Thiophenes rearrangement

An alternative explanation considered for the thiophene rearrangement is that an intermediate of type 154, arising by expansion of the valence shell of the sulfur atom, may be involved.135 136 E. F. Ullman and B. Singh, J. Am. Chem. Soc. 88, 1844 (1966). 

Introduction of a 3-bromosubstituent onto thiophene is accompHshed by initial tribromination, followed by reduction of the a-bromines by treatment with zinc/acetic acid, thereby utilizing only one of three bromines introduced. The so-called halogen dance sequence of reactions, whereby bromothiophenes are treated with base, causing proton abstraction and rearrangement of bromine to the produce the most-stable anion, has also been used to introduce a bromine atom at position 3. The formation of 3-bromotbiopbene [872-31-1] from this sequence of reactions (17) is an efficient use of bromine. Vapor-phase techniques have also been proposed to achieve this halogen migration (18), but with less specificity. Table 3 summarizes properties of some brominated thiophenes. 

The light-induced rearrangement of 2-phenyl- to 3-phenyl-thiophene may occur by a similar mechanism an equilibrium between the bicyclic intermediate (26) and the cyclopro-penylthioaldehyde (27) has been suggested (Scheme 2). The formation of IV-substituted pyrroles on irradiation of either furans or thiophenes in the presence of a primary amine supports this suggestion (Scheme 3). Irradiation of 2-phenylselenophene yields, in addition to 3-phenylselenophene, the enyne PhC=C—CH=CH2 and selenium. Photolysis of 2-phenyltellurophene furnishes solely the enyne and tellurium (76JOM(108)183). 

The acid-catalyzed rearrangements of substituted pyrroles and thiophenes consequent on ipso protonation have been referred to previously (Section 3.02.2.4.2). There is some evidence that these rearrangements are intramolecular in nature since in the case of acid-induced rearrangement of 2-acylpyrroles to 3-acylpyrroles no intermolecular acylation of suitable substrates could be demonstrated (Scheme 10) (8UOC839). 

Interesting results have been obtained in intramolecular acylation reactions involving pyrrole and thiophene derivatives. A muscone synthesis involves selective intramolecular acylation at a vacant a-position (Scheme 18) (80JOC1906). In attempts to prepare 5,5-fused systems via intramolecular acylation reactions on to a jS-position of a thiophene or a pyrrole, in some cases ipso substitution occurs with the result that rearranged products are formed (Scheme 19) (82TH30200). 

Acyl-pyrroles, -furans and -thiophenes in general have a similar pattern of reactivity to benzenoid ketones. Acyl groups in 2,5-disubstituted derivatives are sometimes displaced during the course of electrophilic substitution reactions. iV-Alkyl-2-acylpyrroles are converted by strong anhydrous acid to A-alkyl-3-acylpyrroles. Similar treatment of N-unsubstituted 2- or 3-acyIpyrroles yields an equilibrium mixture of 2- and 3-acylpyrroles pyrrolecarbaldehydes also afford isomeric mixtures 81JOC839). The probable mechanism of these rearrangements is shown in Scheme 65. A similar mechanism has been proposed for the isomerization of acetylindoles. 

Addition reactions with tetracyanoethane have provided access to 2,5-diamino-3,4-dicyano-thiophene and -selenophene (58JA2775,81ZOR1958). Base catalyzed rearrangement gives the isomeric pyrrolethiol (Scheme 53). 

The rearrangement of thiophenes to the isomeric pyrroles has proven synthetically useful (Schemes 98a and 98b). In the absence of a suitable internal nucleophilic nitrogen, so-called degenerate ring transformations may occur (Schemes 98c and 98d). 

The sodium ethoxide catalyzed rearrangement of readily prepared thiazole derivatives provides a facile thiophene synthesis (Scheme 103) (76JPR343). 

JOC1537). The mechanisms of these transformations may involve homolytic or heterolytic C —S bond fission. A sulfur-walk mechanism has been proposed to account for isomerization or automerization of Dewar thiophenes and their 5-oxides e.g. 31 in Scheme 17) (76JA4325). Calculations show that a symmetrical pyramidal intermediate with the sulfur atom centered over the plane of the four carbon atoms is unlikely <79JOU140l). Reactions which may be mechanistically similar to that shown in Scheme 18 are the thermal isomerization of thiirane (32 Scheme 19) (70CB949) and the rearrangement of (6) to a benzothio-phene (80JOC4366). 

The rearrangement (automerization) of Dewar thiophene 5-oxide (61), observed by NMR, occurs so much more rapidly than that of the corresponding episulfide that special mechanisms have been invoked. The one which involves a zwitterionic intermediate (Scheme 108) is favored over a pseudopericyclic sulfur-walk mechanism in which the electrons of the carbon-sulfur o--bond and the pair of electrons on sulfur exchange places as the sulfur atom migrates around the ring (80JA2861). 

Distannacyclodecanes synthesis, 1, 606 Disulfide, benzylpurinyl ribosylation, S, 560 Disulfide, bis(l-alkenyl) rearrangement thiophenes from, 4, 871 Disulfide, bis(4-phenyl-3-butenyl) cyclization, 4, 867-868 Disulfide, dibenzothiazolyl as vulcanization accelerator, 1, 402 Disulfide, di(2,6-dimethoxypyrimidin-4-yl) oxidation, 3, 96 Disulfide, dipyrimidinyl synthesis, 3, 137 Disulfide, di(tetrazol-5-yl)... 

Dithiins rearrangements, 3, 979 synthesis, 3, 989 thermolysis, 3, 979 thiophene synthesis from, 4, 903... 

Beekmann rearrangement, 4, 809 reaetions, 4, 809 toxieity, 1, 136 Thiophene, 2-aeyl-alkylation, 4, 777 synthesis, 4, 917 transaeylations, 4, 760 Thiophene, 3-aeyl-synthesis, 4, 918... 

Thiophene, 2-methyl-5-(mesitylsulfonyl)-Truce-Smiles rearrangement, 4, 825 Thiophene, 2-methyl-3-nitro-acidity, 4, 799... 

Thiophene, 3-pentadeuterophenyl-chemical shifts, 4, 730 Thiophene, 2-phenyl-oxidation, 4, 800 phototranspositions, 4, 743 rearrangement, 4, 42 reduction, 4, 775 synthesis, 4, 865, 914 UV spectrum, 4, 735 Thiophene, 3-phenyl-photochemical rearrangements, 4, 735 phototranspositions, 4, 743 lsmeier formylation, 4, 759 Thiophene, 2-pivaloyl-Birch reduction, 4, 775 Thiophene, polybromo-reactivity, 4, 829 Thiophene, polylithio-synthesis, 4, 831 Thiophene, (propargylthio)-rearrangement, 4, 746 Thiophene, 2-(3-pyridinyl)-synthesis, 4, 781 Thiophene, 2-(5-pyrimidinyl)-synthesis, 4, 781 Thiophene, 3-pyrrolidinyl-cycloaddition reactions, 4, 68 with dimethyl acetylenedicarboxylate, 4, 788-789... 

It now seems definitely proved that the thiothiophthene (86) rearranged reversibly to a thiophene (87) under the influence of alkali and not to a thiepin derivative, The dimethyl ether of (87) has been oxidized to 4-methylsulfonyl-2-thiophenecarboxylic acid (88)... 

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