Where is the Limit to which the Qualitative Theory of Chemical Reactivity can be Extended? Study of Dienophilic Activity of Cyanoethylenes

A qualitative quantum-chemical approach, based on the stabilization energy method, is applied to discuss the mechanism and some kinetic regularities observed in Diels-Alder (DA) reactions of cyanoethylenes with cyclopentadiene and phencyclone. For the former reaction series the polar effect predominates over the localizationeffect and both the regular activity sequence and the great range of the rate constants can be understood as a result of the charge transfer from diene to dienophile in a synchronous transition state (TS). The inversion of the regulaT activity sequence observed for the latter reaction series seems to be caused by localizationeffects in asynchronous TS. The relative weakening of the charge transfer is associated with the increased role of electron correlation. Possible distortions of the TS structure within a reaction series are analyzed. The shift of the TS a