Elimination and addition reactions. Part 33. Formation and behaviour of carbanions derived from sulphones and nitriles bearing β-′onium substituents

Abstract

Kinetics of elimination reactions of β-arylsulphonylethyl- and β-cyanoethyl-ammonium and -sulphonium salts have been measured in ethanolic triethylamine buffers. The reactions show buffer saturation kinetics; at low buffer base concentrations ionisation to form the intermediate carbanion is rate-determining, but at higher buffer base concentrations the intermediate carbanion is formed in a rapidly established pre-equilibrium step and the observed rate constant does not change with increasing base concentration at constant buffer ratio.

The rate data separately yield the ionisation rates of the substrates studied, together with the ratio (k₂: k_–1) of the rate constants for loss of the ′onium leaving group from the carbanion and its reprotonation.

Formation of a carbanion from a sulphone is very sensitive to steric and polar effects; ionisation of nitriles is less sensitive in both respects, particularly the former.

The elimination-reprotonation ratio (k₂: k_–1) depends upon the activating group, but for a given activating group and type of leaving group it is insensitive to the structure of the leaving group. It is also insensitive to substitution at either C_α or C_β.

Comparisons are drawn with unactivated elimination reactions of ′onium salts, and the Hammett ρ value has been obtained for expulsion of the leaving group in the 2-phenylethylammonium series.