869306-31-0Relevant academic research and scientific papers
Unprecedented coordination modes and demetalation pathways for unbridged polyenyl ligands. Ruthenium η1η4-cycloheptadienyl complexes from allyl/alkyne cycloaddition
Older, Christina M.,McDonald, Robert,Stryker, Jeffrey M.
, p. 14202 - 14203 (2005)
Cationic (η6-hexamethylbenzene)ruthenium(II) mediates the [3 + 2 + 2] cycloaddition of allyl and alkyne ligands, leading to the unexpected isolation of η1,η4-cycloheptadienyl complexes, an unprecedented coordination mode for transition metal complexes of simple organic rings. The nonconjugated, η1,η4-coordinated complex is obtained as the kinetic reaction product from treatment of the unsubstituted allyl complex with excess ethyne; this complex rearranges slowly at 80 °C to the thermodynamically more stable conjugated η5-cycloheptadienyl isomer. The η1,η4-coordinated isomer is fluxional at room temperature, undergoing rapid and reversible equilibration with a cycloheptatriene hydride intermediate via facile β-hydride elimination/reinsertion. The reinsertion process is remarkably regioselective, returning the nonconjugated η1,η4-cycloheptadienyl isomer exclusively at room temperature. For reactions incorporating dimethylacetylene dicarboxylate (DMAD) as one or both of the alkyne components, η1,η4-coordination appears to be both kinetically and thermodynamically favored, despite undergoing equilibration among all possible η1,η4-cycloheptadienyl and cycloheptatriene hydride isomers prior to arriving at one observed η1,η4-isomer. For this series, no isomerization to η5-coordination is observed even upon prolonged heating. In contrast, the cyclization incorporating both DMAD and phenylacetylene proceeds directly to the η5-cycloheptadienyl isomer at or below room temperature, indicating that η5-coordination remains energetically accessible to this system. The DMAD-based cyclization reactions produce structurally diverse minor byproducts, including both η1,η4-methanocyclohexadiene and acyclic η3,η2-heptadienyl isomers, which have been isolated and rigorously characterized. The unusual η1,η4-coordination of the seven-membered ring leads to unique new organic products upon oxidative demetalation by iodinolysis. Thus, reactions with excess iodine afford bridged tricyclic cyclopropane-containing lactones or substituted cycloheptatrienes in good but sometimes variable yields, depending on the substrate and specific reaction conditions. The ruthenium in these reactions is returned in high yield as the interesting cationic μ-triiodo pseudodimer of (η6-hexamethylbenzene)ruthenium, which is obtained as a triiodide salt. This Ru(III) complex, along with several representative Ru(II) cyclization products, has been characterized in the solid state by X-ray crystallography. Copyright
