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DOI: 10.1039/A704426G (Paper) Reference Section for: J. Chem. Soc., Perkin Trans. 2, 1997, 2667-2672

Electrophilic heteroaromatic substitutions: the coupling of 4,6-dinitrobenzofuroxan[hair space]† with 3-methoxythiophene in H2O–Me2SO mixtures

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Elyane Kizilian, François Terrier, Alain-Pierre Chatrousse, Khalid Gzouli and Jean-Claude Hallé

Abstract

Rate constants have been determined in three aqueous Me2SO mixtures for the SEAr substitution of 3-methoxythiophene (5) by super-electrophilic 4,6-dinitrobenzofuroxan (DNBF[hair space]). The absence of a significant dependence of the rates of reactions on the hydrogen or deuterium labelling at Cα of the thiophene ring indicates that electrophilic addition (k1DNBF) of DNBF at this position is the rate-limiting step of the substitution process. A notable increase in the k1DNBF rate constant with increasing water content of the solvent mixtures is observed. This trend is consistent with the idea that the addition step proceeds through a highly dipolar transition state where the development of negative charge in the DNBF moiety is concomitant with that of a partial positive charge in the thiophene ring. Assuming that the behaviour of 5 fits the rate-equilibrium, Brønsted-type relationship previously established for the coupling of DNBF with nitrogen heterocycles, a pKa[hair space] H2O value of –6.5 may be estimated for the protonation of 5 in aqueous solution. Interestingly, a similar estimate is obtained when the reactivity of 5 is compared with that of an homogeneous series of enol methyl ethers. Despite a low carbon basicity, comparable to that of a rather weakly basic enol ether, 5 appears to be one of the most reactive thiophene compounds toward electrophilic reagents.

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