Isoxazole Strategy for the Synthesis of 2,2′-Bipyridine Ligands: Symmetrical and Unsymmetrical 6,6′-Binicotinates, 2,2′-Bipyridine-5-carboxylates, and Their Metal Complexes

An effective strategy
was developed for the synthesis of new 2,2′-bipyridine
ligands, symmetrical and unsymmetrical 6,6′-binicotinates,
and 2,2′-bipyridine-5-carboxylates, from 4-propargylisoxazoles.
The synthesis of symmetrical 2,2′-disubstituted 6,6′-binicotinates
was achieved using the Eglinton reaction of 5-methoxy-4-(prop-2-yn-1-yl)­isoxazoles
with Cu­(OAc)2, followed by Fe­(NTf2)2/Au­(NTf2)tBuXPhos-catalyzed isomerization
of the so-formed mixture of isoxazole/azirine-substituted biacetylenic
intermediates. Unsymmetrical 2,2′-disubstituted 6,6′-binicotinates
were prepared via a copper-free Sonogashira coupling of 5-methoxy-4-(prop-2-yn-1-yl)­isoxazoles
with 6-bromonicotinates to give methyl 6-(3-(5-methoxyisoxazol-4-yl)­prop-1-ynyl)­pyridine-3-carboxylates,
followed by a transformation of the propargylisoxazole moiety of the
adduct into the pyridine fragment under Fe­(II)/Au­(I) relay catalysis
conditions. 6-(Pyrid-2-yl)­nicotinates were synthesized by a Stille-type
coupling of 2-(tributylstannyl)­pyridine with 6-bromonicotinates. Several
cyclopalladated complexes of a new series of 6,6′-binicotinates
and 2,2′-bipyridine-5-carboxylates and the homoleptic Cu­(I)
complex were synthesized in high yields.