The kinetics and mechanism of the reaction of cyclopentadienylidenetriarylphosphoranes with cyano-olefins. Part IV. Tricyanovinylation of cyclopentadienylidenetriphenylphosphorane with tetracyanoethylene and tricyanovinyl chloride
Abstract
The reaction of cyclopentadienylidenetriphenylphosphorane (I) with tetracyanoethylene (II) in the presence of an excess of triethylamine using benzene, dichloromethane, or chloroform as solvent gives 2-(1,2,2-tricyanovinyl)-cyclopentadienylidenetriphenylphosphorane (IV), λmax. 465 nm, in quantitative yield. Compound (IV) is also obtained from the reaction of (I) with tricyanovinyl choride in benzene or chloroform in the absence of base. The spontaneous reaction of (I) with (II) in dichloromethane (or chloroform) gave a second product (V)λmax. 482 nm, and the conversion of (V) into (IV) was effected photolytically. The base-catalysed elimination of hydrogen cyanide from a mixture of (I) and (II) in benzene was found to be zero order in (II), close to zero order in (I), but first order in base; by using a number of tertiary amines a linear Brønsted correlation was obtained over a range of five pKa units. When 2,3,4,5-tetradeuterio-(I) was used the reaction with each base showed a slight inverse deuterium isotope effect (average kH/kD= 0·96). Rates are also reported with methylene chloride as solvent and the activation parameters for the reaction in benzene are recorded. The results are interpreted in terms of an E1 cB reaction in which the rate-determining step involves loss of cyanide ion from an ion-pair intermediate. The rate of reaction of (I) with tricyanovinyl chloride was too fast to be followed by standard techniques.