Synthesis of fluorenes via the palladium-catalyzed 5-exo-dig annulation of o-alkynylbiaryls

The direct palladium-catalyzed intramolecular hydro arylation of o-alkynylbiaryls proceeded in a highly stereoselective manner producing fluorenes 2, the products of 5-exo-dig cyclization, in excellent yields. The cascade intermolecular arylation, incorporated in this transformation, allowed for the efficient synthesis of fully substituted fluorenes 12. These cyclizations proceed more rapidly with electron-deficient benzene rings which, in combination with a substantial isotope effect observed, strongly supports a C-H activation mechanism for the key annulation step.