Coupling Of Aryl Bromides Research Articles

A synthetic, catalytic and theoretical investigation of an unsymmetrical SCN pincer palladacycle

The SCN ligand 2-{3-[(methylsulfanyl)methyl]phenyl}pyridine, 1, has been synthesized starting from an initial Suzuki–Miyaura (SM) coupling between 3-((hydroxymethyl)phenyl)boronic acid and 2-bromopyridine. The C–H activation of 1 with in situ formed Pd(MeCN)4(BF4)2 has been studied and leads to a mixture of palladacycles, which were characterized by X-ray crystallography. The monomeric palladacycle LPdCl 6, where L-H = 1, has been synthesized, and tested in SM couplings of aryl bromides, where it showed moderate activity. Density functional theory and the atoms in molecules (AIM) method have been used to investigate the formation and bonding of 6, revealing a difference in the nature of the Pd–S and Pd–N bonds. It was found that S-coordination to the metal in the rate determining C–H bond activation step leads to better stabilization of the Pd(II) centre (by 13–28 kJ mol−1) than with N-coordination. This is attributed to the electron donating ability of the donor atoms determined by Bader charges. The AIM analysis also revealed that the Pd–N bonds are stronger than the Pd–S bonds influencing the stability of key intermediates in the palladacycle formation reaction pathway.

Ru-Catalyzed C-H Arylation of Fluoroarenes with Aryl Halides.

Although the ruthenium-catalyzed C–H arylation of arenes bearing directing groups with haloarenes is well-known, this process has never been achieved in the absence of directing groups. We report the first example of such a process and show that unexpectedly the reaction only takes place in the presence of catalytic amounts of a benzoic acid. Furthermore, contrary to other transition metals, the arylation site selectivity is governed by both electronic and steric factors. Stoichiometric and NMR mechanistic studies support a catalytic cycle that involves a well-defined η6-arene-ligand-free Ru(II) catalyst. Indeed, upon initial pivalate-assisted C–H activation, the aryl-Ru(II) intermediate generated is able to react with an aryl bromide coupling partner only in the presence of a benzoate additive. In contrast, directing-group-containing substrates (such as 2-phenylpyridine) do not require a benzoate additive. Deuterium labeling and kinetic isotope effect experiments indicate that C–H activation is both reversible and kinetically significant. Computational studies support a concerted metalation–deprotonation (CMD)-type ruthenation mode and shed light on the unusual arylation regioselectivity.

Chelate Palladium(II) Complexes with Saturated N-Phosphanyl-N-Heterocyclic Carbene Ligands: Synthesis and Catalysis

N-Phosphanyl-N-heterocyclic carbenes (NHCPs) featuring a saturated imidazolin-2-ylidene or tetrahydropyrimid-2-ylidene ring have been synthesized and characterized. The free carbenes exhibit good stability and can be stored in the solid state for months at ambient temperature without decomposition. Contrary to imidazoline-based NHCPs, which decompose by ring opening, N-phosphanyltetrahydropyrimid-2-ylidenes isomerize to 2-phosphanyl tetrahydropyrimidines upon heating. The free carbenes are capable of acting as chelating ligands toward palladium(II), forming very stable mononuclear complexes that have been structurally characterized. The catalytic potential of the complexes has been preliminarily assessed in cross-coupling reactions, most notably in the Suzuki coupling of aryl chlorides, where these complexes display promising activity, and in the copper- and amine-free Sonogashira coupling of aryl bromides.

ChemInform Abstract: Nickel‐Catalyzed Reductive Coupling of Aryl Bromides with Tertiary Alkyl Halides.

AbstractElectron‐deficient aryl bromides react with tertiary alkyl bromides under mild conditions in the presence of pyridine or DMAP and imidazolium salts.

Microwave-assisted copper- and palladium-catalyzed sonogashira-type coupling of aryl bromides and iodides with trimethylsilylacetylene

An efficient and rapid method was developed for the synthesis of 1-aryl-2-(trimethylsilyl)acetylene. Copper and palladium-catalyzed sonogashira-type coupling of trimethylsilylacetylene and aryl bromides or iodides in the presence of triethylamine as base under microwave irradiation in acetonitrile afforded the desired 1-aryl-2-(trimethylsilyl)acetylene. The use of microwave was found to significantly improve the reaction yield and shorten the reaction time.

ChemInform Abstract: Reactivity of para‐Substituted Fluorobenzenes in Palladium‐Catalyzed Intermolecular Direct Arylations.

AbstractThe arylation proceeds regioselectively in ortho‐position to the fluorine substituent.

