REVIEW ON THE METALLATION OF ?-DEFICIENT HETEROAROMATIC COMPOUNDS; REGIOSELECTIVE ORTHO-LITHIATION OF 3-FLUOROPYRIDINE: DIRECTING EFFECTS AND APPLICATION TO SYNTHESIS OF 2,3- OR 3,4-DISUBSTITUTED PYRIDINES

Article abstract of DOI:10.1016/S0040-4020(01)91919-2

Metallation of ?-deficient heterocyclic compounds is first reviewed, which shows the important recent developments in this research area.A particular aspect of this reaction is then given with the study of the 3-fluoropyridine metallation regioselectivity.Lithiation of 3-fluoropyridine is chemoselective at low temperatures using butyllithium-polyamine chelates or lithium diisopropylamide.Protophilic attack by these strong bases can be directed either at the 2- or 4-position depending on the lithiation conditions.Various reaction parameters are thus studied such as solvent, temperature, reaction time, lithium-chelating agent as well as metallating agent.The high regioselectivity of 3-fluoropyridine lithiation is theoretically discused, in particular in terms of kinetic of thermodynamic control of the metallation.Chelation between butyllithium and 3-fluoropyridine is proposed, which completely modifies the heterocycle reactivity toward the lithiating agent.This is confirmed by theoretical quantum calculations performed on different models of 3-fluoropyridine using the CNDO/2.These results allow to select the best 3-fluoropyridine-metallation conditions which lead to 3-fluoro-2-lithiopyridine on the one hand and to 3-fluoro-4-lithiopyridine on the other hand.Each of the lithiated isomers is then reacted with a great variety of electrophiles which gives very conveniently the corresponding 2,3- or 3,4-disubstituted pyridines.