Experimental and Theoretical Study of Orientation in the Nitration of Dithienopyridine

Article abstract of DOI:10.1021/jo00004a046

The nitration of dithieno<3,4-b:3',4'-d>pyridine was investigated.Despite the similarity of the electrophilic reaction centers, high positional selectivity was found.Thus, the 1-, 8-, and 3-nitro isomers were obtained in the relative amounts 78percent, 20percent, and 2percent, respectively.In spite of the unusually large size of the molecular systems, the structure and electronic properties of the Wheland intermediates (sigma complexes) and the transition states were calculated at the ab initio 3-21 G* level.Harmonic frequencies have been calculated to characterize the stationary points obtained.The Wheland intermediate model failed to predict the site preferences, probably due to steric hindrance.However, the energies found for the transition-state complexes are in good agreement with the experimental findings.The transition-state geometry of the ring system shows greater similarity to the parent structure than to the Wheland intermediate.Orientation effects could be explained by the electron distribution of the transition-state structures.