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Trichloromethylation and tert-butylation of 2,4-disubstituted thiophenes: Electrophilic substitution reactions with sterically hindered rearomatization stages

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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.

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Authors
  1. L. I. Belen'kii
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  2. G. P. Gromova
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  3. B. V. Lichitskii
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  4. M. M. Krayushkin
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Additional information

Devoted to the bright memory of I. N. Goncharova.

N. D. Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, Moscow 117913. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1477–1485, November, 1997.

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Belen'kii, L.I., Gromova, G.P., Lichitskii, B.V. et al. Trichloromethylation and tert-butylation of 2,4-disubstituted thiophenes: Electrophilic substitution reactions with sterically hindered rearomatization stages. Chem Heterocycl Compd 33, 1276–1283 (1997). https://doi.org/10.1007/BF02320327

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  • DOI: https://doi.org/10.1007/BF02320327

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