Nine-Membered Osmacycles Derived from Metathesis Reactions between Alkynes and an Osmafuran

Ring-expansion reactions of the five-membered osmafuran Os{CHC(PPh3)C(O) OEt}Cl2(PPh3)2 (1) via alkyne insertion have been investigated, which lead to the formation of several nine-membered osmacycles. Reaction of 1 with PhCCH gives the nine-membered complexes Os{CPhCHCPh-η2-CHCHC(PPh3)C(O)OEt}Cl2(PPh3) as a mixture of a couple of isomers 2a and 2b with different disposition of the two chloride ligands on the metal centers. The reaction involves a head-to-tail double insertion of PhCCH into osmacycle 1 via [2 + 2] cycloaddition process, which is relevant to the polymerization of alkynes by metathesis reaction. Treatment of the mixture of 2a and 2b with PMe3 gives selectively Os{CPhCHCPh-η2-CHCHC(PPh3)C(O)OEt}Cl2(PMe3) (3), the PMe3-substituted counterpart of 2a. These complexes might serve as the intermediates for alkyne polymerization which are stabilized by the coordination of internal olefin. Heating the mixture of 2a and 2b in CHCl3 under reflux gives complex Os{η2-CHPhCHCPhCHCC(PPh3)C(O)OEt}Cl2(PPh3) (4) by an intramolecular hydrogen shift. In addition, reaction of 1 with HCCCH(OH)Ph affords [Os{OCPhCH2-η2-CHCHC(PPh3)C(O)OEt}Cl(PPh3)2]Cl (5) as a monoinsertion product, which can dissociate a phosphine ligand under reflux in CH2Cl2 to give complex Os{OCPhCH2-η2-CHCHC(PPh3)C(O)OEt}Cl2(PPh3) (6). The one of the α-H of the carbonyl group in complex 5 can be deprotonated by NEt3 to give an η3-allylic structure Os[η3-CH{CPh(O)}CHCHC(PPh3)C(O)OEt]Cl(PPh3)2 (7). Treatment of 7 in CHCl3 with 1 equiv of HCl regenerates 5. All of the complexes can be prepared under mild condition in good yield. Moreover, these reactions provide convenient and efficient routes to synthesize the nine-membered osmacycles. Complexes 1, 3, 4, 5′, 6, and 7 have been characterized by X-ray diffraction analysis.