Regioselective coupling of pentafluorophenyl substituted alkynes: Mechanistic insight into the zirconocene coupling of alkynes and a facile route to conjugated polymers bearing electron-withdrawing pentafluorophenyl substituents

The reaction of Cp₂ZrCl₂ with 2 equiv of BuLi at -78 °C, followed by the addition of an unsymmetrical tetra- or pentafluorophenyl substituted alkyne R¹C≡CAr_f (R¹, Ar_f = (CH₂)₄Me, p-C₆F₄H; Me, p-C₆F₄H; Ph, C₆F₅), resulted in regioselective couplings of these alkynes to zirconacyclopentadienes in which the Ar_f substituents preferentially adopt the 3,4-positions (ββ) of the zirconacyclopentadiene ring. With Cp₂Zr(py)(Me₃SiC≡CSiMe₃) as the zirconocene reagent, the couplings could be carried out at room temperature; however, at higher temperatures significant quantities of the 2,4-fluoroaryl substituted (αβ) isomers were also formed. None of the conditions employed produced the 2,5-fluoroaryl substituted (αα) isomers. These fluoroaryl-substituted zirconacyclopentadienes were readily converted to butadienes via reactions with acids. The zirconacyclopentadiene Cp₂ZrC₄-2,5-Ph₂-3,4- (C₆F₅)₂, which resulted from the coupling of PhC≡C(C₆F₅), was converted to the corresponding thiophene by reaction with S₂Cl₂, and to an arene by reaction with MeO₂CC≡CCO₂Me/CuCl. Mechanistic studies on zirconocene couplings of (p-CF₃C₆H₄)C≡C (p-MeC₆H₄) indicate that the observed regioselectivities are determined by an electronic factor that controls the orientation of at least one of the two alkynes as they are coupled. Additionally, these studies suggest an unsymmetrical transition state for the zirconocene coupling of alkynes, and this is supported by DFT calculations. The reaction of [(C₆F₅)C≡CCH₂] ₂CH₂ with Cp₂Zr(py)(Me₃SiC≡CSiMe₃) resulted in a zirconacyclopentadiene in which the pentafluorophenyl substituents have been forced into the 2,5-positions (αα). Zirconocene coupling of the diyne (C₆F₅)C≡C-1,4-C₆ H₄-C≡C(C₆F₅) provided a route to conjugated polymers bearing electron-withdrawing pentafluorophenyl groups.