3-Fluoropyridyl nickel complexes as useful tools for the selective synthesis of new 2,4,5,6-tetrafluoropyridines: a route complementing the established methods to access fluorinated pyridines

Abstract

Treatment of [Ni(COD)₂] with 3-chlorotetrafluoropyridine in the presence of PEt₃ or PCy₃ effects the formation of the complexes trans-[NiCl(3-C₅NF₄)(PEt₃)₂] (1) and trans-[NiCl(3-C₅NF₄)(PCy₃)₂] (2), respectively. Reaction of 1 with MeLi gives trans-[NiMe(3-C₅NF₄)(PEt₃)₂] (3). Treatment of 3 with air yields 3-methyltetrafluoropyridine. The reaction of 1 with CO affords 1-(2,4,5,6-tetrafluoropyridin-3-yl)ethanone (6), which slowly converts into 1-(2,5,6-trifluoropyridin-3-yl)ethanone (7) in the presence of PEt₃. The structure of the complexes 1 and 3 have been determined by X-ray crystallography. The Ni–C distances to the pyridyl ligand are 1.894(1) and 1.936(2) Å, respectively. The Ni–C bond length to the methyl group in 3 is 1.991(3) Å. The studies reported in this paper demonstrate the synthesis of nickel derivatives of tetrafluoropyridine with the metal in the 3-position as well as the preparation of otherwise not accessible 3-substituted tetrafluoropyridines by C–C coupling reactions.