Nuclear Magnetic Resonance Studies of Bicyclic Thiophene Derivatives. I. Ring Current Effects of the Benzene Ring on the Hα and Hβ Signals of the Thiophene Ring in Benzoylthiophene, Thienopyrimidine and Thienodiazepine Derivatives

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Various thiophene derivatives such as 4-phenyIthienopyrimidines (1), 2-acetylamino-3-benzoylthiophenes (2), 5-phenyl-1,4-thienodiazepines (3) and 5-phenyl-1,4-thienodiazepine-4-oxides (4) have been prepared. Their proton magnetic resonance spectra show that Hα and Hβ of the thiophene ring become more shielded with an increase in bulk of ortho substituent of the benzene ring. This is attributed to the shielding effect of the benzene ring; a steric repulsion between ortho substituent of the benzene ring and some atoms on another moiety of each molecule. Such a repulsion should bring the benzene ring out of the plane of the thiophene ring. The degree of shielding on Hβ by the benzene ring is found to increase in the order 1<2<3<4. This can be taken to indicate that the amount of steric repulsion depends not only on the bulk of ortho substituent of the benzene ring but also on the geometry of each ring system of 1–4.
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