Abstract
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 Density Functional Theory (DFT) calculations at the MPWB1K/6-311G(d) level. Topological analysis of the electron localisation function reveals that AI has a pseudoradical structure, while the conceptual DFT reactivity indices characterises this three-atom-component (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 moderate activation energy, 8.7 kcal·mol−1. A bonding evolution theory 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 activation energy is only 0.4 kcal·mol−1, in complete agreement with the high polar character of the more favourable regioisomeric transition state structure. The current MEDT study makes it possible to extend Domingo’s classification of 32CA reactions to a new pseudo(mono)radical type (pmr-type) of reactivity.
Highlights
Global reactivity indices defined within Conceptual DFT (CDFT) [23,24] are powerful tools to explain the reactivity in cycloaddition reactions
Due to the non-symmetry of azomethine imine (AI) 1b and DCE 6, this 32CA reaction can take place through two regioisomeric channels, the ortho and the meta, i.e., those associated with the initial regioisomeric channels, the ortho and the meta, i.e., those associated with the initial computed at TS1, whose direction is in agreement with the analysis of the corresponding electronic chemical potential μ indices, can be rationalised as a delocalisation of the energetically destabilised electron density of the AI framework into the ethylene one, rather than a global electron density transfer [25] (GEDT) associated to a polar process [14]
Analysis of the electron density pattern of the simplest AI 1b reveals that this TAC presents a pseudoradical structure, characterised by the presence of a V(C1) monosynaptic basin integrating 0.62 e at the C1 carbon atom
Summary
[3+2] cycloaddition (32CA) reactions emerged as a powerful synthetic tool for the construction of five-membered heterocyclic compounds [1,2] These reactions have been experimentally known since the end of the 19th century, it was Huisgen who, in 1961, defined them as “1,3-dipolar cycloadditions” [3,4]. ∆Ed6= , called distortion energy, and ∆Ei6= , called interaction energy [7,8] The applicability of this model was checked in 32CA reactions of nine different TACs, three A-TACs 1a–c and six P-TACs 2a–f, with ethylene 3 and acetylene 4 (see Scheme 2) [7,8].
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