Antiaromaticity to Aromaticity: From Boroles to 1,2-Azaborinines by Ring Expansion with Azides

We have exploited the reactivity of antiaromatic boroles, gaining access to aryl‐substituted monocyclic 1,2‐azaborinines. The observed ring‐expansion reaction of inherently electron‐deficient boroles with organometallic and organic azides is demonstrated for representative examples. This substance class is expected to provide a new avenue into 1,2‐azaborinine chemistry, especially in the area of functional organoboron materials. Our results are based on NMR and UV/Vis spectroscopy as well as single‐crystal X‐ray crystallography and provide a virtually quantitative approach that also offers numerous points of variation. Monocyclic 1,2‐azaborinines can be synthesized through ring expansion of boroles with azides (see figure). The highly substituted heterocycles were characterized by multinuclear NMR measurements, single‐crystal X‐ray crystallography, and UV/Vis spectroscopy. This class of molecules is expected to add fresh impetus to the study of azaborinine chemistry, especially in the area of functional organoboron materials.