Spontaneous and base-catalysed elimination from 1-ferrocenylalkyl cations

Abstract

The rate constants of proton elimination from 1-ferrocenylalkyl cations FcC(R¹)CH₂R² with unhindered tertiary amines in H₂O–MeCN follow the Brønsted catalysis relation; the values of β are 0.32 for FcC(Ph)CH₂Bu^t and 0.45 for Fc₂CMe. Oxygen bases and sterically hindered amines are much less effective reagents than unhindered amines of similar basicity. The overall kinetic hydrogen isotope effects on elimination from FcC(Ph)CX₂Bu^t(X = H and D) go through maxima with increasing strength of base and k_H^ene/k_D^ene values are 4.2, 6.7, and 4.2 for reactions with water, pyridine, and quinuclidine, respectively. Chloride ion speeds the spontaneous reaction of FcC(Ph)CH₂Bu^t and increases the kinetic isotope effect, but inhibits the reaction with HO^– without affecting the isotope effect. These observations suggest that both ion-pairing and initial-state conformation are important in determining overall reactivity, the Brønsted β value, and k_H^ene/k_D^ene.