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

Abstract

ABSTRACTThe 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 non-concerted two-stage one-step mechanism, in contrast to the simplified picture of rationalisation traditionally based on molecular orbitals.