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

The 1H 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, 5J(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 1H 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 5J(HF) coupling constant, a predominant Z conformation was tentatively assigned to these derivatives.