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Published on: June 10, 2021
Synthesis and deprotonation of 2-(halophenyl)pyridines and 1-(halophenyl)isoquinolines
Florence Mongin1, Anne-Sophie Rebstock, François Trécourt
1Laboratoire de Chimie Organique Fine et Hétérocyclique, UMR 6014, IRCOF, Place E. Blondel, BP 08, 76131 Mont-Saint-Aignan Cédex, France. florence.mongin@insa-rouen.fr
Abstract:
The ability of the 2-pyridyl and 1-isoquinolyl groups to direct metalation was studied in the benzene series. For this purpose, 2-(halophenyl)pyridines and 1-(halophenyl)isoquinolines were prepared. Interestingly, nucleophilic addition reactions on the azine ring were not observed under kinetic control using butyllithium, and the substrates were cleanly deprotonated on the benzene ring: the azine ring acidifies the adjacent hydrogen H2' (N-H2' interaction through space and/or inductive electron-withdrawing effect) and probably favors the approach of butyllithium (chelation). Under thermodynamic conditions using lithium dialkylamides, the presence of the azine group makes the lithio derivative at C2' more stable (chelation and/or inductive electron-withdrawing effect). This was evidenced in two ways: (1) syntheses of 2-halophenyllithiums (F, Cl, Br) substituted at C6 by a 2-pyridyl or 1-isoquinolyl group without elimination of lithium halide and (2) iodine migration from C2' to C4' when treating 2-(3-halo-2-iodophenyl)pyridines or 1-(3-fluoro-2-iodophenyl)isoquinoline with LTMP. Comparisons between the 2-pyridyl and fluoro units showed the latter was the stronger directing group for deprotonation.
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