Organopalladium Research Articles

Synthetic Modification of Acyclic Bent Allenes (Carbodicarbenes) and Further Studies on Their Structural Implications and Reactivities

The paper describes the synthetic development of Bertrand-type acyclic carbodicarbene scaffolds derived from an unsymmetrical bis(benzimidazol-2-yl)methane bearing two sterically demanding pendant arms, isopropyl (6a) or cyclohexyl (6b). X-ray crystallographic analysis shows that the impact of these pendant arms on the overall structural parameters of carbodicarbenes is minimal. The chemical reactivity of the carbodicarbenes was evaluated with iodomethane to afford compound 7, illustrating its nucleophilic properties. Finally, experiments were also undertaken to investigate the coordination ability of carbodicarbene toward the formation of rhodium carbonyl (10) and palladium allyl complexes (11). The crystal structures of the metal complexes have been determined, revealing that their metal–carbene distances are elongated only slightly, this fact was rationalized on the basis of geometrical steric considerations with regard to the ligand.

Synthesis of novel palladium allyl complexes bearing heteroditopic NHC–S ligands. Kinetic study on the carbene exchange between bis-carbene palladium allyl complexes

We have synthesized several novel palladium allyl and 1,1-dimethylallyl complexes bearing different heteroditopic NHC–S ligands giving rise to a five-membered chelate ring with the metal center. We were able to synthesize some homoleptic bis-carbene allyl derivatives by taking advantage of the hemilability of the thioetheric sulfur. Attempts at preparing mixed bis-carbene complexes bearing two different heteroditopic carbenes (i.e. NHC–S and NHC–Py) simultaneously coordinated to the palladium center lead to a carbene transmetalation with the formation of a statistically distributed equilibrium mixture of the two pure homoleptic and of the mixed bis-carbene palladium allyl complexes in solution. In two different cases the rate of the equilibrium reaction was measured and a mechanistic hypothesis provided. Finally, we have determined the solid state structures of a complex bearing only one NHC–S heteroditopic carbene and of the bis-carbene (NHC–S, NHC–Py) palladium allyl derivatives.

Mechanistic Investigation of the Generation of a Palladium(0) Catalyst from a Palladium(II) Allyl Complex: A Combined Experimental and DFT Study

Cross-coupling reactions can be efficiently catalyzed using palladium complexes. The formation of low-coordinated, highly reactive Pd(0), which is believed to be the catalytic species, is critical. The mechanism of the reduction of a stable and readily available allyl Pd(II) complex into Pd(0) by a combination of K2CO3 and PhB(OH)2 has been studied. We report on the characterization of the associated reactive solution using a combination of density functional theory and experimental methods. First, the stoichiometric reaction of an (allyl)(phosphine)palladium(II) complex with K2CO3 was first investigated using trandem mass spectrometry. A palladium–carbonate complex could be characterized in the electrospray mass spectrum of the reactive solution. Gas-phase infrared spectra of mass-selected complexes have been recorded, giving further information on the coordination mode (κ1) of the carbonate ligand. This structural information derived from spectroscopy is critical because the relative energy of the two κ1- and κ2-carbonate isomers is difficult to determine theoretically, presumably because of the charge transfers at play between the carbonate and the palladium. Second, the product of the stoichiometric addition of PhB(OH)2 to this carbonate complex was investigated. Both 31P and 1H NMR data provide compelling evidence for the formation of the desired 14-electron Pd(0) complex.

Characterization of Two Stable Degradants of Palladium tBuXPhos Catalyst and a Unique Dearomatization Reaction

Two stable degradants of palladium tBuXPhos catalyst have been synthesized from tBuXPhos and Pd2(dba)3·CHCl3, isolated, and fully characterized. Complex 2 augments the known literature examples of palladacycles from this ligand family but is present as a rare four-membered-ring palladacycle, having activated the top ring of the ligand. Complex 3 is an unusual case of palladium-mediated dearomatization, whereby chloroform functionalizes the bottom ring, generating a palladium allyl complex. The mechanism is assigned to an electrophilic carbene attack where palladium directs attack of dichlorocarbene to the anti face of the bottom arene. The structures have been confirmed by NMR and single-crystal X-ray diffraction.

Synthesis, Structure, Ligand Dynamics, and Catalytic Activity of Cationic [Pd(η3-allyl)(κ2(E,N)-EN-chelate)]+(E = P, O, S, Se) Complexes

A series of cationic palladium allyl complexes of the type [Pd(η3-allyl)(κ2(E,N)-EN-chelate)]+ containing several heterodifunctional EN (E = P, O, S, Se) ligands based on N-(2-pyridinyl)aminophosph...

