C-H bond activation at palladium(IV) centers

This communication describes the first observation and study of C-H activation at a Pd(IV) center. This transformation was achieved by designing model complexes in which the rate of reductive elimination is slowed relative to that of the desired C-H activation process. Remarkably, the C-H activation reaction can proceed under mild conditions and with complementary site selectivity to analogous transformations at Pd(II). These results provide a platform for incorporating this new reaction as a step in catalytic processes.     

Polymerization using palladium (II) salts II: Polymerization of 2-ethynyl-5-bromothiophene

The title monomer was polymerized using a bis(triphenyl phosphine) palladium (II) chloride, cuprous iodide, triphenyl phosphine catalyst system. The products are low-molecular-weight materials with thermal stabilities somewhat inferior to corresponding phenylacetylene-based polymers.     

Formation and coordination modes of the C4E4 moiety (E = COOMe) bound to two palladium(II) moieties

The transmetallation of the palladacyclopentadiene complex Pd{C(COOMe)C(COOMe)C(COOMe)C(COOMe)}(bipy) with the dicationic Pd(II) complex [Pd(bipy)(CH(3)CN)(2)][BF(4)](2) afforded a terminally σ-palladated diene complex [Pd(2){μ-η(1):η(1)-C(COOMe)C(COOMe)C(COOMe)C(COOMe)}(bipy)(2)(CH(3)CN)(2)][BF(4)](2). It was revealed by X-ray crystallographic analysis that replacement of the acetonitrile ligands in a terminally σ-palladated diene complex with PPh(3) ligands resulted in the conformation change of the σ-palladated diene moiety from skewed s-cis to planar s-trans. Treatment of a bis-triphenylphosphine dipalladium complex [Pd(2)(PPh(3))(2)(CH(3)CN)(4)][PF(6)](2) with dimethoxyacetylene dicarboxylate (DMAD) (1 equiv.) in acetonitrile resulted in the insertion of DMAD to the Pd-Pd bond to afford [Pd(2){μ-η(1):η(1)-C(COOMe)C(COOMe)}(PPh(3))(2)(CH(3)CN)(4)][PF(6)](2). Addition of the second DMAD gave the ylide-type complex [Pd(2){μ-η(2):η(3)-C(COOMe)C(COOMe)C(COOMe)C(COOMe)(PPh(3))}(PPh(3))(2)(CH(3)CN)(3)][PF(6)](2) of which the structure was determined by X-ray crystallographic analysis.     

Polymerization using palladium (II) salts. III. Polymerization of 4-bromo-4′-ethynyl biphenyl

The monomer 4-bromo-4′-ethynyl biphenyl was synthesized and subsequently polymerized utilizing a bis (triphenylphosphine) palladium (II) chloride, copper (I) iodide, triphenylphosphine catalyst system in an organic amine solvent(s). The polymers produced were predominantly insoluble materials with reasonable thermal stability.     

Catalytic enantioselective allylation at the activated benzylic position of prochiral aryl cyano esters: access to quaternary stereogenic centers

Palladium catalyzed asymmetric allylic alkylation of prochiral aryl cyano esters has been carried out in the presence of various chiral ligands. The base and additives have been varied and allowed to produce the allyl derivative presenting a highly functionalized quaternary stereogenic center. The chiral pocket ligands of Trost appears the most appropriate to produce the desired chiral derivative.     

Practical preparation method of polymer-incarcerated (PI) palladium catalysts using Pd(II) salts

[reaction: see text]. Polymer-incarcerated (PI) palladium catalyst was practically prepared from inexpensive Pd(II) salts and a polystyrene-based copolymer under reducing conditions. Remarkable effects of alkali metal salts on the palladium loading were observed. PI Pd thus prepared showed high catalytic activity in Mizoroki-Heck reactions and Suzuki-Miyaura couplings with a range of substrates including an aryl chloride. In all cases, the Pd catalyst was recovered quantitatively without leaching, and reused several times without significant loss of activity.     

