Computational Study of Ignored Pericyclic Reactions: Rearrangements of 1,2-Bis(Diazo)Alkanes to 1,2,3,4-Tetrazines...

An electrocyclic ring closure of bis-1,3-dipoles can afford six-membered heterocycles. This 8π-electron process was systematically analyzed by DFT calculations with 1,2-bis(diazo)alkane derivatives as possible precursor compounds and 1,2,3,4-tetrazines as products. The C₂-symmetry of the transition state of the parent system points to a conrotatory ring closing event. The subsequent (6-2-2) cycloreversions of these elusive nitrogen-rich heterocycles to alkynes or nitriles and dinitrogen were also computationally investigated. The results show that the reactions are strongly dependent on the substitution pattern, but all are kinetically easily feasible delivering products of differing stability. The calculations can therefore provide important information for experimental endeavors to generate or even isolate so far unknown 1,2,3,4-tetrazines. The feasibility of carbene or 1,2,3-triazolyl-substituted nitrene intermediates for the formation of alkynes is also discussed. The experimental evidence for the proposed processes is enclosed presenting literature known examples of the fragmentation reactions which can most convincingly be explained by the intermediacy of 1,2,3,4-tetrazine derivatives. Furthermore, the electrocyclic ring closure reactions of five other types of bis-1,3-dipoles are calculated, demonstrating that this so far ignored 8π-electrocyclization process can establish a new route to interestingly composed heterocyclic compounds.