Cyclization of 1-(2-Alkynylphenyl)-3,3-dialkyltriazenes:  A Convenient, High-Yield Synthesis of Substituted Cinnolines and Isoindazoles

A new route to isoindazoles and cinnolines through the cyclization of (2-alkynylphenyl)triazenes under neutral conditions is presented. The products that result from heating the starting triazenes depend on both the type of alkyne ortho to the triazene functionality and the temperature used. Butadiyne moieties ortho to dialkyltriazenes yield bis-isoindazole dimers when heated to 150 °C in MeI. A requirement for cyclization in MeI is that the (2-alkynylphenyl)triazene must contain a suitably electron-withdrawing substituent on the phenyl ring to deactivate the triazene toward methylation-induced decomposition to an iodoarene. Ethynyl moieties ortho to dialkyltriazenes yield both isoindazole dimers as well as 3-formylisoindazoles when subjected to the same conditions. Replacing MeI with 1,2-dichlorobenzene as solvent allows for the general cyclization of (2-ethynylphenyl)dialkyltriazenes. Heating to 170 °C results in a mixture of isoindazole and cinnoline products, whereas the cinnolines are produced exclusively in high yield at 200 °C. Alternatively, the isoindazoles can be obtained in good to excellent yield by stirring a 1,2-dichloroethane solution of the starting triazene with CuCl overnight at 50 °C.