The Cycloaddition-Cycloelimination Pathway to Homotropilidenes − Synthesis and Properties of Homotropilidenes

The cycloaddition-cycloelimination reaction sequence between 3,6-disubstituted 1,2,4,5-tetrazines 9 and various cyclopropenes 10 provides 3,4-diazanorcaradienes 11, which undergo [4+2] cycloadditions with additional cyclopropenes 10 to form tetracyclic azo compounds 12. Photolysis of these compounds, with accompanying loss of nitrogen, affords homotropilidenes (bicyclo[5.1.0]octa-2,5-dienes) 13−26. Substituents at various positions of the homotropilidene skeleton have been observed to exert significant influences on the Cope rearrangement rate and, in cases of unsymmetrically substituted homotropilidenes, on the equilibrium positions.