Unusual activation of 1-ethynyl-1-cyclohexanol by [RuCl(η5-C9H7)(PPh3)2]: synthesis and reactivity of the allenylidene derivative [Ru{CCC(C13H20)}(η5-C9H7)(PPh3)2][PF6]

Abstract

The reaction of [RuCl(η⁵-C₉H₇)(PPh₃)₂] with an excess of 1-ethynyl-1-cyclohexanol and NaPF₆ in refluxing methanol yielded the allenylidene complex [Ru{CCC(C₁₃H₂₀)}(η⁵-C₉H₇)(PPh₃)₂][PF₆] 1 via an unprecedented coupling of two molecules of the propargyl (prop-2-ynyl) alcohol derivative. Complex 1 can also be obtained by reaction of the vinylvinylidene derivative [Ru{CC(H)R}(η⁵-C₉H₇)(PPh₃)₂][PF₆] 2 (R = 1-cyclohexenyl) with 1-ethynyl-1-cyclohexanol or 1-ethynylcyclohexene in refluxing methanol. The behaviour of 2 towards other 1-alkyn-3-ols has been studied but only the replacement of the vinylidene moiety by the propargyl alcohols, via an η¹-vinylidene–η²-alkyne tautomerization process, to generate both vinylvinylidene [Ru{CC(H)R}(η⁵-C₉H₇)(PPh₃)₂][PF₆] (R = 1-cyclopentenyl, 1-cycloheptenyl or 1-cyclooctenyl) or allenylidene [Ru(CCCR₂)(η⁵-C₉H₇)(PPh₃)₂][PF₆] (R = Ph or R₂ = 2,2′-biphenyldiyl) complexes along with 1-ethynylcyclohexene was observed. A similar 1,3-enyne elimination also takes place in the reaction of 2 with phenylacetylene or acetonitrile to afford [Ru{CC(H)Ph}(η⁵-C₉H₇)(PPh₃)₂][PF₆] and [Ru(NCMe)(η⁵-C₉H₇)(PPh₃)₂][PF₆], respectively. On the basis of these observations a mechanism for the formation of 1 is proposed. The allenylidene complex 1 regioselectively reacts with NaR, in THF at –20 °C, to yield the neutral σ-alkynyl derivatives [Ru{CCC(C₁₃H₂₀)R}(η⁵-C₉H₇)(PPh₃)₂] (R = CN or OMe). Protonation of the R = CN derivative with HBF₄·Et₂O, in diethyl ether at –20 °C, afforded the cationic vinylidene complex [Ru{CC(H)C(C₁₃H₂₀)CN}(η⁵-C₉H₇)(PPh₃)₂][BF₄]. In contrast, protonation with R = OMe gives back the starting allenylidene derivative 1.