Toward understanding the regioselectivity of the one‐pot reaction of phenylene diamines with aldehydes and cyclopentadiene (Povarov reaction). Combined experimental and theoretical approaches

Three-component acid-catalyzed cyclocondensation of isomeric phenylenediamines with aldehydes and cyclopentadiene (CPD) was studied. In the M06-2X//B3LYP/6-311 + G(d,p) + IEFPCM(SMD) approximation, a quantum chemical model was developed, energies of the PES stationary points were determined, and activation barriers were found for the CPD addition to the protonated Schiff base (rate-limiting step) and for electrophilic carbocation cyclization to give endo, endo (syn)- or endo, exo (anti)-cyclopentene-annulated octahydropyridoquinoline- and octahydrophenanthroline-based cycloadducts. The calculation results are in full agreement with the experimental data, which indicate that the cyclocondensation involving p-phenylenediamine affords both types of cycloadducts, whereas the reactions of o- or m-phenylenediamine lead to regioselective formation of octahydro-1,10-phenanthroline- or octahydropyridoquinoline-based product, respectively. According to homo- and heteronuclear correlation H-1 and C-13 NMR experiments and X-ray diffraction data, the newly formed asymmetric centers of the cyclopentene-annulated tetrahydroquinoline moiety in the polycyclic products have exo (endo)-cis-orientation.