Storable arylpalladium(II) reagents for alkene labeling in aqueous media

We show that arylpalladium(II) reagents linked to biotin and indocyanine dye residues can be prepared by decarboxylative palladation of appropriately substituted electron-rich benzoic acid derivatives. When prepared under the conditions described, these organometallic intermediates are tolerant of air and water, can be stored for several months in solution in dimethyl sulfoxide, and permit biotin- and indocyanine dye-labeling of functionally complex olefinic substrates in water by Heck-type coupling reactions.     

Development of a decarboxylative palladation reaction and its use in a Heck-type olefination of arene carboxylates

The development of a palladium-catalyzed decarboxylative coupling reaction of arene carboxylates with olefinic substrates is described. The optimized procedure for decarboxylative palladation employs Pd(O2CCF3)2 as catalyst (0.2 equiv) in the presence of Ag2CO3 (3 equiv) in the solvent 5% DMSO-DMF and proceeds at temperatures of 80-120 degrees C with a wide range of arene carboxylates and alkenes as substrates. The process is proposed to proceed by an initial Ar-SE reaction involving ipso attack of an electrophilic Pd(II) intermediate on an arene carboxylate to form an arylpalladium(II) species with loss of carbon dioxide. This intermediate is then proposed to react with an olefinic substrate by steps common to the Heck coupling process. Reoxidation of the liberated Pd(0) in situ is proposed to establish the catalytic cycle.     

C-H bond functionalizations with palladium(II): intramolecular oxidative annulations of arenes

Oxidative annulations for the synthesis of carbocycles were developed using a catalytic palladium(II) system. Indoles with pendant olefin tethers were oxidatively cyclized to form annulated products. Electron-rich aromatic systems were also investigated, culminating in the synthesis of benzofurans and dihydrobenzofurans by a similar protocol. These reactions were demonstrated to proceed by an initial C-H bond functionalization event, followed by olefin insertion and β-hydride elimination.     

Palladium‐Catalyzed Decarboxylative γ‐Arylation for the Synthesis of Tetrasubstituted Chiral Allenes

An enantiospecific palladium‐catalyzed decarboxylative coupling of acyclic β,γ‐alkynoic acids with various aryl iodides to chiral tetrasubstituted allenes is described. The coupling reaction comprises a decarboxylative γ‐palladation of α,α‐disubstituted carboxylic acids to provide the tetrasubstituted allenes with complete point‐to‐axial chirality transfer in excellent yields.     

Insertion of molecular oxygen into a palladium(II) hydride bond

Insertion of molecular oxygen into a palladium(II) hydride bond to form an (eta1-hydroperoxo)palladium(II) complex is reported. The hydroperoxo palladium(II) product has been crystallographically characterized. A second-order rate law (first-order in palladium and first-order in oxygen) is observed for the reaction and a large kinetic isotope effect implicates Pd-H bond cleavage in the rate-determining step. The results of studies with radical inhibitors and light suggest that the reaction does not proceed by a radical chain mechanism.     

A mechanistic study on modern palladium catalyst precursors as new gateways to Pd(0) in cationic Heck reactions

Electrospray ionization mass spectrometry (ESI-MS) was used as a means to directly identify catalytic cationic organopalladium species in ligand-controlled Heck reactions involving electron-rich olefins and different Pd-sources. In these high-temperature Heck arylations, the oxidative addition intermediates were observed as bidentate ligand chelated cationic aryl palladium species, suggesting that the used ligand attaches to the metal center at the very beginning of the catalytic cycle. This was also in agreement with the obtained regioisomeric profile of the isolated products. The investigation supports the standard Pd(0)/Pd(II) Heck mechanism and provides further insight regarding the conceivable composition of fundamental Pd(II) intermediates in an ongoing Heck reaction.     

