NMR studies of sulphur heterocycles. II—substituent effects in the thieno[2,3-b] [1]benzothiophene and thieno[3,2-b] [1]benzothiophene systems

Substituent chemical shifts for the title systems have been determined at infinite dilution in CDCl₃ from a study of 40 derivatives. An excellent correlation is found between the ortho effect of substituents in the thiophene ring and the corresponding effects in thiophene itself, indicating negligible perturbation by the fused rings. Long range effects of 2- and 3-substituents on the 5- and 6-protons are electronic in origin, but cannot be interpreted in terms of simple resonance theory. These long range effects correlate linearly with σ_p°. An O, S-cis conformation for the formyl group in 2-formyl-thieno2,3-6]1]benzothiophene is adduced on the basis of chemical shifts.