Palladium acetate complexes bearing chelating N-heterocyclic carbene (NHC) ligands: Synthesis and catalytic oxidative homocoupling of terminal alkynes

The imidazolium salts 1,1′-dibenzyl-3,3′-propylenediimidazolium dichloride and 1,1′-bis(1-naphthalenemethyl)-3,3′-propylenediimidazolium dichloride have been synthesized and transformed into the corresponding bis(NHC) ligands 1,1′-dibenzyl-3,3′-propylenediimidazol-2-ylidene (L₁) and 1,1′-bis(1-naphthalenemethyl)-3,3′-propylenediimidazol-2-ylidene (L₂) that have been employed to stabilize the Pd^II complexes PdCl₂(κ²-C,C-L₁) (2a) and PdCl₂(κ²-C,C-L₂) (2b). Both latter complexes together with their known homologous counterparts PdCl₂(κ²-C,C-L₃) (1a) (L₃ = 1,1′-dibenzyl-3,3′-ethylenediimidazol-2-ylidene) and PdCl₂(κ²-C,C-L₄) (1b) (L₄ = 1,1′-bis(1-naphthalenemethyl)-3,3′-ethylenediimidazol-2-ylidene) have been straightforwardly converted into the corresponding palladium acetate compounds Pd(κ¹-O-OAc)₂(κ²-C,C-L₃) (3a) (OAc = acetate), Pd(κ¹-O-OAc)₂(κ²-C,C-L₄) (3b), Pd(κ¹-O-OAc)₂(κ²-C,C-L₁) (4a), and Pd(κ¹-O-OAc)₂(κ²-C,C-L₂) (4b). In addition, the phosphanyl-NHC-modified palladium acetate complex Pd(κ¹-O-OAc)₂ (κ²-P,C-L₅) (6) (L₅ = 1-((2-diphenylphosphanyl)methylphenyl)-3-methyl-imidazol-2-ylidene) has been synthesized from corresponding palladium iodide complex PdI₂(κ²-P,C-L₅) (5). The reaction of the former complex with p-toluenesulfonic acid (p-TsOH) gave the corresponding bis-tosylate complex Pd(OTs)₂(κ²-P,C-L₅) (7). All new complexes have been characterized by multinuclear NMR spectroscopy and elemental analyses. In addition the solid-state structures of 1b·DMF, 2b·2DMF, 3a, 3b·DMF, 4a, 4b, and 6·CHCl₃·2H₂O have been determined by single crystal X-ray structure analyses. The palladium acetate complexes 3a/b, 4a/b, and 6 have been employed to catalyze the oxidative homocoupling reaction of terminal alkynes in acetonitrile chemoselectively yielding the corresponding 1,4-di-substituted 1,3-diyne in the presence of p-benzoquinone (BQ). The highest catalytic activity in the presence of BQ has been obtained with 6, while within the series of palladium-bis(NHC) complexes, 4b, featured with a n-propylene-bridge and the bulky N-1-naphthalenemethyl substituents, revealed as the most active compound. Hence, this latter precursor has been employed for analogous coupling reaction carried out in the presence of air pressure instead of BQ, yielding lower substrate conversion when compared to reaction performed in the presence of BQ. The important role of the ancillary ligand acetate in the course of the catalytic coupling reaction has been proved by variable-temperature NMR studies carried out with 6 and 7′ under catalytic reaction conditions.     

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.     

Palladium(II) catalyzed carbonylative dimerization of allenyl ketones: efficient synthesis of difuranylketones

Palladium(II) catalyzed carbonylation of 1,2-allenyl ketones 1 in the presence of p-benzoquinone (1 equiv) under a CO atmosphere (balloon) afforded difuranylketones 4 in moderate to good yields. Mechanistically, the electron-withdrawing nature of the acyl group should enhance the electrophilicity of the acylpalladium species B, and thus promote the oxypalladation of an additional molecule of 1, leading to the difuranyl ketone 4.     

Palladium - catalyzed dimerization of propene

Summary The insertion of the previously coordinated second molecule of propene into the Pd-C bond is the rate-determining step in the dimerization of propene with the Pd(acac)₂ + BF₃OEt₂ catalyst system. Tetracoordinated square planar Pd hydrides bearing two vacant coordination sites are likely to be catalytically active species in the dimerization of propene. The simplicity of the catalyst system composition might be of industrial importance.     

