Palladium catalyzed oxidation of monoterpenes: NMR study of palladium(II)-monoterpene interactions

Reactions of the monoterpenes β-pinene, limonene and myrcene with Pd(II) complexes in acetic acid solutions were studied by ¹H NMR spectroscopy. Various π-allyl palladium complexes were detected in situ and their interaction with CuCl₂ has been investigated. The results clarify the mechanism of allylic oxidation of these substrates mediated by Pd(II)/Cu(II)-based catalytic systems. Originally introduced to regenerate reduced palladium species, CuCl₂ has been shown to play an important role in the formation and/or decomposition of key reaction intermediates - π-allyl palladium complexes. β-Pinene and myrcene readily react with Pd(OAc)₂ giving corresponding π-allyls, with two complexes acyclic and cyclic being formed from myrcene. On the other hand, the formation of π-allyl complexes from limonene occurs at a significant rate only in the presence of CuCl₂. NMR observations, including selective paramagnetic enhancement of spin-lattice relaxation, indicate that π-allyl palladium intermediates specifically interact with Cu(II) ions in the reaction solutions. Such interaction probably involves Cu(II) bonding to Pd(II) via bridging ligands, and seems to be responsible for the accelerative effect of CuCl₂ in the palladium catalyzed oxidation of the monoterpenes. Indeed, most of these reactions do not occur at all in the absence of CuCl₂.     

New binuclear Pd(II) thioamide complexes for the Heck reaction of aryl bromides

The reaction of binucleating thioamide ligands (L₁-L₃) with [PdCl₂(PPh₃)₂] in 1:2 molar ratio in methanol medium afforded a series of binuclear palladium(II) complexes. The synthesized complexes were characterized by elemental analysis and ¹H NMR. The molecular structure of one of the complexes was established by X-ray diffraction method. The binuclear palladium(II) thioamide complex has been shown to be an active catalyst for the Heck reaction of aryl bromides with alkenes.     

Unexpected participation of nucleophiles in the reaction of palladium(II) acetate with divalent 3<Emphasis Type="Italic">d</Emphasis> metals

The kinetics of reactions of palladium(II) acetate with cobalt(II), nickel(II), and copper(II) acetates were studied by spectrophotometry. These reactions produce heterobimetallic complexes Pd^II(μ-OOCMe)₄M^II(OH₂)(HOOCMe)₂, where M = Co, Ni, or Cu. These reactions are very slow in carefully dehydrated (<0.01% H₂O) acetic acid, but are considerably enhanced by water or acetonitrile. Our data indicate that the activation of the kinetically inert ring structure of the initial palladium complex Pd₃(μ-OOCMe)₆ by means of the nucleophilic attack of an H₂O or acetonitrile molecule is the key step of the reaction mechanism.     

Mechanism of the formation of palladium complexes serving as catalysts in hydrogenation reactions : II. Reactivity of palladium phosphine halides towards molecular hydrogen

[(PPh₃)₃(PPh₂)₂Pd₃Cl] Cl, benzene and aniline hydrochloride were isolated as products of the reactions of (PPh₃)₂PdCl₂]₂ or [(PPh₃)PdCl₂]₂ with H₂ in organic amines (Am). Similar products were obtained when (Ph₃P)₂Pd(Ph)Br was treated with H2₃ Both in amines and aromatic solvents. The reaction between H₂ and [(PBu₃)PdCl₂]₂ resulted in the formation of [(PBu₃(PBu_2)PdCl2 ·. 2 Am The kinetic data for H₂ absorption by solutions of palladium(II) complexes are consistent with the heterolytic mechanism of cleavage fo hte HH bond in the coordination sphere of palladium(II); the function of the H⁺ acceptor being performed by the bases (e.g. Am or Ph). The reaction between the palladium complexes and H₂ is autocatalytic. Reduction of the initial Pd^II complexes leads to lower oxidation state palladium complexes, which catalyse the reduction of Pd^II complexes. In the coordination sphere of the lower oxidation state palladium complexes, the oxidative addition of PR₃ to Pd takes place with formation of compounds containing a Pd-R bond. It is the reaction between these complexes and H₂ that yields palladium compounds with PR₂ ligands.     

