The kinetics and mechanism of the reaction of cyclopentadienylidenetriarylphosphoranes with cyano-olefins. Part III. Elimination of hydrogen cyanide from 2-(1-phenyl-1,2,2-tricyanoethyl)cyclopentadienylidenetriphenylphosphorane; an E1CB reaction via an ion-pair
Abstract
Kinetic data on the base-catalysed elimination of hydrogen cyanide from 2-(1-phenyl-1,2,2-tricyanoethyl)-cyclopentadienylidenetriphenylphosphorane in benzene, benzene–acetonitrile, or acetonitrile solvent are reported. The elimination is of the first order in substrate and base (tertiary amine) and the second-order rate constants (k2) increase with increasing polarity of the medium. The deuterium isotope effect (k2H/k2D) is ca. 1·0 in each solvent system and a linear Brønsted plot is observed with a number of tertiary amines over a range of five pKa units. Activation parameters are reported for the reaction with triethylamine and the kinetic results are interpreted in terms of a E1CB reaction involving fast proton loss to form an ion-pair, followed by rate-determining elimination of cyanide ion from the ion-pair intermediate.