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Iodine being the least acidifying of all halogens, proton abstraction from 2,6-difluoro-3-iodopyridine (1) occurred initially, i.e. upon 5\115 min of exposure to LIDA, almost exclusively at the 5- rather than the 4-position. Trapping with dry ice or iodine provided the 2,6-difluoro-5-iodopyridine-3-carboxylic acid (17; 72%) or the 2,6-difluoro-3,5-diiodopyridine (18; 81%) respectively. However, when the reaction time with LIDA was prolonged to 30 h, reversible return to the starting material 1 set the stage for a sporadic deprotonation at the 4-position. This unleashed instan-taneously a basicity gradient-driven heavy halogenmigration producing the isomeric 2,6-difluoro-3,4-diiodopyridine (19; 80%) after iodination.
The LIDA-promoted deprotonation of (2,6-difluoropyridin-3-yl)trimethylsilane (5) also took place at the 5-position. The intermediate was trapped with dry ice or 1,1,2-trichloro-1,2,2-trifluoroethane to give 2,6-difluoro-5-(trimethylsilyl)pyridine-3-carboxylic acid (20; 98%) or (5-chloro-2,6-difluoropyridin-3-yl)trimethylsilane (21; 97%)respectively. Using iodine chloride, the acid 20 was iododesilylated to the 2,6-difluoro-5-iodopyridine-3-carboxylicacid (17; 96%). Under the same conditions, the chlorosilane 21 gave 3-chloro-2,6-difluoro-5-iodopyridine (15; 93%). The latter compound was converted into the 5-chloro-2,6-difluoropyridine-3-carboxylic acid (22; 91%), by halogen/metal permutation followed by carboxylation.
Condensation of the 5-lithiated (2,6-difluoropyridin-3-yl)trimethylsilane (5) with chlorotrimethylsilane provided the (2,6-difluoropyridin-3,5-diyl)bis(trimethylsilane) (23; 94%)
which reacted with iodine chloride smoothly under double iododesilylation to afford the 2,6-difluoro-3,5-diiodopyridine (18; 93%) whereas the bromodesilylation effected with elemental bromine rigorously stopped after a unilateral displacement producing (5-bromo-2,6-difluoropyridin-3-yl)trimethylsilane (24; 95%). 3,5-Dibromo-2,6-difluoro-pyridine (25; 90%) was found to be accessible only indirectly by subjecting 3-bromo-2,6-difluoro-5-iodopyridylsi-lane (26) to element-specific halogen/metal permutation and subsequent bromination. The latter precursor 26 was readily prepared by iododesilylation of the bromopyridyl-silane 24 in 91% yield
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Iodine being the least acidifying of all halogens, proton abstraction from 2,6-difluoro-3-iodopyridine (1) occurred initially, i.e. upon 5\115 min of exposure to LIDA, almost exclusively at the 5- rather than the 4-position. Trapping with dry ice or iodine provided the 2,6-difluoro-5-iodopyridine-3-carboxylic acid (17; 72%) or the 2,6-difluoro-3,5-diiodopyridine (18; 81%) respectively. However, when the reaction time with LIDA was prolonged to 30 h, reversible return to the starting material 1 set the stage for a sporadic deprotonation at the 4-position. This unleashed instan-taneously a basicity gradient-driven heavy halogenmigration producing the isomeric 2,6-difluoro-3,4-diiodopyridine (19; 80%) after iodination.
The LIDA-promoted deprotonation of (2,6-difluoropyridin-3-yl)trimethylsilane (5) also took place at the 5-position. The intermediate was trapped with dry ice or 1,1,2-trichloro-1,2,2-trifluoroethane to give 2,6-difluoro-5-(trimethylsilyl)pyridine-3-carboxylic acid (20; 98%) or (5-chloro-2,6-difluoropyridin-3-yl)trimethylsilane (21; 97%)respectively. Using iodine chloride, the acid 20 was iododesilylated to the 2,6-difluoro-5-iodopyridine-3-carboxylicacid (17; 96%). Under the same conditions, the chlorosilane 21 gave 3-chloro-2,6-difluoro-5-iodopyridine (15; 93%). The latter compound was converted into the 5-chloro-2,6-difluoropyridine-3-carboxylic acid (22; 91%), by halogen/metal permutation followed by carboxylation.
Condensation of the 5-lithiated (2,6-difluoropyridin-3-yl)trimethylsilane (5) with chlorotrimethylsilane provided the (2,6-difluoropyridin-3,5-diyl)bis(trimethylsilane) (23; 94%)
which reacted with iodine chloride smoothly under double iododesilylation to afford the 2,6-difluoro-3,5-diiodopyridine (18; 93%) whereas the bromodesilylation effected with elemental bromine rigorously stopped after a unilateral displacement producing (5-bromo-2,6-difluoropyridin-3-yl)trimethylsilane (24; 95%). 3,5-Dibromo-2,6-difluoro-pyridine (25; 90%) was found to be accessible only indirectly by subjecting 3-bromo-2,6-difluoro-5-iodopyridylsi-lane (26) to element-specific halogen/metal permutation and subsequent bromination. The latter precursor 26 was readily prepared by iododesilylation of the bromopyridyl-silane 24 in 91% yield
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