Electronic effects of icosahedral carboranes: mechanistic alteration in solvolysis of α-(o-carboranyl)benzyl tosylates by electronic effect of substituents

Abstract

Abstract Solvolysis of α-( o - and m -carboranyl)benzyl toluene- p -sulfonates bearing a range of substituents at the 3- or 4-position of the benzyl group has been investigated. The rates of hydrolysis of m -carboranyl derivatives increased linearly with increasing electron-releasing character of the substituent group ( ρ =−5.37 correlated to σ + ), which indicates that the hydrolysis proceeds through a typical S N 1 process. In contrast, the hydrolysis rates of the o -carboranyl derivatives with a wide range of electron-withdrawing substituents did not show significant change, but suddenly began to show a linear increase with increasing electron-releasing character of the substituents ( ρ =−5.91 correlated to σ + ). Moreover, the optical purity of the retentive hydrolysis of the o -carboranyl derivatives rapidly declined with increasing electron-donating effects of the substituents. This indicates that the mechanism of the hydrolysis changes from the retentive mechanism to an S N 1 mechanism.