Monomeric azoboranes.

Monomeric azoboranes, Mes2B-NN-R, were synthesized from silyldiazenes (R-NN-SiEt3) and Mes2BBr. These compounds feature an unusual, nearly linear cumulenic BNN framework reminiscent of a ketenimine, giving rise to complementary reactivity at the boron and nitrogen. Moreover, the monomeric azoboranes behave as BNN 1,3-dipoles, undergoing [3+2] cycloaddition with unsaturated substrates followed by mesityl migration to provide diverse BNN-containing heterocycles. This work establishes monomeric azoboranes as versatile ambiphilic building blocks for boron-nitrogen heterocycle synthesis.