POLYHALOGENATED HETEROCYCLIC-COMPOUNDS - PART 42 - FLUORINATED NITROGEN-HETEROCYCLES WITH UNUSUAL SUBSTITUTION PATTERNS

Reductive replacement of fluorine in 3,5-dichloro-2,4,6-trifluoropyrid ine 2a and pentafluoropyridine 2 using DIBAL and LAH leads to useful s yntheses 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-difluoropyri dine 3. Bromofluoro-pyrimidine and -quinoxaline derivatives have also been efficiently synthesised from perfluorinated precursors. Catalytic hydrogenation of bromofluoro heterocycles gives high yields of the co rresponding fluorinated heterocycles. Reactions of nucleophiles with 3 gives surprising results. Soft nucleophiles, e.g. PhSNa displace brom ine whereas hard nucleophiles, e.g. MeONa, displace fluorine.