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

Abstract

AbstractThe 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 Dalkılıç 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.