Geminal Cross-Coupling of 1,1-Dibromoolefins Facilitating Multiple Topological π-Conjugated Tetraarylethenes

The cross-coupling reactions have been used in C–C bond formation which can be used extensively in optoelectronic materials for organic light-emitting diodes, organic photovoltaics, and chemical biosensing. Here, we report 2-fold geminal C–C bond formation at 1,1-dibromoolefins via cross-coupling reactions of aromatic boronic esters over Pd catalysts for multiple topological configurations of π-conjugated molecules. We employ a series of recipes from a precursor toolbox to produce π-conjugated macrocycles, conjugated dendrimers, linear conjugated polymers, and multiple conjugated microporous polymers and nanoparticles. The π-conjugated macrocycles, dendrimers, and linear polymers show characteristic aggregation-induced emission properties. The conjugated microporous polymers possess unique porosity of 2–3 nm. The microporous polymer nanoparticles can be redispersed in solution. This universal strategy toward definite topological configurations of π-conjugated molecules enables efficient coupling of aryl bromides with various coupling partners under mild conditions affording multiple topological conjugated systems with abundant optical and optoelectronic interest.

ChemInform Abstract: Highly Active Bimetallic Nickel‐Palladium Alloy Nanoparticle Catalyzed Suzuki‐Miyaura Reactions.

AbstractThe Ni‐Pd nanoparticle catalyst with low palladium content efficiently catalyzes the coupling of aryl bromides and iodides with arylboronic acids in water/ethanol solvent at moderate reaction temperatures.

Nickel-Catalyzed Reductive Coupling of Aryl Bromides with Tertiary Alkyl Halides.

A mild Ni-catalyzed reductive arylation of tertiary alkyl halides with aryl bromides has been developed that delivers products bearing all-carbon quaternary centers in moderate to excellent yields with excellent functional group tolerance. Electron-deficient arenes are generally more effective in inhibiting alkyl isomerization. The reactions proceed successfully with pyridine or 4-(dimethylamino)pyridine, while imidazolium salts slightly enhance the coupling efficiency.

Reductive Alkylation of 2-Bromoazoles via Photoinduced Electron Transfer: A Versatile Strategy to Csp(2)-Csp(3) Coupled Products.

Access to Csp(2)-Csp(3)-coupled products is a challenging goal at the forefront of catalysis. The photocatalytic reductive coupling of aryl bromides with unactivated alkenes is introduced as a convenient method that circumvents any need for synthesis of sp(3)-hybridized coupling partners. The reaction takes place via photoinduced electron transfer from a tertiary amine to an aryl bromide that fragments to provide an aryl radical and subsequently reacts with an alkene to form a C-C bond. Conveniently, the amine also serves as the final reductant. The method is operationally simple, functional group tolerant, and takes place with selectivities that will allow it to be used in the context of complex molecule synthesis.

Synthesis of a palladium complex with a β-d-glucopyranosyl-thiosemicarbazone and its application in the Suzuki–Miyaura coupling of aryl bromides with phenylboronic acid

A palladium(II) complex with a known phosphane-free β-d-glucopyranosyl-thiosemicarbazone ligand has been prepared and characterized by spectroscopic studies. The ligand is bonded to the metal in a bidentate coordination mode via the azomethine nitrogen and the thiolate sulfur in the dimeric form. The application of this complex on the Suzuki–Miyaura coupling of aryl bromides with phenylboronic acid in DMF/K2CO3 at 100°C for 24h in air, using 0.1mol% of palladium, afforded the corresponding biaryls in 54–91% yield. Crabtree’s test provided evidence that the real catalyst is mainly heterogeneous rather than homogeneous.

Reactivity of Para‐Substituted Fluorobenzenes in Palladium‐catalyzed Intermolecular Direct Arylations

AbstractThe influence of both electron‐withdrawing and electron‐donating substituents such as amino, bromo, chloro, ketone, methyl, methoxy, and nitro at C‐4 on fluorobenzene derivatives, for palladium‐catalyzed direct arylation at α‐position to the fluorine atom has been explored. With moderate electron‐withdrawing substituents, the reaction proceeds nicely using phosphine‐free PdCl2 catalyst, at a very low loading, and potassium pivalate/dimethylacetamide (PivOK/DMA) as catalytic system. In all cases, a regioselective arylation at the α position to the fluorine atom was observed. Moreover, a wide variety of substituents on the aryl bromide coupling partner, such as formyl, nitro, nitrile, chloro, and methyl, and also heteroaryl bromides was tolerated.

