Elimination and addition reactions. Part 30. Leaving group abilities in alkene-forming eliminations activated by sulphonyl groups

Abstract

Rates of elimination of the group Z from a series of β-substituted sulphones, PhSO₂·CH₂·CH₂Z, in ethanolic sodium ethoxide at 25 °C have been measured. For each substrate, the mechanism of the reaction has been shown to be the reversible carbanion mechanism [(E₁cB)_R] by determination of the primary kinetic deuterium isotope effect or by demonstration that deuterium-hydrogen exchange at C_β is very much faster than elimination.

The k_obs, values obtained for these substrates encompass the equilibrium constant for carbanion formation and the rate constant for ejection of the leaving group. The effect of the leaving group on the equilibrium constant, assessed from a Taft plot for sulphone detritiation, has been taken into account where possible in comparison of leaving group abilities.

All positively charged leaving groups depart very easily. Among neutral leaving groups the following order of familiar groups is found PhSe > PhO > PhS > PhSO₂ > PhSO > NMeTs > NMeAc > CN. The range of reactivity values spans ca. 10¹⁶. Carbon-linked leaving groups such as cyano are exceptionally poor.

No simple correlation is obtained between leaving group order and pK_a of ZH, the dissociation energy of the C–Z bond, the inverse nucleophilicity of Z:, or the polar effect of Z as measured by values of σ_I(CH₂Z) or the ¹³C chemical shift of C_α–Z.

Comparisons are drawn between the present data and earlier, usually qualititative, observations on elimination reactions.