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.