The reaction of homo-coupling of alkynes using zirconium- and titanium-containing reagents is an important basis for the efficient synthesis of polymer, oligomers and macrocycles. It is generally accepted that the homo-coupling takes place through the intermediate formation of “Cp₂Zr” or “Cp₂Ti” species that can be generated by the interaction of Cp₂ZrCl₂ or Cp₂TiCl₂ with alkali and alkaline earth metals, as well as with alkyl derivatives of metals. The most widely used reagents include Negishi reagent (“Cp₂ZrBu₂”), Takahashi reagent (“Cp₂ZrEt₂”), and Rosenthal reagent (Cp₂Zr(py)TMSC≡CTMS). The well-known system of reagents Ti(OPr)₄ - i-PrMgBr should also be mentioned. Recently, we have shown the promise of using a reagent based on tantalum (TaCl₅ – Mg) for the reduction of propargylamines. Continuing these studies, in order to develop new catalytic systems for the homo-coupling of acetylenic compounds, in this work we studied the TaCl₅-catalyzed reaction of dialkyl-substituted acetylenes with EtAlCl₂ and Mg.
We have established for the first time that the reaction of dialkyl-substituted acetylenes with EtAlCl₂ and magnesium in the presence of catalytic amounts of tantalum (V) chloride in a toluene solution after hydrolysis and deuterolysis gives tetraalkyl-substituted (E,E)-buta-1,3-dienes in high yield. A selective method for the preparation of tetraalkyl-substituted (Z,Z)-1,4-dichlorobuta-1,3-dienes based on the reaction of aluminacyclopentadienes formed in situ with methanesulfonyl chloride has been developed. A plausible scheme for the catalytic conversion has been proposed.