Abstract
Comparison of the E parameters of the 3-cyanomethylidene-2-oxindoline derivatives with those of the reference electrophiles benzylidenemalonates and analogously substituted benzylidene Meldrum’s acids.
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A kinetic study of the nucleophilic addition reactions of 3-cyanomethylidene-2-oxindoline derivatives with cyclic amines (namely: piperidine, morpholine and pyrrolidine) in MeCN solution at 20 °C is reported. The second-order rate constants showed of this process fit nicely the Brönsted equation log k1 = βnuc pKa + C, allowing the determination of the βnuc parameter in the range of 0.63 < βnuc < 0.77 that indicates that the degree of formation of N–C bond in the transition state is more half complete. Moreover, the analysis of the kinetic measurements based on a good linear log k (20 °C) = sN (E + N) free enthalpy relationship are used to assess the electrophilic reactivity in term of E parameter of these series of 2-oxindoline derivatives Michael acceptors. Of major interest is that the estimated E values were established to cover a domain of reactivity of ~3 units of E, ranging from −17.5 for 2-(5-chloro-2-oxindolin-3-ylidene)malononitrile (the most reactive electrophile) to −20.3 for ethyl 2-(5-chloro-1-methyl-2-oxindolin-3-ylidene)-2-cyanoacetate (the least reactive electrophile). The theoretical reactivity indices ω based on the conceptual Density Functional Theory (DFT) explains correctly the experimental electrophilicity E ordering founded in terms of experimental scales.