Stoichiometric and Catalytic Conversion of Alkynes to Conjugated (Z,Z)-Dienes and Cyclopentadienes via Palladacyclopentadienes and 1,3-Dienylpalladium(II) Halide and Triorganopalladium(IV) Halide Compounds Containing Chelating Nitrogen Ligands

Abstract

Palladacyclo-2,4-pentadiene compounds containing a chelating bidentate nitrogen ligand Pd{(C(E)C(E)-C(E)C(E)}(NN) 1a−f (E = CO2Me, NN = Ph-bip, Ar-bian, bpy, dcm-bpy, bpym) and 2a,b (E = CF3, NN = Ph-bip, (p-tol)-bian) have been prepared from Pd(dba)2, the appropriate bidentate N-ligand, and electron-deficient acetylenes dimethyl 2-butynedioate or hexafluorobutyne. X-ray crystal structures were obtained for compounds 1a (NN = Ph-bip) and 1d (NN = 2,2‘-bpy). In solution, an equilibrium between the monomer and a dimer exists for compounds 1d and 1e (NN = bipyrimidine); in the solid state, 1d is a monomer. The dimeric form of 1d is of the same type as the zerovalent palladium compound [(μ-3,3‘-dicarbomethoxy-2,2‘-bipyridine)Pd(tcne)]2 in which the two bipyridine derivatives bridge between the two palladium centers, as determined from the X-ray crystal structure of this compound (7). The palladacycles 1 undergo oxidative addition of methyl iodide, benzyl bromide, or iodobenzene. Subsequent reductive eliminati...