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⁻.     

Palladium(II) complexes of aliphatic amines and their oxidation by chloramine‐T in perchloric acid medium

The formation of palladium(II) complexes with aliphatic amines and their oxidation by chloramine‐T in perchloric acid medium has been studied. The spectrophotometric studies showed the formation of 1:1 and 1:2 complexes between palladium(II) and amine in absence of HClO₄. An increase in [HClO₄] in reaction mixture suppresses the complex formation and in presence of [HClO₄] ～10^?3 mol dm^?3 only a 1:1 complex between palladium(II) and amine has been observed. The effect of Cl^? on the complex formation has also been studied. Palladium(II)‐catalyzed oxidation of these amines by chloramine‐T showed a first‐order dependence of rate with respect to each—oxidant, substrate, catalyst, and H⁺. The mechanism consistent with kinetic data for the oxidation process has been proposed in absence as well as in presence of initial [Cl^?]. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 603–612, 2002     

Nickelatetrazoles as Intermediates in the Course of the Photolysis of Nickel(II) Azido Complexes? – A Theoretical Study

The primary photoreactions due to charge transfer excitation of d⁸ transition metal complexes of [MP₂(N₃)₂] constitution (P₂: mono‐ or diphosphane ligands) are strongly influenced by the central ion. While [MP₂(N₃)₂] complexes of both palladium(II) and platinum(II) yield primarily the corresponding metal(I) species and azidyl radicals, nickel(II) complexes of the same constitution lead to intermediate generation of nitrenes as has been indirectly shown by various scavenging reactions. During the course of the succeeding reactions, the intermediate generation of nickelatetrazoles is assumed. Both nitrene and metallatetrazole intermediates could not be identified directly so far. Therefore, the proposed reaction pathway has been modeled theoretically to get indications regarding the probability of the formation of such intermediates. Based on density functional theory (DFT) calculations, the energetics of the proposed pathway is compared with those of possible other pathways. The calculations support the proposed reaction pathway and point to possible reasons for the different behaviour of nickel(II) compared to palladium(II) and platinum(II).     

Synthesis and Crystal Structures of New Palladium(II)—Gallium(III) Complexes with Bridging Halogenide Ligands

The reaction of palladium(II) bromide or palladium(II) iodide with the respective gallium(III) halogenide in the presence of aromatic solvents leads to the formation of palladium(II) tetrabromo— and tetraiodogallate. The compounds are isostructural {monoclinic, C2/m, Pd[GaBr₄]₂: a = 1267(2), b = 808(1), c = 722(1) pm, β = 94.5(1)°; Pd[GaI₄]₂: a = 1363(1), b = 849.9(4), c = 756.6(7) pm, β = 95.38(3)°}. The structures contain mononuclear complexes Pd[GaX₄]₂, where X^— = Br^— ( 1 ), I^— ( 2 ). The crystal structures of 1 and 2 were determined by single‐crystal X‐ray diffraction. Crystals of both compounds turned out to be similarly twinned.     

Cathodic adsorption voltammetry of palladium(II)

The electrochemical behavior of palladium(II) was studied by differential pulse, linear sweep, and alternating-current square-wave voltammetry in HC1, HNO₃, H₂SO₄, and HC1O₄ solutions in the presence of dimethylglyoxime. A peak with a height linearly depending on the concentration of palladium(II) was observed in voltammograms. Typical relationships between the height and potential of a peak and pH, dimethylglyoxime concentration, the potential and time of adsorption accumulation suggested that the observed peak was due to the hydrogen liberation catalyzed by palladium(II) dimethylglyoximate adsorbed on the electrode surface. The detection limits for palladium(II) accumulated for 120 s at -0.2 V were 2 x 10^-8, 5 x 10^-9, and 8 x 10^-10 M for differential pulse, linear sweep, and alternating-current square-wave voltammetry, respectively.     

