Abstract
The kinetics of the reactions of benzylidenecyanoacetates 1a-d (X = H, Me, OMe, and NMe2) with nitroalkyl anions 2a-c have been studied in aqueous solution at 20 degrees C. The second-order rate constants are used to evaluate the electrophilicity parameter E of these series of Michael acceptors 1a-d according to the linear free enthalpy relationship log k (20 degrees C) = s(N) (E + N). The measured E values were found to cover a domain of reactivity, ranging from -10.07 for the most reactive electrophile 1a (X = H) to -14.04 for the less electrophile 1d (X = NMe2). Mayr's approach was found to correctly predict the rate constants for the reactions of these series of olefins 1a-d with the hydroxide ion in water and 50% water-50% acetonitrile at 20 degrees C. Analysis of the kinetic measurements using the Bronsted relationship shows that (nuc) values remain remarkably constant for changes in the nature of the substituent X. A notable finding of this work is perhaps provided by the observed large changes in the electrophilicity parameter E on going from benzylidenecyanoacetates 1 to their analogues benzylidenemalonates 3 (E approximate to 9.06-10.48), whereas the replacement of second CO2Et group by the CN group in 1 to give benzylidenemalononitriles 4 has little effect on electrophilic reactivity, i.e., E approximate to 0.65-0.95. This abnormal pattern in the E values has been attributed to the resonance interaction and salvation effects. On the other hand, the effect of benzylidenecyanoacetate substituents on the electrophilic reactivity was examined quantitatively on the basis of the electrophilicity parameter E, leading to linear correlation of E with Hammett-Brown substituent constants (sigma(+)(p)). More importantly, the four electrophiles have comparable log k(o) values, which are located at a relatively low level, i.e., log k(o) 4-5, in the intrinsic reactivity scale.