Novel [2+2+1] Cyclotrimerization of Alkynes Mediated by Bidentate Cyclopentadienyl-Phosphine Ruthenium Complexes

Abstract

A remarkable change in reactivity of ruthenium cyclopentadienyl phosphine complexes toward alkynes HC⋮CR‘ (R‘ = Ph, C6H9, p-C6H4Me, p-C6H4OMe, ferrocenyl (Fc)) has been observed when the phosphine ligand is tethered onto the Cp ring via a two-carbon linker. That is, we contrast [RuCp(PR3)(CH3CN)2]PF6, dealt with before, with the bidentate cyclopentadienyl-phosphine complex [Ru(η5-C5H4CH2CH2-κ1P-PPh2)(CH3CN)2]PF6. While a metallacyclopentatriene complex (C) generated via oxidative coupling of two alkynes is a common first key intermediate, the onward reaction of C differs greatly. If the phosphine is tethered, a third alkyne molecule can be accommodated, resulting finally in an unusual C−C coupling process involving three alkynes and the tethered phosphine to give the cycloaddition product [Ru(η5-C5H4CH2CH2PH2-κ1C-CH-η4-C5H5)]+. According to DFT/B3LYP calculations this intriguing [2+2+1] alkyne cyclotrimerization proceeds via Ru−P bond dissociation, phosphine attack at the coordinated acetylene to yield a 1-metallacyclopropene, carbene vinyl insertion, and olefin vinyl insertion. On the other hand, for HC⋮CR‘ (R‘ = COOMe, COOEt, COMe) a [2+2+2] cyclotrimerization is favored. In sharp contrast, if the phosphine in C is simply unidentate, alkyne attack is prohibited, with alternative internal rearrangements taking place involving phosphine migration or 1,2-hydrogen shift. In these terms tethering may be considered as amounting to a delayed phosphine migration. Thus, the phosphine ligand in ruthenium chemistry is not necessarily just a spectator ligand but can switch over to an actor ligand.