Involving Palladium Research Articles

Assembling phthalaldehyde and o-aminochalcones by Pd(II) catalyzed tandem reaction for synthesizing isoindoloindonole fluorophores

One-pot synthesis of isoindoloindonoles having various auxochrome, from easily available starting materials phthalaldehyde and o-aminochalcone, involving palladium(II) acetate catalysis in acetic acid is reported. A mechanism based on the formation of 2-aryl-2H-isoindol-1-ol (22) intermediate is proposed for the formation of the tetracyclic core of isoindoloindonoles. The possible intramolecular cyclization via the direct participation of the carbonyl vs the adjacent αβ-conjugated double bond was evaluated. Thus, a detailed study on how the presence of electronically different groups at the β position of chalcones affects the outcome of the cyclization reaction and the photophysical properties of the resulting isoindoloindolones is presented here. Furthermore, the core structure was determined using NMR, mass and single-crystal X-ray diffraction analysis.

Synthesis and Mechanistic Investigation of Bipyrazolo[1,5-a]pyridines via Palladium-Catalyzed Cross-Dehydrogenative Coupling of Pyrazolo[1,5-a]pyridines.

The synthesis of a range of 3,3'-bipyrazolo[1,5-a]pyridine derivatives via direct cross-dehydrogenative coupling of pyrazolo[1,5-a]pyridine precursors is herein presented. This simple and efficient methodology involving palladium(II)-catalyzed C-H bond activation showed good functional group tolerance and product yield (up to 94%). Through the mechanistic insights gained from both kinetic isotope effect experimental studies and density functional theory calculations, a plausible reaction mechanism was outlined. Furthermore, subsequent derivatizations of the resulting 7,7'-diaryl-3,3'-bipyrazolo[1,5-a]pyridines, executed by performing palladium-mediated ortho C-H bond activation followed by hypervalent iodine-induced chlorination, rendered this series of compounds more extended π-conjugation and twisted conformations. Our study on these bipyrazolo[1,5-a]pyridine-based luminogens provides new opportunities for tailor-made organic luminescent materials.

Halogen Bonding Involving Palladium(II) as an XB Acceptor

The half-lantern PdII2 complexes trans-(O,C)-[Pd(ppz)(μ-O∩N)]2 (1) and trans-(E,N)-[Pd(ppz)(μ-E∩N)]2 (E∩N is a deprotonated 2-substituted pyridine; E = S (2), Se (3); Hppz = 1-phenylpyrazole) were cocrystallized with 1,4-diiodotetrafluorobenzene (FIB) to give cocrystals 1·(FIB) and (2−3)·2(FIB); the parent complexes and the cocrystals were studied by X-ray crystallography. The crystal structure of trans-(O,C)-1·FIB is assembled mainly by the I···O halogen bonding (XB) to give the [@PdII2]O···I(areneF)I···O[@PdII2] linkage, while trans-(E,N)-(2–3)·2(FIB) are built by the joint action of I···Pd and I···E XBs, thus furnishing the PdII2···I(areneF)I···E[@PdII2] (E = S, Se) cluster. Detailed theoretical (DFT) studies with the application of the QTAIM, ELF, NBO, IGM, SAPT, and HSAB methods revealed several types of attractive noncovalent interactions in the cocrystals, namely, the I···Pd, I···E (E = O, S, Se), I···C, π–π-stacking, and π···F interactions. I···Pd is a rare type of XB involving the metal center as an XB acceptor, the Pd···Pd communication facilitating the I···Pd bonding. The I···Pd and I···E bonds are comparable in strength (the bond interaction energies being between −7 and −13 kcal/mol), but the former is controlled by dispersion forces, while the latter is mostly governed by an electrostatic term.

