Thiophenes, chloro-, isomerization

A mixture of thiophene and isomeric thienothiophenes (total yield of 85%) was prepared by the gas-phase reaction of dimethyl sulfide, diethyl disulfide, diethyl trisulfide and diethyl tetrasulfide with acetylene (89ZOR2588). It was also found that gaseous allyl 2-thienyl sulfide is transformed into a mixture of thiophene-2-thiol, bis(2-thienyl) sulfide and 2-methylthieno[2,3-i]thiophene at 350-410 °C in the presence of acetylene, the reaction gives these products along with thieno[2,3-i]thiophene (91KGS1312). Thermolysis (at 540 °C) of a mixture of di(prop-l-enyl) sulfide and 2-chloro(or bromo)thiophene affords a mixture of thieno[2,3-Z ]thiophene and thieno[3,2- )]thiophene (91ZOR354). 

Isomeric thienothiophenes can also be synthesized from other thiophene derivatives. In the study of the 3,3-sigmatropic rearrangement of allyl 2-thienyl sulfide (85), 2-methylthieno[2,3-Z)]thiophene (86) and 3-chloro-3,4-dihydro-2/f-thieno[2,3-Z)]thiopyran (87) were isolated in a total yield of up to 60% (77ZOR2624, 79KGS1062, 82DOK97). 

Nitration of decafluoro-l,5-di-(2-thienyl)pentane (60) gave a mixture of isomeric dinitro-derivatives. N.m.r. analysis indicates that competitive substitution had occurred in the 4- and 5-positions in the proportions 1 3. In the presence of iron powder, thiophen reacted with sulphuryl chloride to give a mixture of chlorinated bithienyls as the main product in yields of up to 42%, whereas the reaction without a catalyst gave chloro-thiophens. The reaction was also catalysed by Friedel-Crafts-type catalysts with the usual order of efficiency. It is suggested that the reaction is electrophilic in nature, the electrophilic reagent being thiophen, pro-... 

Thiophenes substituted at C-2 were selectively halogenated at C-5 under biphasic conditions, though in the case of 2-methylthiophene, 10% of isomeric 2-methyl-3-chlorothiophene was also identified [96]. In this way, thiophenes with mixed halogens can be obtained. Chlorination and bromination of 2-iodothiophene gave 2-chloro-5-iodothiophene and 2-bromo-5-iodothiophene, respectively (Scheme 63). 

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