A study of the reactions of methionine- and histidine-containing peptides with palladium(II) complexes: The key role of steric crowding on palladium(II) in the selective cleavage of the peptide bond

¹H NMR spectroscopy was applied to study the reactions of palladium(II) complexes, cis-[Pd(dpa)Cl₂] and cis-[Pd(dpa)(H₂O)₂]²⁺ (dpa is 2,2′-dipyridylamine acting as a bidentate ligand) with the dipeptides methionylglycine (Met-Gly) and histidylglycine (His-Gly), and the N-acetylated derivatives of these dipeptides, MeCOMet-Gly and MeCOHis-Gly. All reactions were carried out in the pH range 2.0–2.5 with equimolar amounts of the palladium(II) complex and the peptide at two different temperatures, 25 and 60 °C. In the reactions of cis-[Pd(dpa)Cl₂] and cis-[Pd(dpa)(H₂O)₂]²⁺ with Met-Gly and His-Gly, no hydrolysis of the peptide bond was observed. The final product in these reactions was the [Pd(dpa)₂]²⁺ complex. The square-planar structure of this complex was confirmed by X-ray analysis. The reaction of the cis-[Pd(dpa)(H₂O)₂]²⁺ complex with the MeCOHis-Gly and MeCOMet-Gly peptides under the previously mentioned experimental conditions was remarkably selective in the cleavage of the amide bond involving the carboxylic group of methionine in the side chain. The modes of coordination of cis-[Pd(dpa)Cl₂] and cis-[Pd(dpa)(H₂O)₂]²⁺ in the reactions with the non-acetylated peptides and the total steric inhibition of the hydrolytic reaction between cis-[Pd(dpa)(H₂O)₂]²⁺ and MeCOHis-Gly can be attributed to the steric bulk of the palladium(II) complex. This finding should be taken into consideration in designing new palladium(II) complexes for the regioselective cleavage of peptides and proteins.     

Interaction of adenine with dichloro-[1-alkyl-2-(naphthylazo)-imidazole]palladium(II) complexes: kinetic and mechanistic studies

Interaction of adenine (A) with dichloro-[1-alkyl-2-(α-naphthylazo)imidazole] palladium(II) [Pd(α-NaiR)Cl₂], 1 and dichloro-[1-alkyl-2-(β-naphthylazo)imidazole] palladium(II) [Pd(β-NaiR)Cl₂], 2 {where R=Me (a), Et (b) or Bz (c)} in MeCN-water (50% v/v) medium to yield [{1-alkyl-2-(α-naphthylazo)imidazole}(adenine)]palladium(II) perchlorates (3a, 3b, 3c) and [{1-alkyl-2-(β-naphthylazo)imidazole}(adenine)]palladium(II) perchlorates (4a, 4b, 4c) was studied. The products were characterized by physico-chemical and spectroscopic methods. The reaction kinetics were second order overall, being first order in both the Pd(II) complex and adenine. The effect of adding chloride was consistent with rate-limiting dissociation of chloride from the complex. Thermodynamic parameters were determined from temperature variation experiments. The second-order rate constant k ₂ corroborates with the experimental Δ^‡H° values, while the negative values of Δ^‡S° indicate that the reaction proceeds through an associative inner sphere mechanism.     

A Heterogeneous‐Catalyst‐Based,Microwave‐Assisted Protocol for the Synthesis of 2,2′‐Bipyridines

A new method of preparing 2,2′‐bipyridines with short reaction times by using microwave assistance and heterogeneous catalysts has been developed. With a Negishi‐like protocol, it was found that Ni/Al₂O₃–SiO₂ afforded 2,2′‐bipyridine products in up to 86 % yield in 1 h. Palladium supported on alumina also provided yields of 2,2′‐bipyridines comparable to those seen for homogeneous PEPPSI^TM (1,3‐diisopropylimidazol‐2‐ylidene)(3‐chloropyridyl)palladium(II)dichloride) and tetrakis(triphenylphosphanyl)palladium complexes.     