Enzyme-mediated oxidative dimerization reactions of cyano-resveratrol analogues

The synthesis and oxidative coupling reactions of two cyano-resveratrol analogues were studied. An 8-5-coupled dihydrobenzofuran-type dimer and an 8-8-coupled diastereoisomeric indane mixture were prepared. The cyano groups of the resveratrol analogues had almost no influence on the regio- or stereoselectivity of the coupling reactions of the two precursors.     

Palladium catalyzed Suzuki-Miyaura coupling with aryl chlorides using a bulky phenanthryl N-heterocyclic carbene ligand

A novel bis-phenanthryl N-heterocyclic carbene (NHC) based palladium acetate catalyst was effective for the coupling of various aryl and vinyl chlorides with organoboron compounds. N,N-Bis-(2,9-dicyclohexyl-10-phenanthryl)-4,5-dihydroimidazolium chloride 8 (H₂ICP·HCl) with Pd(OAc)₂ and KF·18-c-6 in THF at room temperature gave Suzuki-Miyaura coupling of aryl and vinyl chorides, including unactivated and di-ortho substituted substrates in high yields. Hindered tri- and tetra-ortho substituted products were also efficiently produced. Benzyl chloride was also found to be a useful coupling partner and trimethylboroxine was used to give methylated products. The effect of ligand, base, temperature, solvent, and reaction time are reported along with various substrates including halides and triflates.     

Intramolecular arylation of benzimidazoles via Pd(II)/Cu(I) catalyzed cross-dehydrogenative coupling

Electron poor benzimidazole substrates were arylated via an intramolecular cross-dehydrogenative coupling (CDC) reaction. These CDC reactions were catalyzed by a Pd(II)/Cu(I) catalyst system, capable of producing moderate yields on a large library of substrates. The substrate scope consisted of tethered arene-benzimidazoles that upon coupling, produced a fused polycyclic motif.     

Acid catalyzed stereoselective rearrangement and dimerization of flavenes: synthesis of dependensin

Appropriately substituted flavenes undergo stereoselective rearrangement and dimerization when treated with methanolic hydrochloric acid to give benzopyranobenzopyrans. A rationale for the rearrangement is proposed. This synthetic methodology has been used for a high yield synthesis of the natural product dependensin.     

Palladium catalyzed N-alkylation of amines with alcohols

An iron oxide immobilized palladium catalyst was prepared for the N-alkylation of amines with alcohols under base and organic ligand free conditions. Applying the optimized reaction conditions, the coupling reactions of amines and alcohols with various structures could be realized with up to 99% isolated yields. The catalysts were studied by XRD, BET, and XPS and the mechanism was studied by DFT calculations.     

Palladium catalyzed alkylation of indole via aliphatic C-H bond activation of tertiary amine

Alkylation of indoles has been achieved via Pd-catalyzed aliphatic C-H bond activation of tertiary amine coupling with indole followed by C-N bond cleavage and subsequent addition of indole. This method involves the migration of alkane chain from tertiary amine to indole.     

Cu(II)-mediated oxidative dimerization of 2-phenylpyridine derivatives

A Cu(II)/I₂-mediated C-H bond activation is described. A variety of 2-phenylpyridine derivatives are oxidatively dimerized at the ortho-position of the phenyl ring in which a net loss of two hydrogen atoms results in the formation of a biaryl compound via a double C-H activation/C-C bond-forming process. Moderate functional group tolerance was observed on both the aryl and the pyridyl rings. A single electron transfer (SET) or electrophilic metalation process for iodination followed by Ullmann coupling of the intermediate iodinated product is proposed as the operating mechanism for the dimerization process.     

Palladium catalyzed allylation of nitroalkanes with allyl chlorides

The palladium catalyzed synthesis of allylnitro compounds from allyl chlorides and nitroalkanes is described.

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Palladium-katalysierte Allylierung von Nitroalkanen mit Allylchlorden (Kurze Mitteilung)

Zusammenfassung Die Palladium-katalysierte Darstellung von Allylverbindungen aus Allylchloriden und Nitroalkanen wird beschrieben.

