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^†

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 elimination gives rise to the formation of 4-functionalized 1,3-dienylpalladium(II) halide compounds 3−5 (cis arrangement of the ester functions at the double bonds). In the reaction with an excess of 1,4-chloro-2-butyne, a trimerization took place forming 1-(1‘-chloroethenyl)-1,2,3,4,5-pentakis(chloromethyl)-2,4-cyclopentadiene (6). Employing the established kinetic compatibility of the formation of the palladacycles with a successive oxidative addition/reductive elimination of organic halides and subsequent transmetalation with tetramethyltin, a catalytic cycle for the three-component synthesis of (Z,Z)-dienes of the type R−C(E)C(E)C(E)C(E)CH₃ (8, R = alkyl, aryl; E = CO₂CH₃) has been conceived, e.g., from dimethyl 2-butynedioate, an organic halide, and tetramethyltin employing 1% of 1b as the catalyst in DMF. This constitutes the first catalytic synthesis of conjugated dienes from alkynes. Pd(phosphine) compounds do not catalyze this reaction.