Palladium-Catalyzed C-H Benzannulation of Functionalized Furans and Pyrroles with Alkynes

A benzannulation strategy involving activation of two C-H bonds of five-membered heteroarenes was developed. Readily available furans and pyrroles stabilized by synthetically useful electron-withdrawing groups underwent Pd-catalyzed 1:2 annulation reactions with diaryl alkynes. A variety of functional groups, including ester, amide, ketone, aldehyde, and nitrile, on the heterocyclic cores were tolerated in the Pd-catalyzed oxidative reactions. In these reactions, the combination of 2,2-dimethylbutyric acid and its conjugate base facilitated metalation at the heteroaromatic rings and reoxidation of the Pd(0) species using oxygen as the terminal oxidant. This strategy provides fluorescent benzofuran and indole derivatives and is expected to allow for further development of functionalized polycyclic heteroaromatic compounds.