Polyhalogenated heterocyclic compounds. Part 42. Fluorinated nitrogen heterocycles with unusual substitution patterns

Reductive replacement of fluorine in 3,5-dichloro-2,4,6-trifluoropyridine 2a and pentafluoropyridine 2 using DIBAL and LAH leads to useful syntheses of fluoro and chlorofluoro pyridine derivatives. Replacement of fluorine by bromine using a mixture of HBr and AIBr(3) is extremely efficient, giving excellent routes to 2,4,6-tribromo-3,5-difluoropyridine 3. Bromofluoro-pyrimidine and -quinoxaline derivatives have also been efficiently synthesised from perfluorinated precursors. Catalytic hydrogenation of bromofluoro heterocycles gives high yields of the corresponding fluorinated heterocycles. Reactions of nucleophiles with 3 gives surprising results. Soft nucleophiles, e.g. PhSNa displace bromine whereas hard nucleophiles, e.g. MeONa, displace fluorine.