Pd/PR3-catalyzed cross-coupling of aromatic carboxylic acids with electron-deficient polyfluoroarenes via combination of decarboxylation with sp2 C-H cleavage

By using Pd(TFA)(2)/PCy(3) as a catalyst, a broad range of aromatic carboxylic acids, including heteroaromatic carboxylic acids, efficiently underwent decarboxylative coupling with an array of polyfluoroarenes in the presence of stoichiometric amount of silver salts to generate biaryls. Silver salts were adjusted to the reactivity of aromatic carboxylic acids to efficiently suppress the protodecarboxylation and therefore improve decarboxylative cross-couplings. It was established that the palladium complex containing the PCy(3) ligand was capable of catalyzing the decarboxylation of electron-rich aromatic carboxylic acids, and silver salts promoted the decarboxylation of both electron-rich and -deficient ones. To explain the two different decarboxylation processes, two possible reaction pathways are proposed, which were further supported by the facts that the stoichiometric arylpalladium complex can directly arylate pentafluorobenzene in the presence of PCy(3) and the arylpalladium complex can catalyze the decarboxylative coupling of 2,4-dimethoxybenzoic acid with pentafluorobenzene. The kinetic isotope effect of 4.0 clearly showed that the C-H bond cleavage of polyfluoroarenes is involved in the rate-determining step.     

Palladium‐Catalyzed Oxidative Difunctionalization of Alkenes with α‐Carbonyl Alkyl Bromides Initiated through a Heck‐type Insertion: A Route to Indolin‐2‐ones

The oxidative interception of various σ‐alkyl palladium(II) intermediates with additional reagents for the difunctionalization of alkenes is an important research area. A new palladium‐catalyzed oxidative difunctionalization reaction of alkenes with α‐carbonyl alkyl bromides is described, in which the σ‐alkyl palladium(II) intermediate is generated through a Heck insertion and trapped using an aryl C(sp²)? H bond. This method can be applied to various α‐carbonyl alkyl bromides, including primary, secondary, and tertiary α‐bromoalkyl esters, ketones, and amides.     

Aryl to aryl palladium migration in the Heck and Suzuki coupling of o-halobiaryls

A novel 1,4-palladium migration between the o- and o'-positions of biaryls has been observed in organopalladium intermediates derived from o-halobiaryls. The organopalladium intermediates generated by this migration have been trapped either by a Heck reaction employing ethyl acrylate or by Suzuki cross-coupling using arylboronic acids. This palladium migration can be activated or deactivated by choosing the appropriate reaction conditions. Chemical and computational evidence supports the presence of an equilibrium that correlates with the C-H acidity of the available arene positions.     

Palladium(II)‐Catalyzed Enantioselective Arylation of Unbiased Methylene C(sp3)−H Bonds Enabled by a 2‐Pyridinylisopropyl Auxiliary and Chiral Phosphoric Acids

Enantioselective functionalizations of unbiased methylene C(sp³)?H bonds of linear systems by metal insertion are intrinsically challenging and remain a largely unsolved problem. Herein, we report a palladium(II)‐catalyzed enantioselective arylation of unbiased methylene β‐C(sp³)?H bonds enabled by the combination of a strongly coordinating bidentate PIP auxiliary with a monodentate chiral phosphoric acid (CPA). The synergistic effect between the PIP auxiliary and the non‐C₂‐symmetric CPA is crucial for effective stereocontrol. A broad range of aliphatic carboxylic acids and aryl bromides can be used, providing β‐arylated aliphatic carboxylic acid derivatives in high yields (up to 96 %) with good enantioselectivities (up to 95:5 e.r.). Notably, this reaction also represents the first palladium(II)‐catalyzed enantioselective C?H activation with less reactive and cost‐effective aryl bromides as the arylating reagents. Mechanistic studies suggest that a single CPA is involved in the stereodetermining C?H palladation step.     

