A strong hyperconjugative interaction was observed between the out-of-plane C 1 -H 1b sigma bond and the *C 2 C 3 antibonding orbital (E = 8.88 kcal/mol) which was stronger than C 1 -C Me →*C 2 C 3 interactions (about 6.00 kcal/mol) found in the other reactants analysed (See the Supporting Information). This strong interaction might explain why the primary azide 1i showed a lower preference for undergoing a sigmatropic rearrangement compared to secondary azides (1a-h). The results suggest that the conjugative effect plays a significant role in determining the reactivity of the reagents, however, other factors may also contribute. A good linear correlation was found between the E (2) C 2 C 3 →Zb and the free activation energies of the prototropic reaction (R 2 =0.84, Figure ).