Acetolysis of ferrocenylmethyl benzoate. High secondary α-deuterium kinetic isotope effect for primary carbon-oxygen cleavage

Abstract

Secondary α-deuterium kinetic isotope effect ( KIE) in acetolysis of ferrocenyl-1,1-dideuteriomethyl benzoate was determined at 298 K as kH/ kD = 1.50 (22.5% per D). This appears to be one of the largest KIE observed for carbon-oxygen cleavage. The solvolysis exhibits a ‘special salt effect’, and a common ion rate depression effect, indicating the presence of solvent-separated ion pairs and the return to tight pairs. The high value of KIE in acetolysis strongly suggests that the acetolysis is a limiting dissociative process with the carbonium ion transition state, stabilized mainly by conjugation with the π-system. The entropy of activation (−30 J K −1 mol −1) is very similar to those values we previously determined in ethanolyses of ferrocenylmethyl acetate and benzoate (−22 and −25 J K −1 mol −1), where the KIE was only 11.4% per D. In the latter reactions the stabilization of the transition state probably involves some Fe-C exo bond formation. It appears that the mode of stabilization of the ferrocenylmethyl cation depends on the type of solvolysis and on the nature of the media: high KIE is observed in solvents of high ionizing power. Entropies of activation cannot help in distinguishing between the two possible modes of stabilization.