Insights into the Heck reaction with PCP pincer palladium(II) complexes

[reaction: see text] The Heck reaction of phenyl halides with styrene using a series of related PCP pincer palladium(II) complexes was studied in order to evaluate the effect of ligand structure and electronics on the catalytic activity and to investigate the nature of the catalyst species. We suggest these pincer complexes are precatalysts for highly active forms of metallic palladium. This conclusion is based on kinetic studies (induction periods, sigmoidal kinetics), Hg drop tests, quantitative poisoning experiments, and NMR studies.     

Palladium‐Catalyzed Annulation of Diarylamines with Olefins through C?H Activation: Direct Access to N‐Arylindoles

A palladium‐catalyzed dehydrogenative coupling between diarylamines and olefins has been discovered for the synthesis of substituted indoles. This intermolecular annulation approach incorporates readily available olefins for the first time and obviates the need of any additional directing group. An ortho palladation, olefin coordination, and β‐migratory insertion sequence has been proposed for the generation of olefinated intermediate, which is found to produce the expected indole moiety.     

Regioselectivity of styrene carbonylation in the presence of palladium(II) complexes

The influence of a catalyst on regioselectivity of styrene carbonylation in ionic liquids catalyzed by palladium(II) complexes was studied. The mechanism of the migration insertion of styrene at the palladium-hydrogen bond determining the regioselectrivity of the whole reaction, which is treated as electrophilic addition to the multiple bond of styrene, was studied. A study of the structure of intermediate complexes formed at this stage was performed using the B3LYP method of density functional theory.     

Mechanism of oxidation of xylenes with Pd(II) complexes

Conclusions Oxidation of xylenes with Pd(II) complexes in acidic medium is possible according to two pathways: a) electrophilic substitution in the arene ring with the formation of an organometalic intermediate and b) one-electron transfer from the arene to the palladium atom with the intermediacy of the arene cation-radical. The relative contribution of each pathway depends on the nature of the arene and on the acidity of the medium.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2167–2174, October, 1985.     

Heck-type arylation of 2-cycloalken-1-ones with arylpalladium intermediates formed by decarboxylative palladation and by aryl iodide insertion

[reaction: see text] A palladium-catalyzed decarboxylative arylation reaction was shown to produce Heck-type coupling products using a number of different arene carboxylic acid and 2-cycloalken-1-one substrates. The more conventional Heck coupling of an aryl iodide and a 2-cycloalken-1-one reactant was also briefly explored for comparison, where it was found that phosphine-free (Jeffery) conditions afforded the highest yield of product.     

Kinetic investigation of a PC(sp)P pincer palladium (II) complex in the Heck reaction

An investigation of the kinetics of the Heck reaction between 4-iodoanisole and styrene catalysed by {cis-1,3-bis[(di-tert-butylphosphino)methyl]-cyclohexane} palladium (II) iodide (1) has been performed in DMF-d₇ solution. Based on mercury poisoning experiments a heterogeneous palladium catalyst formed from the pre-catalyst is proposed. Saturation behaviour with respect to the olefin concentration suggests a mechanism consisting of a pre-equilibrium association of the olefin followed by a rate determining reaction with aryl halide. The equilibrium constant for the olefin association, K₁, and the rate constant for the subsequent oxidative addition step, k₂, were determined to (5.7 ± 2.5) × 10⁻³ and 18.4 ± 2.7 M⁻¹ s⁻¹, respectively.     

Catalytic oxidation of Fe(II) aqua ions with atmospheric oxygen in the presence of Pd(II) tetraaqua complex and an aromatic compound

The effect of a series of aromatic compounds (toluene, benzyl alcohol, benzonitrile, phenylacetonitrile, and o-cyanotoluene) in a concentration of 0.01 M on the oxidation of Fe(II) aqua ions with oxygen in the presence of Pd(II) tetraaqua complex at 25–70°C was revealed. In the presence of an aromatic compound, palladium black is not formed, which results in an increased yield of Fe(III) in the Pd-catalyzed oxidation of Fe(II) with oxygen in a perchloric acid medium. A scheme involving the formation of a complex of palladium species in an intermediate oxidation state with arene and molecular oxygen was suggested.     