Palladium(II) complexes of phosphane ligands with ammonium-functionalized carbosilane substituents

The synthesis of diphenylarylphosphane and 1,2-bis(diarylphosphanyl)ethane ligands, where the aryl group is -C₆H₄CH₂CH₂SiMe₂CH₂OC₆H₄-3-NMe₂, their palladium(II) complexes, and their corresponding ammonium-quaternized derivatives is described. These new phosphanes were devised as models of potentially water-soluble dendritic carbosilane ligands, although the solubility brought about by the quaternized N-trimethylanilinium groups is scarce. The palladium(II) complexes have been fully characterized by ¹H, ¹³C, and ³¹P NMR spectroscopy and mass spectrometry, and have been tested in the Hiyama cross-coupling reaction between tri(methoxy)phenylsilane and 3-bromopyridine in aqueous sodium hydroxide solution.     

Ethylene Adsorption and Oxidation Kinetics in the Reactions with Pd(II)/SiO2 and Cr(VI)/SiO2: Consideration of Substrate Distribution between the Gas Phase,Silica Gel,and Reaction Centers

The kinetics of ethylene oxidation by PdCl₂ and CrO₃ complexes supported on silica gel (300 K, closed batch reactor) and the adsorption of C₂H₄ by silica gel and metal complex reaction centers (M ⁿ ) were studied. A new version of the kinetic distribution method was applied to determine the rate constants of ethylene reactions with metal complexes with consideration for the equilibrium distribution of C₂H₄ among the reactor gas phase, silica gel, and M ⁿ . The rate constant of a first-order reaction with respect to Cr(VI) (k _e) remained constant as [M ⁿ ] was increased up to 0.15 mol % with the absence of detectable ethylene adsorption by chromium(VI). In the case of Pd(II)/SiO₂, strong ethylene adsorption by palladium(II) was found, and k _e was an exponential function of [M ⁿ ]. This exponential function is indicative of an increase in the specific activity of Pd(II) with palladium concentration on SiO₂. Taking into account the adsorption of ethylene (physisorption on SiO₂ and chemisorption on Pd(II)), we found an analogy between the kinetic behaviors of Pd(II) in reactions with ethylene on silica gel and with ethylene and other hydrocarbons in solutions.     

Palladium(II) complexes with R2edda-derived ligands

Four palladium(II) complexes with R₂edda ligands, dichlorido(O,O′-dialkylethylenediamine-N,N′-diacetate)palladium(II) monohydrates, [PdCl₂(R₂edda)]?H₂O, R = Me, Et, n-Pr, i-Bu, and the new ligand precursor i-Bu₂edda?2HCl?H₂O, O,O′-diisobutylethylenediamine-N,N′-diacetate dihydrochloride monohydrate, were synthesized and characterized by IR, ¹H and ¹³C NMR spectroscopy, and elemental analysis. DFT calculations were performed for the palladium(II) complexes and a high possibility for isomer formation due to stereogenic N ligand atoms was confirmed. Moreover, DFT simulations revealed energetic profile of isomer formation. Computational outcomes are in agreement with spectroscopic instrumental findings, both strongly indicating a non-stereoselective reaction between selected esters and K₂[PdCl₄], forming isomers.     

A series of (NHC)Pd(N˄O)(OAc) complexes: synthesis,characterization and catalytic activities towards desulfinative Sonogashira coupling of arylsulfonyl hydrazides with arylalkynes

A series of well-defined N-heterocyclic carbene palladium (II) complexes with general formula (NHC)Pd(N^˄O)(OAc) were prepared through reaction of Pd (NHC)(OAc)₂(H₂O) with 1-methyl-1H-pyrazole-3-carboxylic acid or 1-methyl-1H-indazole-3-carboxylic acid in the presence of K₂CO₃. These complexes were then used for desulfinative Sonogashira coupling of arylsulfonyl hydrazides with terminal alkynes. With low catalyst loading, all synthesized palladium compounds exhibited moderate to high catalytic activities for the reactions.     

