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)2 Na2 Mn(CH2 SiMe3 )4 ] and [(PMDETA)2 K2 Mn(CH2 SiMe3 )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(CH2 SiMe3 )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)4 Na2 Mn(C≡CPh)4 }∞ ] and [(THF)4 Li2 Mn(C≡CPh)4 ]. Controlled exposure of [{(THF)4 Na2 Mn(C≡CPh)4 }∞ ] and [(THF)4 Li2 Mn(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.
Keywords: 1,3-diynes; alkali-metal effect; homocoupling; manganates; metal - metal cooperativity.
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