Formation of BN Isosteres of Azo Dyes by Ring Expansion of Boroles with Azides^†

Herein, we present the results of our investigations on the effect of ortho substitution of aryl azides on the ring-expansion reaction of boroles, five-membered unsaturated boron heterocycles. These studies led to the isolation of the first 1,2-azaborinine-substituted azo dyes, which are bright yellow solids. One of the derivatives, (E)-2-mesityl-1-(mesityldiazenyl)-3,4,5,6-tetraphenyl-1,2-azaborinine, was found to be unstable in solution and to transform through a Jacobsen-like reaction into an indazole and 1-hydro-1,2-azaborinine. DFT calculations were performed to shed light on possible mechanisms to rationalize the unexpected azo-azaborinine formation and to draw conclusions about the role played by the ortho substituents in the reaction.