A DFT analysis of the participation of zwitterionic TACs in polar [3+2] cycloaddition reactions

A set of seven non-substituted tri-atom-components (TACs) participating in [3+2] cycloaddition (32CA) reactions has been studied using the reactivity indices defined within the conceptual DFT at the B3LYP/6-31G(d) level of theory. This series of TACs shows a zwitterionic structure and low reactivity towards ethylene. The general characteristic of these TACs is their high nucleophilic and a low electrophilic behaviour. Activation energies computed at the MPWB1K/6-311G(d) level in dichloromethane point to that non-substituted TACs react quickly toward dicyanoethylene showing their ability to react towards electron-deficient ethylenes. However, when the TACs are electrophilically activated by an appropriate substitution there seems to be insufficient activation to react toward electron-rich ethylenes. The electrophilic activation of the TAC moiety for nucleophilic attacks was only determined by the coordination with a Lewis acid. All 32CA reactions studied in this work presented high regioselectivity. The polar character of these 32CA reactions is associated with the global charge transfer found at the TS, which is in agreement with a zwitterionic-type (zw-type) mechanism. According to our results, the present theoretical study suggests that the substitution is required in both, TACs and the ethylene species, in order to experimentally perform these zw-type 32CA reactions under mild conditions. (C) 2014 Elsevier Ltd. All rights reserved.