Deprotonation of the α-(N,N-dimethylcarbamoyl)-α-methyl-4-methoxybenzyl carbocation by alkanecarboxylate and halide ions

Abstract

The product rate constant ratios k_elim/k_az[N₃⁻] for partitioning of the α-(N,N-dimethylcarbamoyl)-α-methyl-4-methoxybenzyl carbocation (4⁺+++) in aqueous methanol between deprotonation of the α-methyl carbon to form an alkene and nucleophilic addition of azide ion increase for reactions in the presence of carboxylate and halide ions. An analysis of these data gave second-order rate constants k_B for deprotonation of 4⁺+++ by carboxylate ions which are correlated by a Brønsted coefficient of β = 0.17. The second-order rate constant for deprotonation of 4⁺+++ by solvent water shows a 40-fold negative deviation from this correlation. Values of k_elim/k_az[N₃⁻] in 20% methanol in water increase by ≤30% as the halide ion concentration is increased from 0.0–0.80 mol dm⁻³ (I = 0.80, NaClO₄) due either to direct deprotonation of 4⁺+++ by the halide ion or to a specific salt effect on the partitioning of 4⁺+++. The value for k_B calculated with the assumption that this increase is due to deprotonation of 4⁺+++ by chloride ion shows a small positive deviation from the Brønsted correlation of rate data for the reaction of carboxylate ions. However, all of these data show a satisfactory fit to a single Brønsted correlation with slope β = 0.12. These results are consistent with the conclusion that chloride ion shows a normal reactivity toward deprotonation of this α-methyl carbocation for a base of pK_a ≈ −8 compared to substituted carboxylate anions, and an enhanced reactivity compared to solvent water.