Catalytic oxidation of octene-1 in the presence of palladium(II) salts and heteropolyacids

In the presence of the catalytic system (PdSO₄+HPA), where HPA=H₉PMo₆V₆O₄₀, octene-1 is found to be selectively (95%) oxidized to octanone-2. The optimum concentration ratio for catalyst stability is [HPA]:[PdSO₄]=30–40. The substitution of PdCl₂ for PdSO₄ leads to the formation of all three isomeric octanones without decrease of the overall selectivity. Effective rate constants and the apparent activation energy have been determined in the temperature range from 60 to 80 °C.

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(PdSO₄+), =H₉PMo₆V₆O₄₀, -1 ( 95%) -2. []:[PdSO₄]=30–40. PdSO₄ PdCl₂ . 60–80 °C.

     

The molecular structure of chlorothiomethoxymethyl-bis(triphenylphosphine)palladium(II) methylenechloride solvate at −160°C

The precise molecular structure of [PdCl(CH₂SCH₃)(PPh₃)₂] has been determined from three-dimensional X-ray diffraction data collected at ?160°C. The CH₂Cl₂ solvated crystal ([PdCl(CH₂SCH₃)(PPh₃)₂ · CH₂Cl₂]) belongs to the monoclinic system, space group P2₁/n, with four formula units in a cell of dimensions: a 14.973(3), b 15.333(3), c 17.377(3) Å and β 115.77(1)° at ?160°C. The structure was solved by the conventional heavy atom method and refined by the least-squares procedure to R = 0.035 for observed reflections. The geometry around the palladium atom is square-planar. The phosphorus atoms of the two triphenylphosphine ligands are mutually trans. The CH₂SCH₃ group is bonded to the palladium atom only through the PdC σ-bond and the sulfur atom is not bonded to the metal atom (PdC(1) 2.061(3), SC(1) 1.796(3), SC(2) 1.817(5), Pd?S 2.973(1) Å, PdC(1)S 100.64(14)° and C(1)SC(2) 101.28(18)°). The structure is in contrast to that of [PdCl(CH₂SCH₃)(PPh₃)], in which both the carbon and sulfur atoms of the CH₂SCH₃ group are bonded to the palladium atom.     

A new method for alkylating isoquinolinium salts at the 4 position

Alkylation and acylation of 2,3-dimethylisoquinolinium iodide at position 4- may be carried out in alkaline aqueous acetone. Presumably 1-acetonyl-2,3-dimethyl-1,2-dihydroisoquinoline is an intermediate. Lower yields were obtained in similar reactions with 2-methylisoquinolinium iodide.     

Synthesis of electron-rich platinum centers: Platinum0(carbene)(alkene)2 complexes

The synthesis and molecular structure of the zero-valent platinum-mono-carbene-bis-alkene complexes [Pt⁰(NHC)(dimethyl fumarate)₂] (NHC = 1,3-dimesityl-imidazol-2-ylidene (1a); 1,3-dimesityl-dihydroimidazol-2-ylidene (2a); diphenyl-dihydroimidazol-2-ylidene (2b) are described. Two routes have been evaluated for the synthesis of 1a and 2a, involving reaction of a zero-valent platinum compound either with an isolated carbene ligand, or with an in situ generated carbene ligand. The in situ method proved to be easier and gave similar yields of about 50% after crystallization. Attempts have been made to synthesize similar compounds with N-phenyl and N-alkyl groups, of which the latter met with little success. However, (1,3-diphenyl-dihydroimidazol-2-ylidene)-bis(η²-dimethyl fumarate) platinum(0) (2b) could be obtained in 49% yield, after crystallization, from the appropriate Wanzlick dimer.Compound 1a reacts with H₂ and D₂ in sequences of oxidative addition, migration–insertion involving dimethyl fumarate, and reductive elimination to form neutral hydrido platinum (II) carbene complexes, probably containing a metallacyclic (R)–CO … Pt unit.     

