Soft Fluorination of 4‑Alkylpyridines

We
report the mild and selective mono- and difluorination of 4-alkylpyridines.
The process involves soft-dearomatization of pyridines to the corresponding
alkylidene dihydropyridines and treatment with Selectfluor. The reaction
tolerates a broad range of functional groups, including those bearing
acidic and weak C–H bonds. In addition, selective fluorination
of 4-alkylpyridines attached to 2-alkylpyridines and 2-alkylpyrimidines
can be achieved in good yields, but a 4-alkylpyridine tethered to
a 4-alkylpyrimidine is fluorinated at both heterobenzylic positions.