Understanding the participation of 3-nitropyridine in polar Diels-Alder reactions. A DFT study

Abstract

The reactivity of 3-nitropyridine acting as an electrophilic dienophile in polar Diels-Alder (P-DA) reactions toward three different dienes of increased nucleophilicity has been theoretically studied using DFT methods at the MPWB1K/6-31G(d) level. It has been observed that this aromatic heterocyclic system suffers cycloaddition reactions yielding isoquinoline derivatives. The present DFT study establishes that while the P-DA reactions with isoprene and 1-methoxy-1,3-butadiene take place through a two-stage one-step mechanism, the use of the strong nucleophilic Danishefsky's diene changes the mechanism to a two-step one with formation of a zwitterionic intermediate. These P-DA reactions are completely regioselective allowing the formation of a unique substituted isoquinoline. Analysis of the DFT reactivity indices at the ground state of the reagents correctly explains the reactivity and regioselectivity for these P-DA reactions.