Metal-free regioselective carboboration of arylnitriles with L2PhB: (1: L=oxazol-2-ylidene) catalyzed by Et3B afforded the unprecedented acyclic 2-aza-4-borabutadienes 2, thus demonstrating a new strategy to construct a B,C,N-mixed π-system involving B=C and C=N bonds. Thermal isomerization of 2 gave C-borylimines (3), and diverse reactivity of 2 a towards several substrates, such as H+, F+, O2, S, Se, and isonitriles, allowed construction of boron-containing heterocycles with various ring sizes, thus illustrating the utility of 2 as a synthetic building block. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim