Bridgehead Carbocations: Solvolysis of a Series of 5-Substituted Bicycle [3.1.1]heptyl Bromides. Nucleophilic Solvent Assistance to Ionization of 1-Bromobicyclo[3.1.1]heptane

Abstract

The synthesis of a range of 5-substituted bicyclo [3.1.1] heptyl bromides for solvolytic studies is described. It is found that the substituent has a profound effect on the rate of solvolysis of the system and acts principally in accordance with the magnitude of its inductive/field constant σ_I. The most spectacular example of the effect of the substituent is provided by the COOMe group which leads to a retardation in the rate of methanolysis by a factor of 6.5°10⁵. While a linear relationship in the plot of log k and σ_I is generally obeyed, as expected for a mechanism mediated by the bicyclo [3.1.1] heptyl bridgehead cation, two of the bromides, 1-bromobicyclo[3.1.1] heptane and its 5-methoxy derivative, show deviant behaviour and react more rapidly than predicted on the basis of the Hammett plot. Evidence is presented to show that the enhanced rate of the parent is the result of nucleophilic assistance by the solvent. Anchimeric assistance in the solvolysis of 5-methoxybicyclo[3.1.1] heptyl bromide is attributed to the powerful p-donor property of the methoxy substituent which stabilizes the transition state in a unique concerted ring-opening and ionization step.