Synthesis of Fluorenes via the Palladium-Catalyzed 5-Exo-dig Annulation of o-Alkynyl Biaryls.

The direct Pd-catalyzed intramolecular rapidly with electron-deficient benzene ring, which, in hydroarylation of o-alkynyl biaryls proceeded in highly combination with a substantial isotope effect observed, 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 efficient synthesis of fully-substituted fluorenes 12. These cyclizations proceed more rapidly with electron-deficient benzene ring, which, in combination with a substantial isotope effect observed, strongly supports a C-H activation mechanism for the key annulation step.