Acetylacetonato(phosphane)iridium Complexes: Synthesis and Catalytic Activity in the Cyclization of Alkynoic Acids

The iridium complex [Ir(acac)(eta(2)-coe)(2)] (1) has been easily prepared by the addition of Tl(acac) to a suspension of [Ir(mu-Cl)(eta(2)-coe)(2)](2) in hexane. Complex 1 has been characterized fully including an X-ray diffraction study. It is moderately stable and is a useful precursor to a number of different acetylacetonato(phosphane)iridium complexes via addition of monodentate and bidentate phosphanes. Reactions proceeded at room temperature to give high yields of the desired square planar phosphane complexes [Ir(acac)(P)] (2: P = 2 PPh3, 3: P = 2 PMePh2, 4: P = dppm, 5: P = dppe, 6: P = dppp, 7: P = dppb, 8: P = dppf, 9: P = dcpe) except for those involving dppe, which gave a complicated mixture of products. Complex [Ir(acac)(dppe)] (5) was prepared in a microwave reactor at 125 degrees C. All complexes prepared in this study were efficient in the intramolecular catalyzed cyclization of 4-pentynoic acid to give exclusive formation of the exocyclic gamma-methylene-gamma-butyrolactone. Complex 1 was found to be the most active and selective catalyst precursor for the intramolecular cyclization of both 4-pentynoic acid and 5-hexynoic acid.