Journal of the Chemical Society. Perkin transactions II p. 1485 - 1490 (1988)
Update date:2022-08-25
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Cunningham, Ian D.
para- and meta-substituted aromatic isocyanides have been found to react in alkaline aqueous dioxane yielding the corresponding formamides.From a kinetic analysis of the reaction, the observed rate constants were found to be linearly dependent upon hydroxide concentration; this observation, together with the positive value of ρ, was taken as evidence of direct nucleophilic attack.Decreasing values of ΔH(excit.) were observed as electron-withdrawing substituents were introduced, whereas ΔS(excit.) was fairly constant at -57 J K-1 mol-1; this indicates little variation in transition-state geometry across the range of isocyanides studied.The kinetic evidence is discussed in terms of two mechanisms: nucleophilic attack of hydroxide with concerted proton transfer from water to the isocyano carbon and nucleophilic attack of hydroxide to yield a carbanion intermediate, with protonation occuring in a rapid subsequent step.The reaction rate increases with increasing water content in the solvent; this observation appears to favour the concerted mechanism.In dioxane-deuterium oxide the reaction showed a small inverse isotope effect; the apparent lack of an appreciable primary isotope effect is consistent with a concerted mechanism only if minimal proton transfer from water to carbon has occured in the transition state.
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