A Nitrilium-Type N-Heterocyclic Aryne

The first solution-phase synthesis and reactivity of a nitrilium-type N-hetaryne are described. The 1,2-azaborine-derived 1,6-BN-aryne 2 exhibits [4 + 2], [3 + 2], and [2 + 2] cycloadditions and electrophilic aromatic substitution (EAS) reactivity. The observed regio- and diastereoselectivities of the cycloaddition and EAS products are consistent with a polarized aryne/nitrilium species. The free 1,6-BN-aryne 2 was isolated and characterized under matrix isolation conditions. A Lewis structure description where the 1,6-BN-aryne 2 resonates between two limiting (ketenimine (dominant) vs nitrilium) forms is consistent with DFT calculations. New 1,2-azaborine structures that are functionalized at the C6- and N-positions, including highly strained derivatives that were previously not accessible, can now be accessed using 1,6-BN-aryne 2 as a versatile synthetic building block.