om970977l_si_001.pdf (697.93 kB)

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

Download (697.93 kB)
journal contribution
posted on 03.04.1998 by Ruud van Belzen, René A. Klein, Huub Kooijman, Nora Veldman, Anthony L. Spek, Cornelis J. Elsevier
Palladacyclo-2,4-pentadiene compounds containing a chelating bidentate nitrogen ligand Pd{(C(E)C(E)-C(E)C(E)}(NN) 1af (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 35 (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.