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 degreesC, followed by the addition of an unsymmetrical tetra- or pentafluorophenyl substituted alkyne R(1)CdropCAr(dagger) (R-1, Ar-dagger = (CH2)(4)Me, p-C6F4H; Me, p-C6F4H; Ph, C6F5), resulted in regioselective couplings of these alkynes to zirconacyclopentaclienes in which the Art substituents preferentially adopt the 3,4-positions (betabeta) of the zirconacyclopentacliene 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 (alphabeta) isomers were also formed. None of the conditions employed produced the 2,5-fluoroaryl substituted (alphaalpha) isomers. These fluoroaryl-substituted zirconacyclopentadienes were readily converted to butadienes via reactions with acids. The zirconacyclopentacliene Cp2ZrC4-2,5-Ph-2-3+(C6F5)(2), which resulted from the coupling of PhCequivalent toC(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)Cequivalent toC(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)Cequivalent toCCH(2)](2)CH2 with Cp2Zr(py)(Me(3)SiCequivalent toCSiMe(3)) resulted in a zirconacyclopentacliene in which the pentafluorophenyl substituents have been forced into the 2,5-positions (alphaalpha). Zirconocene coupling of the diyne (C6F5)Cequivalent toC-1+C6H4-Cequivalent toC(C6F5) provided a route to conjugated polymers bearing electron-withdrawing pentafluorophenyl groups.