The nitration of pyridine Py and pyridine-1-oxide PyO have low activation Gibbs free energies, but in the medium acid, the corresponding pyridinium cation PyH+ is strongly deactivated for the electrophilic aromatic substitution.
Phenyl azide only participates in [3 + 2] cycloaddition reactions with strongly nucleophilic enamines. A good correlation between the polar character of the reaction and the relative reaction rate constants kr is found.
The [3+2] cycloaddition reactions of DFA with NI or DM are regiospecific involving the non-substituted C−C double bond of the former, whereas that with MDM is poorly regioselective. MEDT study explains the experimental results.
The mechanism of the addition of 2-aryl-3-nitro-2H-chromenes to pyrazole was evaluated in the framework of molecular electron density theory based on the results of the wb97xd/6-311+G(d,p) (IEFPCM) calculations.
The presence of Mg(II) cation does not substantially accelerate this low polar zw-type [3+2] cycloaddition reaction, but changes the facial diastereoselectivity. These low polar reactions are completely ortho regioselective.