N-aminorhodamne complexes of palladium(II), palladium(IV) and platinum(II)

Summary TheN-aminorhodanine (L) complexes: PdLX, (X = Br or I), ML_1.5Cl₂ (M = Pd or Pt), PtL₂X₂ (X = Br, I or ClO₄), PdL₃(ClO₄)₂, PdL_1.5Cl₄ and PdL₃(ClO₄)₄ have been prepared and investigated. The ligand is bonded to the metal ion through the aminic nitrogen atom as monodentate or through this atom and the thiocarbonylic sulphur atom when it acts as chelating or bridging ligand. The carbonylic oxygen atom is never coordinated.     

The synthesis and catalytic properties of imidazolinium salts and their palladium(II) complexes

Three 1,3-dialkylimidazolinium chloride salts and their palladium(II) complexes have been synthesized and characterized by elemental analysis, ¹H NMR, ¹³C NMR and IR spectroscopy. The catalytic activities of the 1,3-dialkylimidazolinium chloride salts and their complexes in the Heck and Suzuki coupling reactions were investigated.     

Polymerization using palladium (II) salts: Homopolymers and copolymers from phenylethynyl compounds and aromatic bromides

Low-molecular-weight homopolymers of m-bromophenylacetylene, as well as copolymers and networks of select di- and triethynyl benzenes with di- and tribromobenzenes, have been synthesized via a coupling reaction utilizing a palladium (II) salt, triphenylphosphine, cuprous iodide catalyst system.     

Chiral bisimidazolium salts derived from amino acids and their palladium(II)- and platinum(II)-biscarbene complexes

We report new chiral bisimidazolium salts synthesized from naturally occurring l-amino acids. They served as precursors for bidentate N-heterocyclic carbene metal complexes. The chiral imidazoles could be synthesized in good yields via a one-pot ring closing reaction, followed by esterification. The methylene bridged bisimidazolium iodide salts are accessible in moderate yields. Corresponding palladium(II)- and platinum(II)-NHC complexes could be synthesized and fully characterized, but do not show optical activity. We also report a solid state structure of one of the synthesized palladium(II) biscarbene compounds derived from alanine.     

Stereocontrolled palladium(0)-catalyzed preparation of unsaturated azidosugars: an easy access to 2- and 4-aminoglycosides

The palladium-catalyzed substitution of alkyl 4,6-di-O-acetyl-α-d-erythro-hex-2-eno-pyranosides using NaN₃ as the nucleophile gave predominantly the corresponding alkyl 2-azido-2,3,4-trideoxy-α-d-threo-hex-2-enopyranosides in the presence of Pd(PPh₃)₄. However, alkyl 6-O-acetyl-4-azido-2,3,4-trideoxy-α-d-erythro-hex-2-enopyranosides were obtained as the major products using Pd(PPh₃)₄ as the catalyst in the presence of dppb as the added ligand. Conversely, alkyl 6-O-(tert-butyldimethylsilyl)-4-O-methoxycarbonyl-2,3-dideoxy-α-d-hex-2-enopyranosides gave exclusively alkyl 4-azido-6-O-(tert-butyldimethylsilyl)-2,3,4-trideoxy-α-d-erythro-hex-2-enopyranosides in the presence of Pd₂(dba)₃/PPh₃ as the catalyst and Me₃SiN₃ as the nucleophile. The bis-hydroxylation followed by hydrogenation of ethyl 4-azido-2,3,4-trideoxy-α-d-erythro-hex-2-enopyranoside afforded the corresponding 4-amino-α-d-mannopyranoside, when propyl 2-azido-2,3,4-trideoxy-α-d-threo-hex-3-enopyranoside gave the 2-amino-α-d-altropyranoside under the same conditions.     

