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.