Palladium nanoparticles generated from allylpalladium chloride in situ: A simple and highly efficient catalytic system for Mizoroki–Heck reactions

The Mizoroki–Heck reactions of aryl halides catalyzed by palladium nanoparticles generated in situ from a simple allyl palladium precursor were investigated in argon. The high turnover numbers of 9,300,000 have been obtained with 4-bromobenzonitrile as substrate and 3500 with 4-nitrochlorobenzene. When the reaction was performed in air, a low yield was given, but it could be improved obviously by addition of PEG-400. The main reason was that inactive Pd(II) species could be rapidly reduced to the active Pd(0) by PEG. In other word, the existence of air and PEG led to a synergistic effect which the oxidation by air prevents the aggregation of Pd NPs and the reduction by PEG maintains the high activity of Pd(0) species.

Facile synthesis and reactivity study of mixed phosphane–isocyanide Pd(II) and Pd(0) complexes

The reaction between an equimolecular mixture of isocyanide CNR (CNR = di-methylphenyl isocyanide (DIC), tert-butyl isocyanide (TIC), triphenyl phosphane (PPh 3) and a dechlorinated solution of the palladium allyl dimers [Pd(η 3-allyl)Cl] 2 (allyl = 2-Meallyl, 1,1-Me 2allyl) in stoichiometric ratio yields the mixed derivative [Pd(η 3-allyl)(CNR)(PPh 3)] only. Apparently, the mixed derivative represents the most stable species among all the possible ones that might be formed under those experimental conditions. Theoretical calculations are in agreement with the experimental observation and the energy stabilization of the mixed species with respect to the homoleptic derivatives is traced back to an overall push– pull effect exerted by the isocyanide and the phosphane acting synergically. Similar behavior is observed in the case of the synthesis of the palladacyclopentadienyl complexes [Pd(C 4(COOMe) 4)(CNR)(PPh 3)] and of the palladium(0) olefin complexes whose synthesis invariably yields the mixed [Pd(η 2-olefin)(CNR)(PPh 3)] derivatives. The paper includes studies on the reactivity toward allylamination in the case of the palladium(II) allyl complexes. A diffractometric investigation on the solid state structures of four different palladium isocyanide–phosphane complexes is also included.

Synthesis and structures of π-allylpalladium(II) complexes containing bis(1,2,4-triazol-5-ylidene-1-yl)borate ligands. An unusual tetrahedral palladium complex

Four air stable, neutral π-allylpalladium(II) complexes containing bis(1,2,4-triazol-5-ylidene-1-yl)borate ligands [H 2B(RBTz) 2Pd( π-allyl)] (R = n Bu, 2a; t Bu, 2b; 2,6-diisopropylphenyl, 2c; cyclohexyl, 2d) have been prepared and characterized. The molecular structures of 2c and 2d have been confirmed by single-crystal X-ray diffraction. To our surprise, the coordination geometry about the palladium atom in 2d is distorted tetrahedron, in which the allyl group is nearly perpendicular to the plane defined by the Pd and the carbene C atoms. To our knowledge, such configuration has not been reported for a four-coordinated palladium allyl complex.

Cover Picture: Selective Palladium‐Loaded MIL‐101 Catalysts (Chem. Eur. J. 29/2011)

The loading of the porous coordination polymer MIL-101 with size-conform und undersized Pd nanoparticles (NPs) is reported by R. Kempe and J. Hermannsdörfer in their Full Paper on page 8071. Initially, metal–organic chemical vapor deposition is used to deposit an organopalladium compound, which can be reduced by H2 under different conditions (room temperature and 70 °C) to generate these differently sized Pd NPs. The hybrid system Pd@MIL-101 is a size-selective hydrogenation catalyst. The picture in the background is the Eremitage Bayreuth (photo by Jana Kempe).

Bioconjugates of Enantiomerically Pure Organopalladium Compounds by Metal‐Assisted Positional Selective Transesterifications at Palladium Enolates

The synthesis of enantiomerically pure palladatricyclo[4.1.0.0(2,4)]heptanes and their modification by an unprecedented and very efficient positional selective transesterification is described. The mild reaction conditions are most probably based on an acceleration of the transesterification due to assistance by the metal. This novel approach allows straightforward access to a large number of structurally diverse organometallic complexes. The functional groups on the newly installed ester moieties were modified by using standard peptide synthesis protocols, Sonogashira reactions, and nucleophilic substitution reactions. The cellular uptake of these organometallic species was traced by confocal microscopy and their biological activity was evaluated by using different cell lines. Inhibition of cell growth and induction of apoptotic cell death by these novel palladium heterocycles are equivalent to Cisplatin.

