Bridged Boranoanthracenes: Precursors for Free Oxoboranes through Aromatization-Driven Oxidative Extrusion

Abstract

We introduce a novel class of boranobornadiene derivatives, termed boranoanthracene, along with an in-depth study of their structures and reactivities. Using these versatile precursors, we propose a fundamentally novel mechanism for generating free oxoborane species. This pathway enables the formation of aminoxoborane species, which are rarely reported in the literature. The proposed mechanism unfolds via the coordination of an oxygen-Lewis base (dimethyl sulfoxide) to the boron center, triggering a fragmentation cascade propelled by oxidative aromatization. A detailed experimental analysis, NMR measurements, and DFT calculations provide a strong evidence supporting our findings. We explored three distinct reactivities of these species: first, the insertion of oxoborane species into B-C bonds, representing, to the best of our knowledge, the first example of this reactivity. Second, we demonstrated the [3 + 2] cycloaddition reaction of oxoboranes with nitrones, offering viable access to new boranoheterocycles. Third, we reported the first example of a [5 + 2] cycloaddition between oxoboranes and azomethine imines, leading to the formation of a seven-membered boracycle. The diverse reactivities and facile generation of aminoxoboranes highlight their immense potential as versatile tools in organic chemistry.