Iridium-Catalyzed Annulative Coupling of 2‑Arylbenzoyl Chlorides with Alkynes: Selective Formation of Phenanthrene Derivatives

2-Arylbenzoyl
chlorides undergo annulative coupling with internal
alkynes in the presence of a catalyst system of [IrCl­(cod)]2/P­(t-Bu)3 to selectively afford the corresponding
phenanthrene derivatives accompanied by elimination of carbon monoxide
and hydrogen chloride. The reaction occurs without addition of any
external base. Deuterium-labeling experiments using 2-(d5-phenyl)­benzoyl chloride suggest that the rate-determining
step does not involve the C2′–H bond cleavage. Formation
of a [(t-Bu)3PH]­[(biphenyl-2,2′-diyl)­Ir­(CO)­Cl2] complex dimer, of which the structure was determined by
single-crystal X-ray analysis, from a stoichiometric reaction at 60
°C without addition of alkyne also supports the facile C–H
cleavage.