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

Article abstract of DOI:10.1016/0022-1139(96)03394-5

Treatment of pentafluoropyridine (1) in diethyl ether with approximately 1 M equiv. of the hydrazonides Ph₂C=NNHLi (3a) and Ph₂C=NNLiPh (3b) under mild conditions gives good yields (62% and 83%) of 4- and 2-(Ph₂C=NH)C₅F₄N (5a and 6a) and 4- and 2-(Ph₂C=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 ortho-quinonoidal σ-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 HC1 aq.) of the 5a/6a mixture yields tetrafluoro-4-hydrazinopyridine (9) and, depending on the conditions, tetrafluoro-2-hydrazinopyridine (10) or 2-aminotetrafluoropyridine (11).