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, 1JCC and 1JCH coupling constants and 19F NMR chemical shifts have been measured. In all cases, a dramatic increase of the 1JC2C3 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 1JC3C4 measured in 3-fluoro derivatives. T 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 19F NMR shieldings are well reflected in the calculated DFT data.