Nucleophilic addition of amines to the activated ethylene bond. Part 3. Kinetics and mechanism of the addition of amines to trans-(2-furyl)nitroethylene

Abstract

The kinetics of addition of a number of primary and secondary aliphatic amines to trans-(2-furyl)nitroethylene in acetonitrile at 25 °C have been studied. It has been shown that the reaction is catalysed both by the amine reagent and by tertiary amines and in the case of primary amines the dependence of the observed second-order rate constant (k_obs) on amine concentration takes the form of a hyperbola. The Arrhenius activation energy has been found to be negative for the catalytic reaction route. On the basis of analysing the observed kinetic regularities a stepwise reaction mechanism has been proposed which involves formation of the zwitterionic addition complex (1) at the first equilibrium step (k₁, k_–1) which is then converted into the reaction product by means of proton transfer in parallel routes, i.e. the non-catalytic one (k₂) and that catalysed by the initial (k₃) or tertiary (k₄) amine. The rate constants of the non-catalytic [k=k₁k₂/(k_–1+k₂)] and catalytic [k_b=k₁k₃/(k_–1+k₂)] reaction routes have been determined for a number of proton-containing amines; for primary amines the nucleophilic attack rate constants (k₁) and relations for the constants of conversion of the zwitterionic intermediate (1) into the reaction product and the disintegration of the former into initial compounds (k₂/k_–1 and k₃/k_–1) have been determined. A mechanism for proton transfer in the intermediate (1) is discussed.