Theoretical Studies on the Mechanism of the Formation of Cyclopentadienes and Dihydropyridazines

The synthesis of cyclopentadiene derivatives via the domino reaction of norbornadienes with dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate, followed a retro-Diels-Alder reaction, was reported by Dalkilic few years ago (Tetrahedron 2015;71 : 1966). In some cases, the formation of an unexpected dihydropyridazine (DHP) was observed, and explained by a 1,3-hydrogen shift. A density functional theory study is herein reported giving insights into the proposed mechanisms with different substituents on norbornadiene. Based on the kinetic and thermodynamic data, the retro-Diels-Alder reaction is confirmed to explain the formation of cyclopentadienes plus pyridazines. The intramolecular 1,3-hydrogen shift mechanism was not sufficient to explain the exclusive formation of two DHPs. Thus, an alternative mechanism involving a stepwise tautomerisation is being reported for formation of DHPs.