Lewis acid catalyzed Z to E isomerization of 1,2-bis(diphenylphosphino)ethene

Z-1,2-Bis(diphenylphosphino)ethene, cis-Ph2PCH=CHPPh2, forms 1:2 adducts with GaBr3 and GaI3, the former of which has been identified in an X-ray diffraction study as the complex of the isomerized ligand, E-Ph2PCH=CHPPh2. The GaI3 complex is believed to be analogous on the basis of analytical and spectroscopic data. InBr3 affords a 1:1 complex with an ionic structure [(Ph2PCH=CHPPh2)(2)InBr2](+) [InBr4](-) in which the cation contains the original cis-ligand. With InI3 also a 1:1 adduct is obtained, where the metal triiodide unit is attached to only one phosphorus atom of the non-isomerized (cis) ligand in the solid state. There is rapid site exchange of the InI3 unit in chloroform solution as followed by NMR spectroscopy. The metal halide induced Z/E isomerization of Ph2PCH=CHPPh2 has been studied in various solvents and at variable temperature with stoichiometric and catalytic amounts of AIX(3) and GaX3 Lewis-acids. InX3 compounds proved ineffective (X = Cl, Br, I). Anhydrous AlBr3 was found to be most efficient, giving a 90% Z/E conversion in 10 min at 100 degrees C in toluene. iii mechanism is proposed which is compatible with the experimental data.