Abstract
The synthetic scope of asymmetric 5′,6-disubstituted-2,2′-bipyridines in the preparation
of flexible multitopic ligands has been investigated. The preparation of the pivotal
6-bromo-5′-bromomethyl-2,2′-bipyridine building block involved a Kröhnke protocol
followed by a radical bromination reaction which has been optimized. Functionalization
of 6-bromo-5′-bromomethyl-2,2′-bipyridine has been realized by both a Delepine and
a Gabriel reaction, providing after hydrolysis the corresponding amino compound, whereas
a nucleophilic attack of acetate followed by a saponification reaction provided the
corresponding alcohol. Multifunctionalized bipyridine ligands have been prepared in
one step from 6-bromo-5′-bromomethyl-2,2′-bipyridine and acyclic amines or primary
alcohol via base-assisted nucleophilic substitutions. In all cases, transformation
of the bromo-function to the corresponding ethyl esters has been made possible under
mild conditions using a palladium promoted carboethoxylation reaction, while saponification
under basic conditions provided the corresponding acids after protonation. The utility
of the herein reported protocols has been extended to the synthesis of an ethylenediamine
platform bearing an appended 4-nitrobenzyl group for potential linking to other molecules.
Key words
bromination - pyridines - bipyridines - ligands - radical reaction - nucleophilic
additions
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