Pyrolysis mechanisms thiophene

Benzo [6]thiophene is a minor product of the pyrolysis of thiophene.368-370 Three mechanisms have been advanced for its forma-... 

Electrophilic replacement constants crXr have been obtained for all the positions of benzo[6]thiophene from the solvolysis of isomeric l-(benzo[ >]thienyl)ethyl chlorides in 80% ethanol-water. These constants signify replacement of the entire benzene ring by another aromatic system (74JOC2828). The positional order of reactivity was determined to be 3>2>6>5>4>7, all positions being more reactive than benzene. The same order was also derived from the kinetic data for pyrolysis of the isomeric l-(benzo[6]thienyl)ethyl acetates (78JCS(P2)1053). A modified extended selectivity treatment has been developed to correlate electrophilic substitution data in benzo[Z> ]thiophene, which assumes a dual activation mechanism (79JOC724). 

The well-known Elbs reaction has been applied to thiophene syntheses, but the reaction may involve some isomerization. Thus pyrolysis of 3-o-toluoylbenzo[6]thiophene (347) for 3 hours at 340-360 °C gave naphtho[2,1 -b]benzothiophene (348) in 35% crude yield, instead of the expected naphtho[2,3-Z>]benzothiophene. A radical mechanism (Scheme 26) was proposed to explain this result (56JCS3435). 

Two other minor products formed in the pyrolysis, not present in the starting benzothiophene, were phenylthiophene and bithienyl. According to our proposed scheme, these seem to demand the formation of thiophyne and/or thiophene from benzothiophene. Mass spectra of some aromatic dicarboxylic anhydrides give a clue to a possible mechanism. 

Co-pyrolysis of pentaduorobenzenethiol (6) and tetrafluoroethene (7) in a flow system at 310— 370 C affords, via an interesting mechanism, perfluoro-2,3-dihydrobenzo[6]thiophene (8), albeit in low yields.39... 

SimUar [4 + 4] dimers have been reported during the flash vacuum pyrolysis of 2-methyl-3-furylme-thyl benzoate in which 4//,5//,9//,l(iF/-cycloocta[l,2-b 6,5-b difuran (117) is produced in virtually quantative yield.A stepwise mechanism has been forwarded to rationalize the high-yield formation of the corresponding head-to-head [4 + 4] dimer.Other examples of the [4 -h 4] dimerization of 2,3-di-methylene furan and 2,3-dimeAylene thiophene have been reported as well. 

Benzyne (from pyrolysis of phthalic anhydride) reacts with thiophen at 690°C to give mainly naphthalene, benzo [b] thiophen (54), 2- and 3-phenyl-thiophens, and bithienyl.57 The bithienyl (probably the 2,2 -isomer) is also formed from thiophen alone under the same conditions. The phenylthiophens arise by insertion of benzyne into C—H bonds of thiophen. The source of naphthalene is most likely to be via a Diels-Alder reaction (Scheme 7, X = H), and Fields and Meyerson assume a mechanism involving initially 1,2-cyclo-addition of benzyne to a C—S bond of thiophen to account for the formation of 54 (Scheme 8).2,57,58 However, there is no evidence to support such a route, which seems unconvincing, not least because benzo [b]thiepin (53)... 

Thiophen Dioxides and their Dihydro and Tetrahvdro Derivatives. - Flash vacuum pyrolysis of cobalt complexes (194) of thiophen 1 1-dioxides led to extrusion of sulphur dioxide and the formation of cyclobutadiene complexes (195). Stereochemical labelling experiments were employed in order to determine the mechanism of this transformation.114 A detailed publication on the reaction of... 

The pyrolysis of 3,8-bis(TMS)-l,2,5,6-tetrathiocane (47) was studied by pulse pyrolytic GC/MS. Compound (47) undergoes thermal decomposition via two mechanisms. The first pathway involves the formation of S2 and trimethylvinylsilane, which reacts further to give CSj, SiS2, and lower alkanes. The second pathway involves loss of three sulfur atoms to form TMS-substituted thiophenes <85JOM(289)23l>. Compound (47) also undergoes reaction with Fc3(CO)i2 to give a variety of sulfur-iron carbonyl complexes <86ZOK1540>. 

FIGURE 8.23 The proposed reaction mechanism of thiophene pyrolysis. 

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