Structural and synthetic insights on oxidative homocouplings of alkynes mediated by alkali-metal manganates

Exploiting bimetallic cooperation alkali-metal manganate (II) complexes can efficiently promote oxidative homocoupling of terminal alkynes furnishing an array of conjugated 1,3-diynes. The influence of the alkali-metal on these C−C bond forming processes has been studied by preparing and structurally characterizing the alkali-metal tetra(alkyl) manganates [(TMEDA)₂Na₂Mn(CH₂SiMe₃)₄] and [(PMDETA)₂K₂Mn(CH₂SiMe₃)₄]. Reactivity studies using phenylacetylene as a model substrate have revealed that for the homocoupling to take place initial metalation of the alkyne is required. In this regard, the lack of basicity of neutral Mn(CH₂SiMe₃)₂ precludes the formation of the diyne. Contrastingly, the tetra(alkyl) alkali-metal manganates behave as polybasic reagents, being able to easily deprotonate phenylacetylene yielding [{(THF)₄Na₂Mn(C≡CPh)₄}_∞] and [(THF)₄Li₂Mn(C≡CPh)₄]. Controlled exposure of [{(THF)₄Na₂Mn(C≡CPh)₄}_∞] and [(THF)₄Li₂Mn(C≡CPh)₄] to dry air confirmed their intermediary in formation of 1,4-diphenyl-1,3-butadiyne in excellent yields. While the Na/Mn(II) partnership proved to be the most efficient in stoichiometric transformations, under catalytic regimes, the combination of MC≡CAr (M= Li, Na) and MnCl₂ (6 mol %) only works for lithium, most likely due to the degradation of alkynylsodiums under the aerobic reaction conditions.