Polyfluorobipyridyls

Abstract

The chlorination of pyridine and 2-methylpyridine, using phosphorus pentachloride has been developed to give good yields of pentachloropyridine. The reaction between pentachloropyridine and potassium fluoride gave high yields of pentafluoro-, 3-ohlorotetra-fluoro-, and 3,5-dichlorotrifluoro-pyridines. Octachloro-2,2'- bipyridyl, obtained from the reaction between 2,2'-bipyridyl and phosphorus pentachloride has been reacted with potassium fluoride affording octafluoro-, 3-chloroheptafluoro-, 3,3'-dichlorohexa- fluoro-, and 3,3',5,5'-tetrachlorotetrafluoro-2,2'-bipyrldyls. Octafluoro-3,3'-, and octafluoro-4,4'-bipyridyls have been synthesized from 3-chloro-, and 4-bromo-tetrafluoropyridines respectively, using the Ullmann technique. Octafluoro-4,4’-bipyridyl has also been prepared from the reaction between 2,3,5,6-tetrafluoropyridyl- magnesium bromide and pentafluoropyridine. Under similar conditions, 3-chloroheptafluoro-, and 3,5-dichlorohexafluoro-'4, 4'-bipyridyls were obtained from the reaction between the Grignard reagent and 3-chloro-tetrafluoro-, and 3,5-dichlorotrifluoro-pyridines respectively. The polyfluorobipyridyls have been reacted with several nucleophilic reagents and the orientations of the products determined from nuclear magnetic resonance spectregraphic measurements. The reaction between octafluoro-, 3-chloroheptafluoro-, and 3,3'- dichlorohexafluoro-2,2'-bipyridyls and sodium methoxide in methanol afforded the mono- and di-ethers, the fluorine atom para to the ring nitrogen being replaced in each case. The reaction between octafluoro-3,3'-bipyridyl and nucleophilic reagents led to replacement of the fluorine atoms ortho (6), and para (4) to the ring nitrogen. With the nucleophile, X = OCH(_3), in methanol, substitution at the 4-position (>95%) occurred when equi-raolar amounts of octafluoro-3,3'-bipyridyl and sodium methoxide were used. When a 2:1 molar ratio of sodium methoxide to the bipyridyl was used, an equi-molar mixture of 4,4'-, and 4,6'-dimethoxyhexafluoro-3,3'- bipyridyls was obtained. When X = OC(_2)H(_5), n-OC(_4)H(_7), n-OC(_4)H(_9) , i-OC(_3)H(_7) and t-OC(_4)H(9) replacement of both the 4- and 6-fluorine atoms took place, the amount of 4-substitution decreasing with a corresponding increase in 6-substitution, as the size of the nucleophilic reagent increased. When X = OCH(_3), in t-C(_4)H(_9)OH, replacement of the 6-fluorine atom (>80%) occurred and when X = NH(_3) and CH(_3)Li, substitution at the 6-position took place. In the reaction between octafluoro-3,3'-bipyridyl and sodium i-propoxide, an increase in the extent of replacement of the 6-fluorine atom occurred when ether was added to the reaction medium. This solvent effect has also been demonstrated in nucleophilic substitution in 3,5-dichlorotrifluoropyridine. An attempt has been made to rationalize the orientations by consideration of steric and electrostatic interactions, and the solvation of the transition state. Nucleophilic substitution in octafluoro-4.,4-bipyridyl led to replacement of the fluorine atoms ortho to the ring nitrogen. Nucleophilic substitution in 3-chloroheptafluoro-'-4-,4'-bipyridyl takes place at the 6-position ( >95%).