A novel magnetically recoverable palladium nanocatalyst containing organoselenium ligand for the synthesis of biaryls via Suzuki-Miyaura coupling reaction

In the present work, a novel heterogeneous catalytic system involving palladium(II) complex as moisture- and air-stable organoselenium ligand supported on Fe3O4 nanoparticles modified by SiO2/azidopropyltriemethoxy silane was designed, synthesized and characterized using various physicochemical methods inclusive VSM, EDX, FE-SEM, TG, XRD, and FT-IR spectroscopy. The catalytic activity of the synthesized magnetic nanocatalyst which named Fe3O4@SiO2-T-Se/Pd(II) was evaluated in Suzuki-Miyaura coupling reactions for the preparation of corresponding biaryls using diverse aryl halides and arylboronic acids. The mild reaction conditions, variety of substrate scope, good yield, low reaction time, high stability, utilization of organoselenium compound as an air and moisture insensitive ligand and its immobilization on solid support, and more importantly effortless recovery and recyclability of the catalyst up to seven consecutive runs with no remarkable change in its activity are some of the interesting features of this protocol that makes it more beneficial from both industrial and environmental points of view.

Activation of diverse carbon-heteroatom and carbon-carbon bonds via palladium(II)-catalysed β-X elimination.

Chemists' ability to synthesize structurally complex, high-value organic molecules from simple starting materials is limited by methods to selectively activate and functionalize strong alkyl C(sp3) covalent bonds. Recent activity has focused on the activation of abundant C-O, C-N and C-C bonds via a mechanistic paradigm of oxidative addition of a low-valent, electron-rich transition metal. This approach typically employs nickel(0), rhodium(I), ruthenium(0) and iron catalysts under conditions finely tuned for specific, electronically activated substrates, sometimes assisted by chelating functional groups or ring strain. By adopting a redox-neutral strategy involving palladium(II)-catalysed C-H activation followed by β-heteroatom/carbon elimination, we describe here a catalytic method to activate alkyl C(sp3)-oxygen, nitrogen, carbon, fluorine and sulfur bonds with high regioselectivity. Directed hydrofunctionalization of the resultant palladium(II)-bound alkene leads to formal functional group metathesis. The method is applied to amino acid upgrading with complete regioselectivity and moderate to high retention of enantiomeric excess. Low-strain heterocycles undergo strong-bond activation and substitution, giving ring-opened products.

Potentiometric Studies of Ternary Complexes Involving Palladium(II) with Fluoxetine Drug and Some Bio-ligands

The complex formation equilibria of [Pd(FLX)(H2O)2]2+, where FLX = the drug fluoxetine, with the bio-ligands (L), glycine, serine, cysteine, histidine, glycylglycine and glycylvaline, were studied and their formation constants were determined. Stoichiometries and stability constants of the complexes were determined at 25 °C and at constant 0.10 mol·dm−3 NaNO3 ionic strength. The concentration distribution of the complexes in solution was evaluated. Palladium(II) complexes were synthesized and characterized by the 1H NMR and electronic spectral studies. The structures consist of monomeric units in which the Pd(II) atoms exhibit square planar geometry.

Palladium(II)-Catalyzed Tandem Synthesis of Acenes Using Carboxylic Acids as Traceless Directing Groups.

A straightforward synthetic strategy for generating useful anthracene derivatives was developed involving palladium(II)-catalyzed tandem transformation with carboxylic acids as traceless directing groups. Carboxyl-directed C-H alkenylation, carboxyl-directed secondary C-H activation and rollover, intramolecular C-C bond formation, and decarboxylative aromatization are proposed as the key steps in the tandem reaction pathway. This novel synthetic route utilizes a broad range of substrates and provides a convenient synthetic tool that allows access to acenes.

Palladium(II)‐Catalyzed Tandem Synthesis of Acenes Using Carboxylic Acids as Traceless Directing Groups

AbstractA straightforward synthetic strategy for generating useful anthracene derivatives was developed involving palladium(II)‐catalyzed tandem transformation with carboxylic acids as traceless directing groups. Carboxyl‐directed C‐H alkenylation, carboxyl‐directed secondary C‐H activation and rollover, intramolecular C−C bond formation, and decarboxylative aromatization are proposed as the key steps in the tandem reaction pathway. This novel synthetic route utilizes a broad range of substrates and provides a convenient synthetic tool that allows access to acenes.

Triplet–Triplet Excitation Transfer in Palladium Porphyrin–Fullerene and Platinum Porphyrin–Fullerene Dyads

Covalently linked donor–acceptor dyads involving palladium(II) and platinum(II) porphyrins as triplet sensitizers and fullerene as an acceptor have been newly synthesized. These dyads were characterized by optical absorbance, emission, and electrochemical methods. In contrast to the earlier reported zinc(II) porphyrin and free-base porphyrin-based dyads of similar structures, photoinduced electron transfer from the short-lived singlet and long-lived triplet excited metalloporphyrin to the fullerene was not observed, although these processes are energetically possible according to the energy level diagrams. That is, diagnostic transient bands corresponding to MP•+ [M = Pd(II) or Pt(II)] in the 600–650-nm range and C60•– in the 1000-nm range were absent in the femtosecond and nanosecond transient absorption spectra. Interestingly, excited energy transfer from the triplet excited metalloporphyrin to the fullerene was witnessed in both palladium and platinum porphyrin-derived dyads by nanosecond transient absorption studies. Three solvents with different polarities were employed to visualize the medium effects. The determined rate of energy transfer, kEnT, was found to be higher for the PdP-based dyad than the PtP-based dyad in a given solvent and that the rates were higher for polar solvents than for nonpolar solvent. The present investigation demonstrates how the heavy-metal ion in the porphyrin cavity modulates photoinduced processes and the solvent-dependent kinetics of these events.

Pd(ii)–SDP-catalyzed enantioselective 5-exo-dig cyclization of γ-alkynoic acids: application to the synthesis of functionalized dihydofuran-2(3H)-ones containing a chiral quaternary carbon center

The Pd(II)-SDP-catalyzed first enantioselective intramolecular cyclization of α,α-disubstituted γ-alkynoic acids is described. This 5-exo-dig cyclization afforded dihydrofuran-2(3H)-ones bearing a chiral quaternary carbon center in excellent yields with enantioselectivities up to 71%. A mechanism involving palladium(II) species is proposed to rationalize the outcome of the reaction.

Insertion of Molecular Oxygen into a Palladium(II) Methyl Bond: A Radical Chain Mechanism Involving Palladium(III) Intermediates

The reaction of (bipy)PdMe(2) (1) (bipy = 2,2'-bipyridine) with molecular oxygen results in the formation of the palladium(II) methylperoxide complex (bipy)PdMe(OOMe) (2). The identity of the product 2 has been confirmed by independent synthesis. Results of kinetic studies of this unprecedented oxygen insertion reaction into a palladium alkyl bond support the involvement of a radical chain mechanism. Reproducible rates, attained in the presence of the radical initiator 2,2'-azobis(2-methylpropionitrile) (AIBN), reveal that the reaction is overall first-order (one-half-order in both [1] and [AIBN], and zero-order in [O(2)]). The unusual rate law (half-order in [1]) implies that the reaction proceeds by a mechanism that differs significantly from those for organic autoxidations and for the recently reported examples of insertion of O(2) into Pd(II) hydride bonds. The mechanism for the autoxidation of 1 is more closely related to that found for the autoxidation of main group and early transition metal alkyl complexes. Notably, the chain propagation is proposed to proceed via a stepwise associative homolytic substitution at the Pd center of 1 with formation of a pentacoordinate Pd(III) intermediate.

Critical Effect of Phosphane Ligands on the Mechanism of Carbon–Carbon Bond Formation Involving Palladium(II) Complexes: A Theoretical Investigation of Reductive Elimination from Square‐Planar and T‐Shaped Species

AbstractA theoretical ONIOM study has been carried out to understand the influence of phosphane ligands on the structure of Pd complexes and their reactivity in C–C bond formation. The calculations were performed for Me–Me reductive elimination with the ligands L = PPh3, PCy3, PMe3, PH3, and vinyl–vinyl, Ph–Ph, ethynyl–ethynyl, vinyl–Me, vinyl–Ph and vinyl–ethynyl couplings with L = PPh3 for [PdR2Ln] complexes (n = 1, 2). The calculations revealed critical changes in the reactivity of palladium complexes depending on the mechanism and ligand type. In the case of the standard four‐coordinate pathway (n = 2) the relative reactivity in carbon–carbon bond formation follows the order: L = PPh3 > PH3 > PCy3 > PMe3. However, for reductive elimination involving T‐shaped complexes by the ligand predissociation pathway (n = 1), the relative reactivity changes in the order: L = PCy3 > PPh3 > PH3 > PMe3. The theoretical study suggested that the steric effect of phosphane ligands has the largest impact on the structure of the initial palladium complexes, while the electronic effect is most influential on the transition states of C–C coupling in these complexes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Recent Advances Involving Palladium (II) Complexes for the Cancer Therapy

