The Fluorination of Simple Five-Membered Heteroaromatic Compounds with Elemental Fluorine

The reaction of elemental fluorine with five-membered heteroaromatics under controlled conditions proceeds cleanly, yielding major substitution products in the case of thiophene (1) and N-methylpyrrole (2) or 1,4-addition products in the case of furan (3), while pyrrole (4) gives only tars. The site of fluorine substitution on N-methylpyrrole remarkably favours the beta-carbon atoms by a factor of ca. 3 over the alpha-carbons. The alpha/beta substitution ratio for 1 (1.9) and 2 (0.28) and the intermolecular reactivity order N-methylpyrrole > thiophene > benzene agrees with a polar electrophilic substitution mechanism. Also the predominantly syn 1,4-addition to furan conforms to the polar syn addition of F2 to double bonds.