Experimental Observation of a Terminal Borylene‐Dinitrogen Adduct via Cleavage of a 1,2,3,4,5‐Diboratriazoline

While azides do not react with simple alkenes except under harsh conditions, a diboron alkene analogue, the doubly cyclic alkyl(amino)carbene (CAAC)‐stabilized dicyanodiborene 1, reacts spontaneously with organic azides (7‐10 equiv.) at room temperature to yield two equivalents of stable CAAC‐stabilized imino(cyano)boranes (2‐R). NMR‐spectroscopic monitoring of the reaction mixtures shows the initial formation of a 1:1 mixture of 2‐R and a relatively long‐lived intermediate (Int), which in the presence of excess azide is converted into a second equivalent of 2‐R. In the absence of excess azide, however, Int decomposes to 3, the product of an intramolecular C‒H activation by a putative dicoordinate borylene intermediate “(CAAC)B(CN)”. Mechanistic insights from trapping experiments, NMR‐spectroscopic and high‐resolution mass spectrometry data, as well as DFT computations reveal that Int is the terminal borylene end‐on‐dinitrogen adduct [(CAAC)B(CN)(η1‐N2)]. The formation of the iminoboranes 2‐R from diborene 1 and RN3 proceeds via an azide‐diborene Huisgen‐type [3+2] cycloaddition reaction, followed by a retro‐[3+2] cycloaddition, yielding 2‐R and [(CAAC)B(CN)(η1‐N2)]. The latter then undergoes either N2 extrusion and intramolecular C‒H activationto generate 3, or a Staudinger‐type reaction with a second equivalent of azide to generate a second equivalent of the iminoborane 2‐R.