A facile route to symmetrical and unsymmetrical cycloalkeno[c]fused 2,2′-bipyridine ligands via inverse electron demand Diels-Alder reaction of 5,5′-bi-1,2,4-triazines

A series of 6,6'-bisfunctionalized symmetrical cycloalkeno[c]fused 2,2'-bipyridines 4a-h or unsymmetrical cycloalkeno[c]fused 2,2'-bipyridines 6a-g is described. They were prepared by the Diels-Alder reaction of 3,3'-disubstituted 5,5'-bi-1,2,4-triazines 2a,b with cyclic enamines 3a-d. The regioselective reaction of I-vinylimidazole (3e) with 2a leads to 5-(2-pyridyl)-1,2,4-triazine 9 which undergoes Diels-Alder reaction with cyclic enamines 3a-d to give unsymmetrical 2,2'-bipyridines 10a-d with one cycloalkene ring attached.