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

Abstract

Rate constants have been determined in three aqueous Me₂SO mixtures for the S_EAr substitution of 3-methoxythiophene (5) by super-electrophilic 4,6-dinitrobenzofuroxan (DNBF). The absence of a significant dependence of the rates of reactions on the hydrogen or deuterium labelling at C_α of the thiophene ring indicates that electrophilic addition (k₁^DNBF) of DNBF at this position is the rate-limiting step of the substitution process. A notable increase in the k₁^DNBF 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 pK_a ^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.