Reactions of 1,2-Azaborinine, a BN-Benzyne, with Organic π Systems.

Ortho-benzyne and 1,2-azaborinine are related by the formal exchange of the CC triple bond by the isoelectronic BN unit. The (2 + 2) and (2 + 4) cycloaddition reactions of 1,2-azaborinine with the different organic π systems (ethene, ethyne, 1,3-butadiene, 1,3-cyclopentadiene, furan, benzene) were examined computationally using density functional, second-order perturbation, and coupled-cluster methods. All reactions of 1,2-azaborinine with the studied substrates are highly exothermic and involve the formation of Lewis acid-base complexes of 1,2-azaborinine and respective π systems. The interaction between the π bond of the substrates and the empty p orbital of the boron atom in these complexes is remarkably strong, resulting in two-step mechanisms for the (2 + 2) and (2 + 4) cycloaddition reactions. Cycloaddition reactions have lower barriers than CH insertion reactions, and (2 + 4) reactions are favored over (2 + 2) cycloadditions.