The kinetics and mechanism of the reaction of cyclopentadienylidenetriaryl-phosphoranes with cyano-olefins. Part I. Nucleophilic addition to tricyanovinylbenzene

Abstract

Cyclopentadienylidenetriphenylphosphorane (I) reacts with tricyanovinylbenzene (II) to give 2-(1,2,2-tricyano-1-phenylethyl)cyclopentadienylidenetriphenylphosphorane (III). The reaction was studied kinetically in benzene (and methylene dichloride) solvent and was shown to involve rate-determining nucleophilic attack by the phosphorus ylide on the cyano-olefin followed by a rapid proton transfer. Evidence is presented which indicates the proton transfer is an intramolecular process. The nucléophilic addition was also shown to be acid-catalysed and the non-linearity of a Brønsted plot was interpreted in terms of acid dimerisation in the aprotic medium. Activation parameters are reported for the catalysed and non-catalysed reactions together with deuterium isotope effects for each process. Substituents in the phenyl groups of the ylide gave rise to rate variations which showed a Hammett correlation with the constants for σ-substituents, giving a ρ value of –1·8 for the acid-catalysed reaction. It is proposed that the mechanism of the acid-catalysed reaction involves rate-determining nucleophilic addition of the ylide to tricyanovinylbenzene via a π-complex, the function of the acid being to assist polarisation of this molecular complex.