The mechanistic aspects of iron(III) porphyrin catalyzed oxidation reactions in mixed solvents

In the iron(III) porphyrin catalyzed oxidation reactions, the formation of various reactive intermediates have been observed to depend upon the nature of the catalyst, the oxidant and the solvent used for the study. The various iron(III) porphyrin catalysts such as F 20TPPFeCl, F 16TPPFeCl, F 12TPPFeCl and F 8TPPFeCl have been used in the present study to understand the effect of solvent system in the activation of the catalysts. As the terminal oxidant t-BuOOH has been used. It has been observed that acetonitrile contaminated with water activates all the catalysts. It has been noted that ∼9% of water in acetonitrile is the best solvent system for the activation of all the catalysts. The results obtained have been applied to successfully oxidize cyclohexene and cyclohexane by these oxidizing systems. It has also been observed that CH 3OH mixed with CH 2Cl 2 play a very important role in the activation of catalyst in hydroperoxide oxidizing system. The 33 ± 3% ratio of CH 3OH in CH 2Cl 2 acts as the most suitable solvent system to convert organopalladium compound 1a– c to 2a– c.

Ugi amine-derived P, N- and P, P-ligands with N-alkyltriethoxysilyl tethers: synthesis and evaluation of mesoporous silica-supported Pd complexes in asymmetric allylic substitution reactions

A facile approach to the synthesis of several types of chiral P, N- and P, P-ligands bearing triethoxysilyl groups starting from N, N-diemthyl-1-ferrocenylethylamine (Ugi amine) has been developed. Allyl palladium complexes of these ligands have been prepared, characterized, and grafted on mesoporous silica. The resulting silica-supported complexes have been shown to catalyze asymmetric allylic alkylation and aminations reactions with moderate enantioselectivities.

Pyridyl and pyridiniumyl β-diketones as building blocks for palladium(ii) and allyl–palladium(ii) isomers. Multinuclear NMR structural elucidation and liquid crystal behaviour

A series of novel β-diketone ligands bearing alkyloxyphenyl R = C6H4OCnH2n+1 and pyridyl substituents at 1 and 3 positions of the β-diketone core [HLR(n)py] have been prepared and structurally characterized. The corresponding β-diketonate complexes [Pd(κ2-LR(n)py)2] were isolated as a mixture of cis and trans isomers each of them exhibiting the ligand in a NO- or OO-bidentate coordinative mode which were established by multinuclear NMR studies (1H, 13C and 15N). However none of those characteristics were determining to achieve liquid-crystalline properties on the new complexes. By contrast reactions of the aforementioned ligands [HLR(n)py] or those containing the pyridiniumyl substituent [HLR(n)pyH]+ towards the allyl–palladium fragment were successful to afford ionic metallomesogenic materials isolated as PF6 salts of [Pd(η3-C3H5)(κ2-HLR(n)py)][PF6] (I) and [Pd(η3-C3H5)(κ2-LR(n)pyH)][PF6] (II) types. Structural characterization and mesomorphic properties were established by multinuclear NMR and the use of DSC and POM techniques. In complexes I and II the corresponding β-diketone ligands were coordinated as neutral bidentate NO-donor or zwitterionicOO-donor systems, respectively. Complexes of type II exhibit a range of the SmC mesophases significantly greater than that found in the type I.

Synthesis, characterization, dynamics and reactivity toward amination of η3-allyl palladium complexes bearing mixed ancillary ligands. evaluation of the electronic characteristics of the ligands from kinetic data

On the basis of an original protocol, we have synthesized several complexes of the type [Pd(η(3)-C(3)H(3)R(2))(LL')]ClO(4) (R = H, Me; L, L' = PPh(3), P(OEt)(3), 2,6-dimethylphenylisocyanide, t-butylisocyanide, 1,3-dimesitylimidazolidine, 1,3-dimesitylimidazol-2-ylidene). The complexes, some of which are completely new species, were fully characterized and their behaviour in solution was studied by means of (1)H NMR. The reactions of the complexes bearing the symmetric allyl moiety [Pd(η(3)-C(3)H(5))(LL')]ClO(4) with piperidine in the presence of the olefin dimethylfumarate were followed under kinetically controlled conditions. Formation of allyl-amine and of the palladium(0) derivatives [Pd(η(2)-dmfu)(LL'] was observed. The reaction rates k(2) proved to be strongly dependent on the ancillary ligand nature and allowed a direct comparison among the electronic characteristics of the ligands. The reactivity trend determined appears to be mainly influenced by the capability of the ancillary ligands in transferring electron density to the metal centre and consequently on the allyl fragment.

The Reaction of Carbon Dioxide with Palladium Allyl Bonds.

