1 H NMR spectra of the 2‐trifluoroacetyl derivatives of benzo[ b ]furan and benzo[ b ]thiophene

The 1 H NMR spectra of the 2‐trifluoroacetyl derivatives of benzo[ b ]furan and benzo[ b ]thiophene were recorded at 200MHz in two solvents, chloroform and acetone. A long‐range coupling constant, 5 J (HF), between the fluorine nuclei of the trifluoroacetyl group and H‐3, of a value higher than 1 Hz, was measured. From the comparison of the 1 H chemical shifts of, and the solvent effects on, the trifluoroacetyl compounds and those of the corresponding 2‐acetyl derivatives, and on the basis of an empirical interpretation of the 5 J (HF) coupling constant, a predominant Z conformation was tentatively assigned to these derivatives.