Reaction of pentafluoropyridine with lithium hydrazonides; Competing monosubstitution at the 2- and 4-positions

Treatment of pentafluoropyridine (1) in diethyl ether with approximately 1 M equiv. of the hydrazonides Ph(2)C=NNHLi (3a) and Ph(2)C=NNLiPh (3b) under mild conditions gives good yields (62% and 83%) of 4- and 2-(Ph(2)C=NH)C5F4N (5a and 6a) and 4- and 2-(Ph(2)C=NPh)C5F4N (5b and 6b), respectively, containing unusually large amounts of 2-substituted products (5a/6a = 50:50; 5b/6b = 65:35). The increased ease of displacement of a 2-F substituent from 1 (--> 6a and 6b) in these cases is ascribed to chelation of the lithium cation in the transition state involved in the rate-determining step leading to formation of an orrho-quinonoidal sigma-complex. Catalytic hydrogenation of a 1:1 mixture of hydrazones 5a and 6a affords the corresponding hydrazines, 4- and 2-H2NNHC5F4N (7 and 8) in good yield (78%); acidic hydrolysis (hot HCl aq.) of the 5a/6a mixture yields tetrafluoro-4-hydrazinopyridine (9) and, depending on the conditions, tetrafluoro-2-hydrazinopyridine (10) or 2-aminotetrafluoropyridine (11).