1,4-Cycloheptadienes: preparation and comparison of their proton magnetic resonance spectra with those of the corresponding cycloheptatrienes

A general route to 3-substituted cyclohepta-1,4-dienes is described in this paper. 7-Substituted cycloheptatrienes were used as starting materials and these were reacted with 4-phenyl-1,2,4-triazoline-3,5-dione to give 4 + 2 cycloaddition products of the norcaradiene form of the cycloheptatrienes. Catalytic reduction of these adducts gave 9-substituted 4-phenyl-2,4,6-triazatetracyclo[5.3.2.0 2,6 .0 8,10 ]dodeca-3,5-diones in good yield. Basic hydrolysis of the hetero-cyclic ring of these products followed by Cu 2+ oxidation of the resulting hydrazo compounds gave the copper complexes of substituted 6,7-diazatricyclo[3.2.2.0 2,4 ]non-6-enes. The azo compounds, formed on decomplexation of the copper complexes, readily lost nitrogen to give 3-substituted cyclohepta-l,4-dienes. Care had to be taken during these last steps as the cyclo-heptadienes with carboxylate substituents very readily isomerized to the conjugated isomers. The pmr spectra of the cyclohepta-l,4-dienes and related cycloheptatrienes are compared and discussed in terms of the presence of an induced diamagnetic ring current in the latter systems.