Transfer Reagents 4: Retro-Diels-Alder Routes to 3,6-Di(2-pyridyl)pyridazinonorbornadiene, a Test Bed for the Relative Dienofugacity of Isobenzofuran, Isoindole, and Anthracene

Norbornadiene cycloadducts reacted with 3,6-di(2-pyridyl)-s-tetrazine to produce pyridazines (diene-protected alkenes), following dehydrogenation (by DDQ) of the intermediate dihydropyridazines. The title 3,6-di(2-pyridyl)-pyridazinonorbornadiene was produced under flash vacuum pyrolysis conditions by ejection of anthracene (590 degrees C), isobenzofuran (630 degrees C) or isoindole (580 degrees C) from the corresponding pyridazines, establishing the following dienofugacity order: isoindole > anthracene > isobenzofuran. Calculations of the respective retro-Diels-alder activation energies at the B3LYP/6-31G* level of theory correctly predicted the experimentally found isobenzofuran > anthracene > isoindole order. Cavity bis-3,6-di(2-pyridyl)-pyridazine (dppn) and chevron-shaped bis-dppn ligands were prepared from the title compound by coupling at the norbornene -bond.