Studies of electronic effects of modified pyridine-imine ligands utilized in cobalt-catalyzed meta-selective Diels-Alder reactions

The regioselectivity of cobalt-catalyzed Diels-Alder reactions can be controlled by the choice of ligands on the cobalt center. Ligands of the pyridine-imine type favor the formation of the 1,3-disubstitution pattern on the dihydroaromatic product. The investigation was aimed to elucidate the factors for controlling the regioselectivities and reactivity induced by electronic effects of the pyridine-imine ligands in the cobalt-catalyzed Diels-Alder reaction. For that, electron-withdrawing as well as electron donating substituents were introduced in the 4-position on the pyridine moiety and on the 4′-position of the aniline derivative used in the imine subunit of the ligands. In close synergy DFT calculations and the comparison with experimental results proved that electronic variation of the substituents at both positions have a negligible influence on the regioselectivity. However, the kinetic data for the cobalt-catalyzed Diels-Alder reactions revealed that there are great differences in the lengths of the induction periods when different cobalt pyridine-imine complexes are applied. These results could be elucidated by conductivity experiments showing that ionic homoleptic complexes Co(L) ₂²⁺/CoBr₄^2- are in equilibrium with their corresponding neutral heteroleptic complexes of type Co(L)Br₂ in solution. The equilibrium position depends on the electronic characteristics of the pyridine-imine ligands (L), thereby influencing the length of the induction period.