Isolation and Reactivity of a BN-Embedded Heterocumulene

The incorporation of B═N units into cumulenes offers a promising strategy to modulate their electronic structures and chemical properties, yet free hybrid heterocumulenes containing both C═C and B═N bonds remain elusive. Herein, we report the synthesis of a hybrid [4]heterocumulene achieved through the strategic use of an N-heterocyclic-substituted diazoalkene and an N-heterocyclic imino-substituted borane as precursors. Single-crystal X-ray diffraction analysis and DFT calculations unequivocally confirm that the BN-embedded heterocumulene features a bent C═C═B═N═C framework, exhibiting exceptional π-delocalization along the cumulenic backbone. Notably, this hybrid system exhibits remarkable reactivity toward unsaturated substrates, including isocyanide, carbon dioxide, and organic azide, leading to the selective formation of three-, four-, and five-membered boron-containing heterocycles. These transformations highlight the unique electronic properties imparted by the BN incorporation and underscore the potential of such heterocumulenes as versatile synthons in boron chemistry.