Theoretical study of orientation in the nitration of dithieno[2,3- b:3',2'- d] pyridine

Abstract

The nitration of dithieno[2,3- b:3',2'- d]pyridine was investigated. The structure and electronic properties of the Wheland intermediates (σ complexes) and the transition states (TSs) were calculated at the ab initio 3-21G ∗ level. Harmonic frequencies were calculated to characterize the stationary points obtained. The computational results suggest a joint TS for position 1 and 2 nitrations which can explain the similar reactivity of these positions. The TS energies obtained are in good agreement with the experimental regioselectivity between the two thiophene rings. The TS geometry of the ring system shows greater similarity to the parent structure than to the Wheland intermediate. The reaction profile, TS structures and electrostatic potential maps obtained are compared with those found for the more reactive dithieno[3,4- b:3,4'- d]pyridine. The TS energies are in good correlation with the experimentally measured relative nitration rates. The reaction profiles are in accord with Hammond's postulate. The difference in reactivity between the two ring systems can be explained by comparison of the electron distribution of the TS structures.