Mechanisms for formation of diazocinones, pyridazines, and pyrazolines from tetrazines--oxyanion-accelerated pericyclic cascades?

A computational approach is utilized to study the diazocinone- and pyridazine-forming cascade reactions resulting from the reaction of 1,2,4,5-tetrazines with cyclic enolates. Many of the proposed reaction steps can be formulated as oxyanion-accelerated pericyclic processes. In examining these, a unique stepwise version of a formal (4 + 2) cycloaddition/(4 + 2) cycloreversion was discovered. For the key ring-opening step in these cascades, theoretical evidence for two distinct processes is reported. Of these two possibilities, an allowed six-electron electrocyclic ring-opening is predicted to be highly favored both kinetically and thermodynamically. Evidence for an unexpected oxyanion-accelerated 1,2-sigmatropic shift was also found for certain systems, leading to the theoretical prediction that seven- and eight-membered ring-fused pyrazoline systems could be formed experimentally under conditions similar to those for diazocinone and pyridazine formation.