Insight on the Factors Controlling the Equilibrium of Allylic Azides

Several allylic azides with different double bond substitution were studied to understand the factors governing their equilibrium using density functional theory along with quantum theory of atoms in molecules, Non-covalent Interactions and Natural Bond Orbitals approaches. The results showed the hydroxyl group or heteroatoms in allylic azides interact with the molecule through an electrostatic weak interaction in each pair of regioisomers. The equilibrium shifts of substituted allylic azides, compared to non-substituted allylic azides, are not attributed to the presence of specific interactions, such as hydrogen bond. The observed equilibrium shifts stem mainly from the strengthening and weakening of negative hyperconjugative interactions, which is affected by the weak interaction involving the proximal substituent in each regioisomer. A good linear correlation was obtained between the hyperconjugative energies of pC=C→s*Z_b interactions and the calculated percentages of secondary azide and tertiary azides in the equilibrium mixture. Also, the effect of aromatic ring substituent was analysed using such approaches. This study not only provides insight into the factor controlling the stabilities of the substituted allylic azides, but also settle the basis to predict the regioisomer predominance in the equilibrium mixture.