Synthesis, structure, theoretical studies, and Ligand exchange reactions of monomeric, T-shaped arylpalladium(II) halide complexes with an additional, weak agostic interaction

A series of monomeric arylpalladium(II) complexes LPd(Ph)X (L = 1-AdPtBu2, PtBu3, or Ph5FcPtBu2 (Q-phos); X = Br, I, OTf) containing a single phosphine ligand have been prepared. Oxidative addition of aryl bromide or aryl iodide to bis-ligated palladium(0) complexes of bulky, trialkylphosphines or to Pd(dba)2 (dba = dibenzylidene acetone) in the presence of 1 equiv of phosphine produced the corresponding arylpalladium(II) complexes in good yields. In contrast, oxidative addition of phenyl chloride to the bis-ligated palladium(0) complexes did not produce arylpalladium(II) complexes. The oxidative addition of phenyl triflate to PdL2 (L = 1-AdPtBu2, PtBu3, or Q-phos) also did not form arylpalladium(II) complexes. The reaction of silver triflate with (1-AdPtBu2)Pd(Ph)Br furnished the corresponding arylpalladium(II) triflate in good yield. The oxidative addition of phenyl bromide and iodide to Pd(Q-phos)2 was faster than oxidative addition to Pd(1-AdPtBu2)2 or Pd(PtBu3)2. Several of the arylpalladium complexes were characterized by X-ray diffraction. All of the arylpalladium(II) complexes are T-shaped monomers. The phenyl ligand, which has the largest trans influence, is located trans to the open coordination site. The complexes appear to be stabilized by a weak agostic interaction of the metal with a ligand C-H bond positioned at the fourth-coordination site of the palladium center. The strength of the Pd.H bond, as assessed by tools of density functional theory, depended upon the donating properties of the ancillary ligands on palladium.     

Mechanism of the cobalt oxazoline palladacycle (COP)-catalyzed asymmetric synthesis of allylic esters

The catalytic enantioselective S(N)2' displacement of (Z)-allylic trichloroacetimidates catalyzed by the palladium(II) complex [COP-OAc](2) is a broadly useful method for the asymmetric synthesis of chiral branched allylic esters. A variety of experiments aimed at elucidating the nature of the catalytic mechanism and its rate- and enantiodetermining steps are reported. Key findings include the following: (a) the demonstration that a variety of bridged-dipalladium complexes are present and constitute resting states of the COP catalyst (however, monomeric palladium(II) complexes are likely involved in the catalytic cycle); (b) labeling experiments establishing that the reaction proceeds in an overall antarafacial fashion; (c) secondary deuterium kinetic isotope effects that suggest substantial rehybridization at both C1 and C3 in the rate-limiting step; and (d) DFT computational studies (B3-LYP/def2-TZVP) that provide evidence for bidentate substrate-bound intermediates and an anti-oxypalladation/syn-deoxypalladation pathway. These results are consistent with a novel mechanism in which chelation of the imidate nitrogen to form a cationic palladium(II) intermediate activates the alkene for attack by external carboxylate in the enantiodetermining step. Computational modeling of the transition-state structure for the acyloxy palladation step provides a model for enantioinduction.     

C-H Bond Functionalizations with Palladium(II): Intramolecular Oxidative Annulations of Arenes

Oxidative annulations for the synthesis of carbocycles were developed using a catalytic palladium(II) system. Indoles with pendant olefin tethers were oxidatively cyclized to form annulated products. Electron-rich aromatic systems were also investigated, culminating in the synthesis of benzofurans and dihydrobenzofurans by a similar protocol. These reactions were demonstrated to proceed by an initial C-H bond functionalization event, followed by olefin insertion and β-hydride elimination.