The kinetics and mechanism of the reaction of cyclopentadienylidenetriarylphosphoranes with cyano-olefins. Part II. Nucleophilic addition to benzylidenemalononitriles; a linear free energy relationship between rate and equilibrium constant for π-complex formation

Abstract

Third-order rate constants (k₃) for the acid-catalysed nucleophilic addition of cyclopentadienylidenetriphenylphosphorane to a series of benzylidenemalononitriles in benzene solvent are reported together with the corresponding activation parameters. The rates show a Hammett correlation of the form, log k=σ+R(σ⁺–σ) with R= 0·7 and this result is compared with Hammett correlations for similar nucleophilic addition reactions. The λ_max. and equilibrium constant (K_π) values for π-complexes of the benzylidene malononitriles with NNN′N′-tetramethyl-p-phenylenediamine in benzene are also reported and a linear free energy relationship between the rate of the nucleophilic addition and K_π is demonstrated. A mechanism is suggested involving π-complex formation between the phosphorus ylide and the cyano-olefin.