preprints.org > chemistry > organic chemistry > doi: 10.20944/preprints201705.0005.v1

Preprint Article Version 1 This version is not peer-reviewed

Version 1 : Received: 28 April 2017 / Approved: 1 May 2017 / Online: 1 May 2017 (08:13:26 CEST)

The electronic structure and the participation of the simplest azomethine imine (AI) in [3+2] cycloaddition (32CA) reactions have been analysed within the Molecular Electron Density Theory (MEDT) using DFT calculations at the MPWB1K/6-311G(d) level. Electron localisation function (ELF) topological analysis reveals that AI has a pseudoradical structure, while the conceptual DFT reactivity indices characterise this TAC as a moderate electrophile and a good nucleophile. The non-polar 32CA reaction of AI with ethylene takes place through a one-step mechanism with low activation energy, 5.3 kcal/mol-1. A bonding evolution theory (BET) study indicates that this reaction takes place through a non-concerted [2n+2τ] mechanism in which the C–C bond formation is clearly anticipated prior to the C–N one. On the other hand, the polar 32CA reaction of AI with dicyanoethylene takes place through a two-stage one-step mechanism. Now, the more favourable regioisomeric transition state structure (TS) is located 8.5 kcal•mol⁻¹ below the reagents, in complete agreement with the high polar character of the TS. The current MEDT study makes it possible to extend Domingo’s classification of 32CA reactions to a new pra-type of reactivity.

azomethine imine; [3+2] cycloaddition reactions; molecular electron density theory; conceptual density functional theory; electron localisation function; bonding evolution theory; electron density; molecular mechanisms; chemical reactivity.