Boryl Azides in 1,3-Dipolar Cycloadditions

The 1,3-dipolar cycloaddition reaction of boron azides with alkynes has been investigated experimentally and computationally. At room temperature pinBN3 (pin = pinacolato) reacts with the strained triple bond of cyclooctyne with formation of an oligomeric boryl triazole. Alcoholysis of the oligomer yields the parent 4,5,6,7,8,9-hexahydro-2H-cyclooctatriazole, which could be characterized as a hydrate by X-ray crystallography. A computational analysis of the reaction of tri- and tetracoordinate boron azides R2BN3 (R = H, Me, pin, cat; cat = catecholato) and IMe·H2BN3 (IMe = 2,6-dimethylimidazole-2-ylidene) reveals significant differences in the reactivity depending on the coordination number: tricoordinate boron azides behave as type II 1,3-dipoles, while the tetracoordinate IMe·H2BN3 is an electron-rich 1,3-dipole (type I) that strongly prefers reactions with electron-poor alkynes.