Synthesis of Flexible Polydendate Ligands Bearing 5′-Substituted-6-carboxylic-2,2′-bipyridine Subunits

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.