Alkyne Coupling Reactions Mediated by Tris(pyrazolyl)borate Ruthenium Vinylidene Complexes: X-Ray Crystal Structures of [TpRuCCHPh(PEt₃)₂][BPh₄] and [TpRuCC(COOMe)CHCHCOOMe(PEt₃)₂][BPh₄]

Abstract

A series of cationic vinylidene complexes of the type [TpRuCCHR(PR3)2][BPh4] (Tp = hydrotris(pyrazolyl)borate; PR3 = PEt3, PMeiPr2; R = Ph, But, COOMe) have been prepared. The reaction of [TpRuCCHCOOMe(PEt3)2][BPh4] with a further HC⋮CCOOMe molecule yields the E-stereoisomer of the vinylvinylidene derivative [TpRuCC(COOMe)CHCHCOOMe(PEt3)2][BPh4], which has been structurally characterized. This C−C coupling reaction has been interpreted in terms of a [2 + 2] cycloaddition of the alkyne to the Cα−Cβ bond of the vinylidene ligand to yield a cyclobutenylidene intermediate, followed by a concerted ring opening. The neutral σ-η1-butadienyl complex [TpRu(CHC(COOMe)CHCHCOOMe(PEt3)2] was prepared by reaction of the vinylvinylidene complex with NaBH4 in MeOH. The reaction of [TpRuCl(PMeiPr2)2] with 1-alkynes leads to neutral vinylidene derivatives [TpRuCCHR(Cl)(PMeiPr2)] (R = Ph, But, SiMe3, H). These complexes react with LDA and diphenylacetylene yielding the enynyl species [TpRu(PhCC(Ph)C⋮CR)(PMeiPr2)] (R = Ph, But). These complexes, which were isolated and characterized, are efficient catalysts for alkyne dimerization reactions, as well as for cross-coupling reaction of terminal alkynes.