Isomer Dependence on the Reactivity of Diazenes with Pentaphenylborole.

Reactions of the anti-aromatic pentaphenylborole with diazenes indicate that both the substitution and the isomer influence the reaction outcome. With respect to trans isomers, azobenzene underwent coordination and C-H addition across the diene of the borole, and 2',6'-dimethylazobenzene furnished a fused tricyclic system. Under photolytic conditions, both of the aforementioned diazenes generate the first 1,3,2-diazaborepin heterocycles, rationalized through reactivity with the cis isomers. This notion is corroborated by the reaction of pentaphenylborole with benzo[c]cinnoline, the tethered variant of azobenzene, that only exists as the cis conformer as the corresponding 1,3,2-diazaborepin was produced regardless if the reaction is conducted in the dark or light. The more aromatic pyridazine proved to be less reactive, forming a resilient adduct.