Electrophilic Substitution of Compounds Containing Several Thiophene Rings

Electrophilic Substitution of Compounds Containing Several Thiophene Rings 

The acetylation and formylation of 2,2 -bithienyl give monosubstitution in the 5-position, - Disubstitution is easily achieved and, in some cases, difficult to avoid as the deactivating effect of a substituent is not very strongly transmitted to the other ring. The nitration of 2,2 -bithienyl has recently been studied.  

Acetylation of 2,3 -bithienyl occurs in the 5-position, which is in accordance with considerations on the localization energies, The structure was proved by hypochlorite oxidation followed by Raney nickel desulfurization of the 5-carboxy-2,3 -bithienyl.  

Bromination of di-2-thienylmethane is best achieved with a bro-mide-bromate mixture in a heterogenous system. Conventional bromination with bromine in CCU was accompanied by marked tar formation. Bromination occurs in the 5- and 5 -positions of the rings as proved by the preparation of di-(5-bromo-2-thienyl) methane 

The position of substitution in disubstituted thiophenes can, in most cases, easily be deduced from the directing effect of each substituent. Thus with a - -M-substituent in the 2-position and a —M-substituent in the 5-position, both substituents direct the entering group to the 3-position as is exemplified by the nitration of methyl 2-bromo-5-thiophenecarboxylate to methyl 2-bromo-3-nitro-5-thio-phenecarboxylate (109) or in the chlororaethylation of methyl 2-methyl-5-thiophenecarboxylate to methyl 2-methyl-3-chloromethyl-5-thiophenecarboxylate (110). ° 

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C. Electrophilic Substitution of Compounds Containing Several Thiophene Rings... 

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