Palladium nanoparticle anchored polyphosphazene nanotubes: preparation and catalytic activity on aryl coupling reactions

Highly accessible-supported palladium (Pd) nanoparticles anchored polyphosphazene (PPZ) nanotubes (NTs) having average diameter of 120 nm were synthesized rapidly at room temperature and homogeneously decorated with Pd nanoparticles. The resultant PPZ–Pd nanocomposites were morphologically and structurally characterized by means of transmission electron microscope equipped with energy-dispersive X-ray spectroscopy and X-ray diffraction analysis. Characterization results showed that the Pd nanoparticles with good dispersibility could be well anchored onto the surfaces of the PPZ NTs. The PPZ–Pd NTs show enhanced catalytic activity for the Suzuki coupling of aryl bromides with arylboronic acid. In addition, these PPZ–Pd NTs show excellent behaviour as reusable catalysts of the Suzuki and Heck coupling reactions.

ChemInform Abstract: Suzuki—Miyaura Reactions Promoted by a PdCl2/Sulfonate‐Tagged Phenanthroline Precatalyst in Water.

AbstractA water soluble sulfonate‐functionalized phenanthroline‐type ligand is synthesized and successfully applied in the Pd‐catalyzed Sukuzi—Miyaura coupling of aryl bromides and iodides with arylboronic acids.

Enantioselective cross-coupling of meso-epoxides with aryl halides.

The first enantioselective cross-electrophile coupling of aryl bromides with meso-epoxides to form trans-β-arylcycloalkanols is presented. The reaction is catalyzed by a combination of (bpy)NiCl2 and a chiral titanocene under reducing conditions. Yields range from 57 to 99% with 78–95% enantiomeric excess. The 30 examples include a variety of functional groups (ether, ester, ketone, nitrile, ketal, trifluoromethyl, sulfonamide, sulfonate ester), both aryl and vinyl halides, and five- to seven-membered rings. The intermediacy of a carbon radical is strongly suggested by the conversion of cyclooctene monoxide to an aryl [3.3.0]bicyclooctanol.

ChemInform Abstract: Palladium Nanoparticles Supported on Poly(N‐vinyl‐ imidazole‐co‐N‐vinylcaprolactam) as an Effective Recyclable Catalyst for the Suzuki Reaction.

AbstractThe heterogeneous catalyst is generated in situ from H2PdCl4 and a vinylimidazol‐vinylcaprolactam copolymer and applied to the Suzuki—Miyaura cross coupling of aryl bromides with phenylboronic acid.

Arylation of diethyl malonate and ethyl cyanoacetate catalyzed by palladium/di-tert-butylneopentylphosphine

α-Arylated carbonyl derivatives are important structural motifs in many natural products and pharmaceutically active compounds. Although arylation of simple monocarbonyl compounds is a well-established methodology, metal-catalyzed arylation of β-dicarbonyl derivatives is significantly more challenging. The ability of β-dicarbonyl anions to bind to palladium in a κ2-O,O mode, rather than the κ1-C-bound mode required for bond formation, often results in the deactivation of catalyst systems. The C-bound form of the enolate can be favored through the use of sterically demanding ligands. Herein, we report that the sterically demanding di-tert-butylneopentylphosphine (DTBNpP) ligand in combination with Pd(dba)2 provides an effective catalyst for the coupling of aryl bromides and chlorides with diethyl malonate. The Pd/DTBNpP system also catalyzes the coupling of aryl bromides with ethyl cyanoacetate.

1,1′-Methylene-3,3′-bis[(N-(tert-butyl)imidazol-2-ylidene] and Its Effect in Palladium-Catalyzed C–C Coupling

A catalytic system utilizing a chelate carbene ligand containing bulk <i>tert</i>-butyl groups is described for palladium-catalyzed Heck and Suzuki coupling reactions. The Heck reaction focused on the coupling of different aryl bromides with mono- and 1,1-disubstituted olefins while the Suzuki reaction involved the coupling of aryl bromides and phenylboronic acid to afford the corresponding biphenyls. The catalyst system performs well with low Pd(OAc)₂ levels (0.025 mol% Pd). In all cases with monosubstituted olefins, the <i>trans</i>-configured products were obtained, while the results of Heck reaction of 1,1-disubstituted olefins exhibited a high selectivity favoring the terminal product.

C–H activation dependent Pd-catalyzed carbonylative coupling of (hetero)aryl bromides and polyfluoroarenes

The carbonylative coupling of aryl and heteroaryl bromides with polyfluoroarenes via palladium-catalyzed C-H activation is presented. This transformation proceeds efficiently at moderate reaction temperatures and does not require strong base or reactive intermediates. A near stoichiometric amount of CO is sufficient and the methodology can thus be easily expanded to include the preparation of [(13)C]-acyl labeled benzopolyfluorophenones.

ChemInform Abstract: Copper(II)‐Catalyzed C—O Coupling of Aryl Bromides with Aliphatic Diols: Synthesis of Ethers, Phenols, and Benzo‐Fused Cyclic Ethers.

AbstractSwitching from aryl iodides (XII) to the corresponding aryl bromides affords the coupling products (XIV) in slightly lower yields.