This review deals with the most important results published on the structures, reactivity and biomedical applications of palladium(II) complexes in the cancer therapy in the last five years. Biological mechanisms of palladium(II) complexes, especially of palladacycle compounds were boarded. Among the most recent advances in the studies involving correlations between chemical structures of palladacycle compounds and biological activities we mention i) the synthesis of metallic isomer complexes containing ligands derivatives of pyridine and imines in trans position having high antitumoral activities, ii) the development of cationic complexes with biological activity, iii) the discovery of preferential nucleotides for the intercalation of metallic complexes in the double helix of DNA of cancerous cells, configuring irreparable lesions in the macromolecule and iv) the interaction of metallic complexes with other biological molecules, like proteins and peptides, through terminal amine groups, carboxylate groups, imidazolic group of histidine and mostly with the thiol group of methionine. Some of these interactions are related to drug nefrotoxicity effect, which understanding is of fundamental importance. v) Novel ortho-cyclopalladated compounds synthesized from p-isopropylbenzaldehyde thiosemicarbazone have been described with specific cytotoxic properties in tumor cells sensitive to cis-diamminedichloroplatinum(II). The lysosomal cysteine proteinases cathepsins B and L have been implicated in a variety of pathological conditions, especially in diseases involving tissue-remodeling states, such as tumor metastasis. Our research group is studying inhibition of Cathepsin B by new palladacycle compounds derived from N, N-dimethyl-1-phenethylamine and having biphosphine ligands. New palladacycle compounds derived from N,N-dimethyl-1-phenethylamine and the ligand bis(diphenylphosphine)ferrocene were presented as effective antitumoral agents.

Equilibrium studies on binary and ternary complexes of palladium and cadmium with some nitrogen and oxygen donor ligands

The metal-ligand equilibrium reactions involving palladium(II), cadmium(II) and 1,2-dihydroxybenzene, 1-amino-2-naphthol-4-sulfonic acid, 8-amino-1-naphthol-3,6-disulfonic acid, o-aminophenol and 4,5-dihydroxybenzene-1,3-disulfonic acid have been studied in solution at 25 and at ionic strength of 0.10 M (KNO 3 ). Formation of the binary, ternary and hydroxo complex species has been inferred in solution and the corresponding equilibrium constants have been evaluated and discussed.

Equilibrium studies of complex formation involving palladium(II)-(1,3-diaminopropane) and cyclobutane dicarboxylic acid, DNA units, amino acids or peptides

Complex formation equillibria of [Pd(DAP)(H2O)2]2+ (DAP = 1,3-diaminopropane) with Cl−, OH−, cyclobutane dicarboxylic acid (CBDCA), amino acids, peptides and DNA unit constituents have been investigated. Stoichiometries and stability constants of the complexes were determined at 37°C and 0.16 M NaNO3 ionic strength. The results showed the formation of 1:1 complexes with amino acids and CBDCA. Peptides form both 1:1 complexes and the corresponding deprotonated amide species. DNA constituents form both 1:1 and 1:2 complexes. [Pd(DAP)(CBDCA)] was isolated and characterized. The concentration distribution of the complexes in solution was evaluated.

C-C-Bond Formation by the Palladium-Catalyzed Cycloisomerization/Dimerization of Terminal Allenyl Ketones: Selectivity and Mechanistic Aspects.

