High secondary α-deuterium kinetic isotope effect in the acetolysis and formolysis of dideuterioferrocenylmethyl benzoate

Abstract

Acetolysis and formolysis of dideuterioferrocenylmethyl benzoate exhibit large secondary α-deuterium kinetic isotope effects and an abnormal temperature dependence. In the presence of LiClO₄(ca. 0.1 mol dm^–3), which prevents the reversion from solvent-separated to contact ion-pairs, k_H/k_D at 25 °C amount to 1.53 ± 0.02 (acetolysis) and 1.48 ± 0.03 (formolysis). In the presence of LiClO₄ the ratios of Arrhenius pre-exponential factors, A_H/A_D, are significantly less than unity and amount to 0.49 ± 0.01 (acetolysis) and 0.38 ± 0.04 (formolysis). In the absence of LiClO₄ the A_H/A_D ratios are much smaller (0.02 both in acetolysis and formolysis). We suggest that these surprisingly low values result from a change in rate-determining step over the temperature range, from formation of the solvent-separated ion-pair at low temperatures to reaction of the dissociated carbocation with solvent at the highest temperatures. Whether tunnelling plays any role in these solvolyses is discussed.