Synthesis and NMR studies of 2-and 3-fluorosubstitued five-membered heterocycles

A set of 2-fluoro- and 3-fluoro-substituted thiophenes, pyrroles and furans has been synthesized by a treatment of the corresponding lithio derivatives with N-fluorodibenzenesulfonamide. For all these compounds, (1)J(CC) and (1)J(CH) coupling constants and F-19 NMR chemical shifts have been measured. In all cases, a dramatic increase of the (1)J(C2C3) couplings has been observed in 2-fluoro- and 3-fluoro-substituted compounds in comparison with those measured for the parent compounds. The same is valid for (1)JC(3)C(4) measured in 3-fluoro derivatives. The DFF calculations performed for 2- and 3-fluoro-substituted compounds reproduce very well the experimental coupling values and show that the Fermi contact contribution is the main factor determining their magnitude. Also the trends observed in the F-19 NMR shieldings are well reflected in the calculated DFF data. (C) 2003 Elsevier B.V. All rights reserved.