Nucleophilic attacks on carbon–carbon double bonds. Part XV. Further evidence for base-catalysis in the reaction of 1,1-dicyano-2-p-dimethylaminophenyl-2-fluoroethylene with substituted anilines in acetonitrile

Abstract

The third-order reaction of 1,1-dicyano-2-p-dimethylaminophenyl-2-fluoroethylene (I) with p-toluidine in acetonitrile is catalysed by pyridines. These compete with p-toluidine in proton expulsion from the zwitterionic intermediate Ar²(Ar¹N⁺H₂)CF–C^–(CN)₂. Two parallel Brønsted lines with α= 0·29 are obtained: the higher for 3- and 4-substituted and the lower for 2-substituted pyridines. The reaction of o-phenylenediamine with (I) is nearly of the second order in the amine; 97% of the catalysis is by external amine and 3% by the o-amino-group of the intermediate. In their reaction with (I), o-, m-, and p-phenylenediamine are on the same Brønsted (α= 1·67) and Hammett (ρ=–3·88) lines for the other substituents. It is concluded that the second amine operates as a base and not as an electrophilic catalyst for the fluorine expulsion. The isotope effects with N-deuteriated anilines are small and they are discussed in relation to the symmetry of the transition state.