The [3+2] cycloaddition (32CA) reactions of cyclic azomethine ylides with a series of acetylene derivatives have been studied within the Molecular Electron Density Theory (MEDT). Analysis of the Conceptual DFT indices allows the characterisation of the ambiphilic character of these cyclic AYs, thus participating as strong electrophiles and strong nucleophiles in polar 32CA reactions. The activation enthalpies associated with the 32CA reactions of these cyclic AYs are found in reasonable agreement with the experimental outcomes. These reactions
Reactions between (E)-2-aryl-1-cyano-1-nitroethenes and diazafluorene lead to acyclic 2,3-diazabuta-1,3-diene derivatives, instead of the expected pyrazoline systems. DFT calculations suggest that this is a consequence of formation of zwitterionic structure in the first stage of the reaction. It must be noted that this is a specific property of the (E)-2-aryl-1-cyano-1-nitroethenes group, in contrast to most other conjugated nitroalkenes.Graphical abstract
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The regioselective zw-type [3 + 2] cycloaddition (32CA) reactions of a series of aryl-substituted nitrile N-oxides (NOs) with trichloronitropropene (TNP) have been both experimentally and theoretically studied within the Molecular Electron Density Theory (MEDT). Zwitterionic NOs behave as moderate nucleophiles while TNP acts as a very strong electrophile in these polar 32CA reactions of forward electron density flux, which present moderate activation Gibbs free energies of 22.8–25.6 kcal·mol−1 and an exergonic character of 28.4 kcal·mol−1 that makes them irreversible and kinetically controlled. The most favorable reaction is that involving the most nucleophilic MeO-substituted NO. Despite Parr functions correctly predicting the experimental regioselectivity with the most favorable O-CCCl3 interaction, these reactions follow a two-stage one-step mechanism in which formation of the O-C(CCl3) bond takes place once the C-C(NO2) bond is already formed. The present MEDT concludes that the reactivity differences in the series of NOs come from their different nucleophilic activation and polar character of the reactions, rather than any mechanistic feature.
In the contrast to all known [3 + 2] cycloadditions between nitrones and conjugated nitroalkenes, reactions of (E)‐3,3,3‐trichloro‐1‐nitroprop‐1‐ene with (Z)‐C‐(9‐anthryl)‐N‐arylnitrones are proceeding in a fully regioselective and stereoselective manner. Additionally, density functional theory calculations suggest stepwise, zwitterionic mechanism of these cycloadditions.
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