Interaction of 2-aminopyrimidine with dichloro-[1-alkyl-2-(naphthylazo) imidazole]palladium(II) complexes : Kinetic and mechanistic studies

Background  

The anticancer properties of cisplatin and palladium(II) complexes stem from the ability of the cis-MCl₂ fragment to bind to DNA bases. However, cisplatin also interacts with non-cancer cells, mainly through bonding molecules containing -SH groups, resulting in nephrotoxicity. This has aroused interest in the design of palladium(II) complexes of improved activity and lower toxicity. The reaction of DNA bases with palladium(II) complexes with chelating N,N^/donors of the cis-MCl₂ configuration constitutes a model system that may help explore the mechanism of cisplatin's anticancer activity. Heterocyclic compounds are found widely in nature and are essential to many biochemical processes. Amongst these naturally occurring compounds, the most thoroughly studied is that of pyrimidine. This was one of the factors that encouraged this study into the kinetics and mechanism of the interaction of 2-aminopyrimidine (2-NH₂-Pym) with dichloro-{1-alkyl-2-(α-naphthylazo)imidazole}palladium(II) [Pd(α-NaiR)Cl₂, 1] and dichloro-{1-alkyl-2-(β-naphthylazo)imidazole}palladium(II) [Pd(β-NaiR)Cl₂, 2] complexes where the alkyl R = Me (a), Et (b), or Bz (c).     

Palladium(II)-N-heterocyclic carbene-catalyzed direct C2- or C5-arylation of thiazoles with aryl bromides

Herein we report, a series of new benzimidazolium chlorides as N-heterocyclic carbene (NHC) ligand and their corresponding palladium(II)-NHC complexes with the general formula [PdCl₂(NHC)₂] were synthesized. All new compounds were characterized by ¹H NMR, ¹³C NMR, IR spectroscopy and elemental analysis techniques. The catalytic activity of palladium(II)-NHC complexes was investigated in the direct C2- or C5-arylation of thiazoles with aryl bromides in presence of palladium(II)-NHC at 150?°C for 1?h. These complexes exhibited the good catalytic performance for the direct arylation of thiazoles. The arylation of thiazoles regioselectively produced C2- or C5-arylated thiazoles in moderate to high yields.     

Palladium(II)‐catalyzed cyclization of W‐(2′,4′‐dienyl)‐2‐alkynamides to α‐alkylidene‐γ‐butyrolactams

In the presence of CuCl₂, N‐(2′, 4′‐dienyl)‐2‐alkynamides can be converted to α‐alkylidene‐σ‐butyrolactams under the catalysis of palladium(II). In this reaction, CuCl₂ is used to oxidize Pd(0) to regenerate Pd(II), or the carbon‐palladium bond is quenched by the oxidative cleavage reaction of CuCl₂.     

Polystyrene bis[N-(2-pyridyl) sulfonamide] palladium(II): Synthesis,characterization as a catalyst for coupling reactions

The [N-(2-pyridyl)] para-styrene sulfonamide (PSS) was prepared as a monomer, from the reaction of para-styrene sulfonyl chloride and 2-amino pyridine in the presence of potassium hydroxide solution 0.5 M as a base, and CH₃Cl. Polystyrene [N-(2-pyridyl) sulfonamide] (PPSS) was synthesized from the polymerization of [N-(2-pyridyl)] para-styrene sulfonamide (PSS). The Polystyrene bis [N-(2-pyridyl) sulfonamide] palladium (II) as a polymer- supporting palladium complex was also prepared from the reaction of PdCl₂ (CH₃CN)₂ with PPSS in the presence of KOH 0.5 M. Polystyrene bis [N-(2-pyridyl) sulfonamide] palladium (II) is produced as a novel heterogeneous catalyst for coupling reactions for C-C bond formation. This method includes higher yield and has an easier work-up procedure. The structures of the monomer, polymer and its Pd complex were confirmed by using FT-IR and ¹H-NMR spectroscopy. Elemental analysis of Pd by inductively coupled plasma (ICP) technique and hot filtration test showed loading of the metal into solution from the catalyst The heterogeneous catalyst was recycled without any loss in its properties.