Understanding the [2n+2n] reaction mechanism between a carbenoid intermediate and CO₂

The mechanism of the cycloaddition reaction of CO₂ with a nucleophilic carbenoid intermediate has been theoretically studied by using the bonding evolution theory (BET) at the B3LYP/6- 31G(d) level of theory. BET combines topological analysis of the electron localisation function and catastrophe theory along a reaction path. This cycloaddition reaction is characterised by 16 structural stability domains, associated to the following sequence of catastrophes: C₈H₉NO₄ + CO₂: 16- CF^†CF^†F^†CFF^†C^†C^†[FF^†]F^†FCC^†-0: C₉H₉NO₆. Formation of the two new C-C and C-O single bonds evolves after the transition state structure is reached. The high nucleophilic character and the electronic structure of carbenoid intermediate together with the specific approaching mode of the CO₂ molecule enable in fact the first C- C single bond formation after a very low activation enthalpy, 2.2 kcal/mol, without any external electrophilic activation. The subsequent ring closure via formation of the C - O single bond occurs at the end of the reaction. We propose thereafter a rationalisation of such a polar reaction process in terms of a [2n+2n] cycloaddition reaction following a nonconcerted two-stage one-step mechanism, in contrast to the simplified picture of rationalisation traditionally based on molecular orbitals.