     

Palladium catalyzed cross-dimerization of terminal acetylenes and acrylates

A palladium catalyzed oxidative cross-coupling reaction between terminal alkynes and acrylates was developed. In the presence of palladium catalyst and copper(II) acetate as oxidant, the cross dimerization products with were formed. This protocol provides a novel method to construct the tetrasubstituted functionalized alkenes.     

Sulfinate as cocatalyst 2 : Palladium catalyzed dimerization of butadiene with acyloins

Toluenesulfinate serves as a cocatalyst for the selective formation of α-hydroxy α-octa-2,7-dienyl ketones (2, the products of C-C bond formation) in the reaction of palladium catalyzed dimerization of 1,3-butadiene with acyloins, while the same reaction using triphenylphosphine, instead of toluenesulfinate, as a cocatalyst provides a mixture of 2 and 2-oxy octa-2′,7′-dienyl ethers (3, the products of C-O bond formation).     

Oxidant-controlled regioselectivity in the oxidative arylation of N-acetylindoles

N-Acetylindoles can be oxidatively coupled with arenes such as benzene or pentafluorobenzene in dioxane. The use of Cu(OAc)₂ as the stoichiometric oxidant produces selective arylation at the 3-position of indole while AgOAc produces selective arylation at indole’s 2-position.     

Intramolecular oxidative coupling of 3-indolylarylketones with Pd(II)-catalysis under air: convenient access to indenoindolones

A Pd-catalyzed method has been developed for the intramolecular oxidative coupling of 3-indolylarylketones under open air as terminal oxidant toward the synthesis of indeno[1,2-b]indol-10(5H)-ones. This reaction represents an intramolecular coupling with dual activation of C-H bonds for electron-deficient arenes, while such reactions are common for electron-rich arenes. Pd(II)-catalysis with pivalic acid as co-catalyst has been found to be crucial. The reaction undergoes without indole N-H-protection.     

Palladium catalyzed thiol cross-coupling of cystein derivatives with aryl and alkenyl halides

Palladium catalyzed cystein thiol cross-coupling reactions with aryl and vinyl halides have been investigated: Pd₂dba₃-CHCl₃ and dppf are the key choice in these reactions. The role of the base in these reactions was also questioned: it has been shown that base can be replaced by an HX-scavenger such as propylene oxide.     

Sulfinate as cocatalyst 3 : Palladium catalyzed dimerization of butadiene with acylamino ketones

A catalyst generated in situ from PdCl₂ (1 mole) and p-CH₃C₆H₄SO₂-Na·4H₂O (5 mole) promotes the dimerization of butadiene with the incorporation of α-acylamino ketones (1a～1h) to provide α-acylamino α-(2,7-octadienyl) ketones (2a～2h) in good yields. This new type of C-C bond forming reaction is not effected by the catalyst of Pd(PPh₃)₄ except for one example (with 1e).     

Generally applicable and efficient oxidative Heck reaction of arylboronic acids with olefins catalyzed by cyclopalladated ferrocenylimine under base- and ligand-free conditions

A mild and efficient method for oxidative Heck-type reaction of arylboronic acids with olefins catalyzed by cyclopalladated ferrocenylimine was developed. The results represent the first examples involving the palladacycle as the catalyst for such couplings. Moreover, the catalytic system could tolerate various functional groups, such as F, Cl, Br, NO₂ and CH₃O, and the substrates could be extended to a wide range of olefins from acrylic esters, α,β-unsaturated ketones to alkenes. Furthermore, the olefination could proceed well under base- and ligand-free conditions and employ oxygen as the environmentally benign oxidant.     

Semiconductor-mediated oxidative dimerization of 1-naphthols with dioxygen and O-demethylation of the enol-ethers by SnO2 without dioxygen

Oxidative dimerization of 1-naphthols 1 with dioxygen in the presence of a semiconductor, such as SnO₂, ZrO₂, or activated charcoal, as a catalytic mediator takes place selectively to give the corresponding 2,2′-binaphthols 2 or 2,2′-binaphthyl-1,1′-quinones 3 in excellent yields without light irradiation. The catalytic activity of SnO₂ could be fully restored by appropriate reactivation treatment after the oxidation. In addition, SnO₂ without dioxygen catalyzes selective O-demethylation of the enol-ethers 3 to 4.