Pd-Catalyzed C–H Annulation of Five-Membered Heteroaryl Halides with Norbornene Derivatives

Complementary to Catellani-type reactions and 1:1 coupling of six-membered halo­(hetero)­arenes and norbornene (NBE) derivatives, Pd-catalyzed 1:2 coupling of five-membered haloheteroarenes with NBEs was achieved to afford rigid nonplanar heterocycles. Pyrazole, thiophene, furan, and indole underwent exo- and trans-selective annulation. Two strained alkene groups of the resulting products were further manipulated to afford 1-alkylindazoles and ladder polymers. The type of heteroarenes and position of halides along with the choice of ligands and bases were critical to set a preference between C–H annulation and Catellani reactions, which will be useful for the development of Pd-catalyzed, NBE-mediated reactions of heteroarenes.