Stable carbonium ions. Part III. Reactions involving 1,3-diarylallyl and 1-ferrocenyl-3-arylallyl cations

Abstract

Equilibrium constants have been measured for the acid-catalysed equilibration of a series of 1,3-diarylpropenols (chalcols) and 1(3)-ferrocenyl-3(1)-arylallyl ethers. The thermodynamic stability of the styryl system is increased by substitution in the aromatic ring of groups exerting either a positive or a negative resonance effect (e.g., p-OMe, p-NO₂) or a positive inductive effect (e.g., p-Me). The styryl system is destabilised by the introduction of groups exerting a negative inductive effect (e.g., p-Cl, m-NO₂). Conjugation of a carbon–carbon double bond with a ferrocenyl group confers greater thermodynamic stability than that provided by conjugation with an aryl group. In all of these cases, the effects are small. The rate of acid-catalysed chalcol equilibration is accelerated by electrondonating substituents and retarded by electron-withdrawing substituents in the aromatic rings. Analysis of the isomeric chalcol mixtures formed under kinetic control in the solvolysis (80% aqueous acetone) of a series of 1,3-diarylallyl p-nitrobenzoates has shown that positive charge distribution in the intermediate 1,3-diarylallyl cations is moderately sensitive to the nature of the substituents present in the aromatic rings.