L. I. Belen’kii, G. P. Gromova, B. V. Lichitskii
Nov 1, 1997
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Journal
Chemistry of Heterocyclic Compounds
Abstract
The interaction of 2,4-disubstituted thiophenes with AlCl3 and CCl4 or tert-BuCl proceeds through cationic σ-complexes corresponding to electrophilic substitution; the deprotonation of these complexes to form 2,3,5-trisubstituted products is sterically hindered, since this stage (rearomatization) involves a change in hybridization of the C(2) atom from sp3 to sp2, which leads to an increase of repulsion between the substituent R in position 3 and the bulky trichloromethyl or tert-butyl group that has entered into position 2 and is coplanar with R. It has been concluded tentatively that in the case of 2,4-alkylthiophenes that have even one hydrogen atom on the α-C atom of the substituent, deprotonation of the σ-complexes that are formed leads to the corresponding 3-alkylidene-5-R-2-dichloromethylene-2,3-dihydrothiophenes or 2-alkylidene-4-R-5-dichloromethylene-2, 5-dihydrothiophenes, which then readily undergo oligomerization, as is also observed for such compounds under conditions of trichloromethylation. The hydrogen chloride that is evolved forms a stable 2H-thiophenium ion with the unreacted 2,4-dialkylthiophene. In the example of 2-tert-butyl-4-methylthiophene, formation of the stable 2,5-di-tert-butyl-3-methylthiophenium ion is observed; upon treatment with water, this ion does not lose a proton, but rather a tert-butyl group, thus being converted to the original 2,4-dialkylthiophene.