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. To B or not to B? Reactions of the anti‐aromatic pentaphenylborole with diazenes furnished a range of unique boracycles with the outcome influenced by whether the trans or cis diazene isomer is reacted.