Divergent Strategies for the π-Extension of Heteroaryl Halides Using Norbornadiene as an Acetylene Synthon.

Pd-catalyzed multicomponent coupling reactions of five-membered heteroaryl halides and norbornadiene (NBD) were developed. Either direct addition of (benzo)azoles or 2:1 annulation was achieved depending on the propensity of the intermediate complex to undergo palladacycle formation, determined by the nature and substitution pattern of the heteroarene. The obtained exo- and cis-diheteroaryl norbornenes underwent epimerization and retro-Diels-Alder reactions to afford the corresponding trans-isomers and π-extended heteroaromatic systems, respectively, demonstrating the versatility of NBD as an acetylene synthon.