Reductive coupling of carbon monoxide with alkynes leading to a novel case of carbon-oxygen bond formation. The first examples of 3-metallafuran complexes

Article abstract of DOI:10.1021/om00016a028

The edge-sharing bioctahedral dirhenium complexes Re₂(μ-X)(μ-CO)X₃(L)(μ-dppm)₂ (X = Cl when L = CO, xylNC; X = Br when L = CO; dppm = Ph₂PCH₂PPh₂) react with terminal alkynes RCCH (R = H, n-Pr, n-Bu, Ph, p-tol) at room temperature in the presence of TlPF₆ to afford the novel series of diamagnetic complexes [Re₂(μ-X)(μ-COC(R)CH)X₂(L)(μ-dppm) ₂]-PF₆ in which the reductive coupling of a bridging CO ligand and the alkyne leads to carbon-oxygen bond formation and the generation of a 3-metallafuran ring. The reactions are regioselective to give the R substituent in the 5-position. These complexes were found to undergo anion-exchange reactions when L = xylNC to afford the analogous BF₄^- and BPh₄^- salts. They can be reduced to their neutral congeners Re₂(μ-X)(μ-COC(R)CH)(L)(μ-dppm)₂ by cobaltocene. In the case of the reaction between Re₂(μ-Cl)(μ-CO)Cl₃(CNxyl)(μ-dppm)₂ and acetylene, the reaction intermediate [Re₂Cl₃(CO)(η²-HCCH)(CNxyl)(μ-dppm) ₂]PF₆ has been prepared and characterized spectroscopically. A combination of NMR spectroscopy and representative single-crystal X-ray structure determination has been used to confirm the structures of the coupled species and the connectivity within the 3-metallafuran rings. The identities of [Re₂(μ-Cl)(μ-COCHCH)Cl₂(CO)(μ-dppm) ₂]PF₆ (3a) and [Re₂(μ-Cl)(μ-COC(p-tol)CH)Cl₂(CNxyl)(μ-dppm) ₂]BF₄ (5h) have been established by X-ray crystallography. Crystal data for 3a: space group Cc (No. 9) with a = 19.396(2) A?, b = 15.495(2) A?, c = 20.001(7) A?, β = 99.66(2)°, V = 5926(4) A?³, and Z = 4. The structure was refined to R = 0.025 (R_w = 0.031) for 3390 data with I > 3.0σ(I). Crystal data for 5h: space group C2/c (No. 15) with a = 37.402(8) A?, b = 15.302(4) A?, c = 28.578(8) A?, β = 119.06(3)°, V = 14 296(15) A?³, and Z = 8. The structure was refined to R = 0.066 (R_w = 0.085) for 5158 data with I > 3.0σ(I). In these structures the Re-Re distances are very similar (2.565(1) A? for 3a and 2.558(1) A? for 5h).