Aromatic reactivity. Part XXXV. Alkali cleavage of aryltrimethylstannanes: an unusual electrophilic aromatic substitution

Abstract

We have measured spectrophotometrically the rates of cleavage by aqueous methanolic alkali of the aryl–tin bonds in a series of X·C₆H₄·SnMe₃ compounds. In this reaction, the rate-determining step of which probably involves separation of the carbanion X·C₆H₄ ^–, the effects of meta substituents are in accord with the Hammett σ constants (σ= 2·18), but the effects of para substituents show an abnormal pattern. For example, there is rate-acceleration not only by electron-withdrawing groups such as p-CF₃ and p-Cl, but also by normally electron-releasing groups such as p-OMe and p-NMe₂. For the para substituents, the effects are roughly in line with their σ¹ constants. The analogy with base-catalysed aromatic hydrogen-exchange is discussed, and substituent effects in the two reactions are compared. An explanation previously advanced for the abnormalities in the hydrogen exchange is examined in the light of the results for the cleavage.