Theoretical study on reaction mechanism of an N-heterocyclic carbene boryl azide with electron-deficient alkynes and nitriles

A theoretical investigation of the [3+2] cycloaddition reaction of the NHC-boryl azide with alkynes and nitriles has been presented by using the DFT (B3LYP) method. Solvent effects on these reactions have been explored by calculation that included a polarizable continuum model (PCM) for the solvent (C6H6). The title reaction could produce two different five-membered products (1,4-regioisomer and 1,5-regioisomer). The reaction pathway involves a one-step mechanism through a [3+2] addition where two nitrogen atoms of the N-heterocyclic carbene boryl azide adds to the C≡A (A=C or N) bond to form two new C–N or N–N bonds. For alkynes, the reactions can take place more easily to give 1,4-regioisomer product, while the reactions proceed for nitriles along the 1,5-regioisomer pathway. The reaction systems have high chemical reactivity with low barriers and could be favored. The calculations indicated that the cycloaddition reaction of alkynes and nitriles has the better regioselectivity. Our computational results are good consistent with the experimental observations of Merling and co-workers for [3+2]-dipolar cycloaddition reaction of N-heterocyclic carbene boryl azide.