The scope of the palladium-catalyzed cyclization/dimerization of terminal allenyl ketones 1 to the 2,4-disubstituted furans 3 has been investigated. Simplified and improved conditions almost exclusively provided the dimer 3, accompanied by only traces of the easily separable monomer 2. The formation of an isomer of 3, the unconjugated ketone 4, was completeley suppressed. Under these mild conditions, besides the normal functional group tolerance known for palladium-catalyzed reactions, an interesting selectivity was observed with functional groups that are known to react either in palladium-catalyzed reactions or reactions catalyzed by other transition-metals. Thus aryl halides, terminal alkynes, 1,6-enynes, and alpha-allenic alcohols were tolerated. In the latter example the selective reaction of only one out of two different allenes was achieved. Mechanistic investigation indicated a Pd(II)/Pd(IV)-cycle involving palladium(II)-gamma-alkoxyvinylcarbene and furylpalladium(IV) hydride intermediates, although a second pathway for the formation of the dimer 3 which also involves Pd(IV)-intermediates like the 3,4-dimethylenepalladacyclopentane 23 and the 3-methylenepalladacyclobutane-like structure 15 (respectively 25) could not completely be excluded.

Sigma-Bond Metathesis Reactions Involving Palladium(II) Hydride and Methyl Complexes: A Theoretical Assessment.

sigma-Bond metathesis reactions of water and methanol with palladium hydride and methyl complexes to yield hydrogen and methane, respectively, have been studied using ab-initio molecular orbital methods at the second-order Møller-Plesset (MP2) perturbation level. The calculations show that such reactions are feasible and that they can in some instances be competitive with an oxidative addition/reductive elimination sequence. A key factor is the presence of an additional lone pair not engaged in the initial bonding of the metal atom with the reacting water or alcohol. Another important controlling factor is the respective charges of the reacting entities. The importance of the choice and of the disposition of the ancillary ligands in the coordination sphere is emphasized.

Photochromic reactions involving palladium(II) octabutoxynaphthalocyanine and molecular oxygen

The palladium complex of 1,6,10,15,19,24,28,33-octabutoxynaphthalocyanine, PdNc(OBu) 8 , is shown to undergo reversible addition of O 2 in benzene solution at ambient temperature (22±1 o C) during steady-state photolysis with an Ar ion laser. On the basis of 1 H-NMR and electronic absorption spectral evidence, the photoadduct is postulated to be an endoperoxide which has molecular oxygen added to one of the benzene rings at the sites a to the point of fusion with the tetrapyrrole ring, i.e., the positions bearing butoxy substituents. The formation of this product probably involves O 2 ( 1 Δ g )

Ternary α-Amino Acid-Palladium(II) Complexes with Ligand-Ligand Hydrogen Bonding

Abstract Histidinate(His)-containing ternary α-amino acid-palladium(II) complexes have been studied by synthetic and spectroscopic methods. The following complexes with imidazolate (im), l- and d-His, l-asparaginate (l-Asn), and l-glutaminate (l-Gln) have been isolated as crystals: [Pd(im)2]·0.5H2O; [Pd(l-His) (d-His)]·H2O; [Pd(l-Asn)-(l-His)]·2H2O; [Pd(l-Gln) (l-His)]·2H2O. The ternary systems involving palladium (II), l-His, and an amino acid l-AA with an OH or a CONH2 group (AA=Asn, Gln, serinate, threoninate, or homoserinate) exhibit absorption maxima at 300–311 nm with ε 310–440 and negative and positive circular dichroism (CD) peaks at 307–328 nm and ≈300 nm, respectively. 13C and 1H NMR spectra gave the amounts of the cis and trans isomers of Pd(His)-(AA) in solution and confirmed that Pd(His)(AA) has an N3O donor set in the coordination plane. Whereas the CD magnitude additivity holds for the ternary systems Pd (l-His)(B) with B=glycinate, l-alaninate, or l-valinate and the systems with l-histidine methyl ester (l-HisOMe) and l-AA, Pd(l-HisOMe)(l-AA), the magnitudes for Pd (l-His) (l-AA) deviate significantly from those estimated on the basis of the additivity. The CD magnitude anomaly as well as the geometrical and rotational isomer populations calculated from the 1H NMR spectra establishes the existence of the ligand-ligand interaction between the carboxylato group of His and the hydroxyl or amido group of AA in solution.

Catalytic reactions involving palladium(II) salts supported on amberlyst

Palladium(II) salts supported on Amberlyst-A21 have been characterised by elemental analyses, infrared and electronic spectra, thermal analysis and Mössbauer measurements. The catalytic properties of these materials have been studied for ester interchange involving vinylacetate and vinyl propionate and the reaction of ethylene and oxygen in acetic acid; the results are compared with a previous study.