1-Methyl-4-ferrocenylmethyl-3,5-diphenylpyrazole: A versatile ligand for palladium(II) and platinum(II)

The syntheses and characterization of two novel ferrocene derivatives containing 3,5-diphenylpyrazole units of general formula [1-R-3,5-Ph₂-(C₃N₂)-CH₂-Fc] {Fc = (η⁵-C₅H₅)Fe(η⁵-C₅H₄) and R = H (2) or Me (3)} together with a study of their reactivity with palladium(II) and platinum(II) salts or complexes under different experimental conditions is described. These studies have allowed us to isolate and characterize trans-[Pd{1-Me-3,5-Ph₂-(C₃N₂)-CH₂-Fc]}₂Cl₂] (4a) and three different types of heterodimetallic complexes: cis-[M{1-Me-3,5-Ph₂-(C₃N₂)-CH₂-Fc]}Cl₂(dmso)] {M = Pd (5a) or Pt (5b)}, the cyclometallated products [M{κ²-C,N-[3-(C₆H₄)-1-Me-5-Ph-(C₃N₂)]-CH₂-Fc}Cl(L)] with L = PPh₃ and M = Pd (6a) or Pt (6b) or L = dmso and M = Pt (8b) and the trans-isomer of [Pt{1-Me-3,5-Ph₂-(C₃N₂)-CH₂-Fc]}Cl₂(dmso)] (7b). In compounds 4a, 5a, 5b and 7b, the ligand behaves as a neutral N-donor group; while in 6a, 6b and 8b it acts as a bidentate [C(sp²,phenyl),N(pyrazole)]⁻ group. A comparative study of the spectroscopic properties of the compounds, based on NMR, IR and UV-Visible experiments, is also reported.     

Reduction of palladium(II) monoglycinate complexes at rotating disc palladium electrode in acid media

Reduction of palladium(II) glycinate complexes in strongly acid 0.5 M NaClO₄ solutions (pH 0.6 and 1.0) with variable palladium(II) complex and free glycine concentration was studied by the taking of cyclic voltammograms at palladium rotating disc electrode. It is shown that it was a chelate monoglycinate palladium(II) complex that was present in all studied solutions and underwent the reduction. The diffusion coefficient of the chelate monoglycinate palladium(II) complex D = (6.5 ± 0.5) × 10⁻⁶ cm²/s was determined from the limiting diffusion current of the complex reduction. The monoglycinate palladium(II) complex reduction occurred in the double-layer segment of the palladium charging curve; it was not complicated by hydrogen adsorption at electrodes. The palladium(II) complex reduction half-wave potential was determined (E _1/2 = ∼0.300 to 0.330 V (SCE)). It is shown that the decreasing of the number of ligands coordinated by palladium via nitrogen atom facilitates the complex reduction process. In particular, the reduction potentials of palladium(II) complexes with different ligand number at palladium electrode shifted markedly toward negative potentials in the series: Pdgly⁺ < Pd(gly)2 < Pd(gly)₄²⁻.     

Carbonylation of aryl- and vinyl-tellurium compounds with carbon monoxide in the presence of palladium(II) salts

Aryl- and vinyl-tellurium(II or IV) compounds react with carbon monoxide (CO) in suitable organic solvents to give the corresponding carboylic acids in moderate to quantitative yields in the presence of a stoichiometric amount of a palladium(II) salt. Treatment of (Z)-styrylphenyl telluride with atmospheric pressure of CO at room temperature in the presence of palladium(II) chloride or lithium chloropalladate(II) affords predominantly (E)-cinnamic acid, while in the presence of palladium(II) acetate similar reaction gives the (Z)-acid highly selectively. Under higher CO pressures (5–50 atm), however, the (Z)-acid becomes the major product, even when palladium(II) chloride is used. The following pathways are proposed for this carbonylation: (1) in the first step organotellurium compounds form the monomeric and/or dimeric palladium complexes such as [(R₂Te)PdCl₂]₂ and/or (R₂Te)₂PdCl₂ (R = aryl, vinyl), then (2) the migration of R moiety from tellurium to palladium (transmetallation) occurs to afford the reactive aryl- or alkenyl-palladium compounds, and (3) the compounds react with CO to give the corresponding acylpalladium compounds, after alkaline hydrolysis, the carboxylic acids are formed. The presence of an ionic carbene-like organopalladium complex is proposed for the formation of the (Z)-acid from (Z)-telluride.