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

ABSTRACT

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

GRAPHICAL ABSTRACT

KEYWORDS:

• 1,3-dipolar cycloaddition
• bonding evolution theory (BET)
• electron localisation function (ELF)
• catastrophe theory
• two-stage one-step mechanisms

Acknowledgments

We are indebted to Professor Bernard Silvi for his kind advice and insightful discussions concerning technical details and practical considerations associated to the BET methodology. We also acknowledge an anonymous reviewer for providing very constructive criticism and comments that have helped us to improve the quality of the presentation of our results and discussion. This work is dedicated to Professor Andreas Savin on the occasion of his 65th birthday.

Disclosure statement

No potential conflict of interests was reported by the authors.

Additional information

Funding

Ministerio de Economía y Competitividad of the Spanish Government: project CTQ2013-45646-P, and European Social Fund BES-2014-068258; FONDECYT (Chile): Projects 1140341, and 1140343; Millennium Nucleus of Chemical Processes and Catalysis (CPC): grant number NC 120082; Universidad Andres Bello (UNAB): grants DI-806-15/R and DI-793-15/R.