Reaction of Pentafluoropyridine with Lithium Hydrazonides; Competing Monosubstitution at The 2- and 4-Positions

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Journal Title, Volume, Page: 
Journal of Fluorine Chemistry, Volume 77, Issue 1, March 1996, Pages 87-92
Year of Publication: 
1996
Authors: 
Dr. Waheed J. Jondi
Current Affiliation: 
Department of Chemistry, Faculty of Science, An-Najah National University, Nablus, Palestine
Preferred Abstract (Original): 

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 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).