A family of palladium allyl complexes of the type bis(2-methylallyl)Pd(L) (L = PMe(3) (1), PEt(3) (2), PPh(3) (3) or NHC (4); NHC = 1,3-Bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene) have been prepared through the reaction of bis(2-methylallyl)Pd with the appropriate free ligand. Compounds 1-4 contain one η(1) and one η(3)-2-methylallyl ligand and 3 was characterized by X-ray crystallography. These complexes react rapidly with CO(2) at low temperature to form well defined unidentate palladium carboxylates of the type (η(3)-2-methylallyl)Pd(OC(O)C(4)H(7))(L) (L = PMe(3) (6), PEt(3) (7), PPh(3) (8) or NHC (9). The structure of 9 was elucidated using X-ray crystallography. The mechanism of the reaction of 1-4 with CO(2) was probed using a combination of experimental and theoretical (density functional theory) studies. The coordination mode of the allyl ligand is crucial and whereas nucleophilic η(1)-allyls react rapidly with CO(2), η(3)-allyls do not react. We propose that the reaction of η(1)-palladium allyls with CO(2) does not proceed via direct insertion of CO(2) into the Pd-C bond but through nucleophilic attack of the terminal olefin on electrophilic CO(2), followed by an associative substitution at palladium.

ChemInform Abstract: Phenylation of Cationic Allyl Palladium(II) Complexes by Tetraphenylborate. Synthesis of α-Diimine Olefin Palladium(0) Complexes and Mechanistic Aspects.

ChemInform Abstract: Phenylation of Cationic Allyl Palladium(II) Complexes by Tetraphenylborate. Synthesis of α-Diimine Olefin Palladium(0) Complexes and Mechanistic Aspects.

ChemInform Abstract: Synthesis of β-Resorcylic Macrolides via Organopalladium Chemistry. Application to the Total Synthesis of (S)-Zearalenone.

ChemInform Abstract: Synthesis of β-Resorcylic Macrolides via Organopalladium Chemistry. Application to the Total Synthesis of (S)-Zearalenone.

ChemInform Abstract: Direct Alkoxylation of Organopalladium Compounds Based on a New Type of C-O Coupling Mediated by Molybdenum Peroxides.

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ChemInform Abstract: Development of Organopalladium(IV) Chemistry: Fundamental Aspects and Systems for Studies of Mechanism in Organometallic Chemistry and Catalysis

Palladium is one of the most widely studied elements in organometallic chemistry, partly owing to the important role of palladium complexes in organic synthesis and catalysis. Since the initial report of PdIMe3(bpy), a rich and diverse organopalladium(IV) chemistry has evolved, including reaction systems that are ideal for studying topics of contemporary interest, e.g., mechanisms of oxidative addition and reductive elimination reactions, selectivity in reductive elimination, and studies of alkyl group exchange between metal centers. An account of each of these topics is presented here, followed by an assessment of some of the possible roles of Pd(IV) in organic synthesis and catalysis. Palladium(IV) chemistry has developed rapidly since 1986 to include the isolation of alkyl-, benzyl-, (eta-l-allyl)-, and arylpalladium complexes. It is providing new comparisons of structure, solution dynamics, and reactivity among the nickel triad elements and is providing new systems for mechanistic studies.

Mechanisms of the CO2 Insertion into (PCP) Palladium Allyl and Methyl σ-Bonds. A Kinetic and Computational Study

The reaction of the σ-bonded (PCP)Pd-Me complex (PCP = 2,6-bis[(di-tert-butylphosphino)methyl]phenyl) with CO2 is first-order in palladium and first-order in CO2 with a rate constant ks = 8.9 ± 0.8 M−1 s−1 at 353 K. Activation parameters are ΔH⧧ = 73 ± 7 kJ/mol and ΔS⧧ = −118 ± 19 J/K mol. Based on this and theoretical calculations we propose an SE2 mechanism where the coordinated methyl group attacks a completely noncoordinated carbon dioxide molecule in a bimolecular reaction. The PCPPd−crotyl complex was synthesized in an 65:35 E:Z mixture, and it was shown to react with CO2 to give the complex PCPPd−O(CO)CH(CH3)CHCH2 as a single isomer, where the former γ-carbon has been carboxylated. Theoretical calculations again suggest an SE2 mechanism with a noncoordinated carbon dioxide reacting with the terminal carbon on the allyl group, forming an η2-bonded olefin complex as an intermediate. The rearrangement of this intermediate to the O-bonded product is concluded to be rate determining. The crystal structure of PCPPd−O(CO)C(CH3)2CHCH2 is reported and as well as the solubility of carbon dioxide in benzene-d6 at different pressures and temperatures.