Biaryl Formation Research Articles

Synthesis of dictyomedins using phosphazene base catalyzed diaryl ether formation

Dictyomedins isolated from dictyostelium cellular slime molds were synthesized by using diaryl ether derivatives as key intermediates for the cyclization to dibenzofuran followed by palladium catalyzed intramolecular biaryl formation.

An unusual decarboxylative benzannulation and biaryl formation during copper(I)-promoted halogen atom transfer radical cyclization of 2-allylaryl trichloroacetates

CuCl/bpy-promoted halogen atom transfer radical cyclization of 2-allylaryl trichloroacetates in refluxing benzene gave benzannulated chloroarenes and benzannulated symmetrical biaryls along with reductive dehalogenation products. The unusual decarboxylative benzannulation and biaryl formation might be explained by a further intramolecular radical addition on the benzene ring of the eight-membered lactone intermediate, initially formed through 8- endo- trig halogen atom transfer radical cyclization, followed by decarboxylation, radical dimerization and dehydrochlorination reactions.

Intermolecular Cross-Coupling of Simple Arenes via C−H Activation by Tuning Concentrations of Arenes and TFA

Formation of unsymmetrical biaryls from simple arenes has been achieved successfully in a catalytic system of Pd(OAc)2/CF3CO2H(TFA)/K2S2O8 just by tuning the concentrations of arenes and TFA under mild conditions. A proposed mechanism containing two-step aromatic C−H activation on this novel intermolecular cross-coupling of arenes is also suggested and partly supported by experiments.

C−C Coupling Reactions of Aryl Bromides and Arylsiloxanes in Water Catalyzed by Palladium Complexes of Phosphanes Modified with Crown Ethers

[reaction: see text] The complexes [PdCl2L2], where L is a crown-ether-containing triarylphosphane, catalyze the formation of biaryls from arylsiloxanes and aryl bromides with high yields in water as solvent and under air. The water-insoluble catalysts [PdCl2(PhCN)2] and [PdCl2(PPh3)2] are also efficient, although they decompose more quickly to form black Pd0.

Mechanism of the Palladium-Catalyzed Homocoupling of Arylboronic Acids: Key Involvement of a Palladium Peroxo Complex

The mechanism of the palladium-catalyzed homocoupling of arylboronic acids ArB(OH)(2) (Ar = 4-Z-C(6)H(4) with Z = MeO, H, CN) in the presence of dioxygen, leading to symmetrical biaryls, has been fully elucidated. The peroxo complex (eta(2)-O(2))PdL(2) (L = PPh(3)), generated in the reaction of dioxygen with the Pd(0) catalyst, was found to play a crucial role. Indeed, it reacts with the arylboronic acid to generate an adduct (coordination of one oxygen atom of the peroxo complex to the oxophilic boron atom of the arylboronic acid) characterized by (31)P NMR spectroscopy and ab initio calculations. This adduct reacts with a second molecule of arylboronic acid to generate trans-ArPd(OH)L(2) complexes. A transmetalation by the arylboronic acid gives trans-ArPdArL(2) complexes. The biaryl is then released in a reductive elimination. This reaction is at the origin of the formation of biaryls as byproducts in palladium-catalyzed Suzuki-Miyaura reactions when they are not conducted under oxygen-free atmosphere.

Use of “Homeopathic” Ligand‐Free Palladium as Catalyst for Aryl‐Aryl Coupling Reactions

AbstractWe have previously shown that the use of ligand‐free palladium employing Pd(OAc)2 as catalyst precursor in the Heck reaction of aryl bromides is possible if low catalyst loadings, typically between 0.01–0.1 mol % are used. We have now tested this phenomenon, which we have dubbed “homeopathic” palladium, in biaryl formation using the Suzuki, the Negishi and the Kumada cross‐coupling reactions. The Suzuki reaction of aryl bromides, both activated and deactivated, is possible using 0.02–0.05 mol % of Pd(OAc)2. In this reaction turnover frequencies up to 30,000 have been reached with activated substrates. Even aryl chlorides could be reacted if strongly electron‐withdrawing substituents were present. The Negishi coupling with a variety of arylzinc halides was possible on aryl bromides containing electron‐withdrawing substituents. The Kumada reaction only gave low yields of products under “homeopathic' conditions.

Product Formation in the Laser Irradiation of Doped Poly(methyl methacrylate) at 248 nm: Implications for Chemical Effects in UV Ablation

This paper examines the modifications that are induced to photolabile haloaromatic compounds (bromo- and iodonaphthalene, iodophenanthrene, and bromoanthracene) dispersed within poly(methyl methacrylate) (PMMA) films upon irradiation with intense nanosecond laser pulses at 248 nm. To this end, laser-induced fluorescence is employed for the characterization and quantification of the emitting aryl products that remain in the substrate following irradiation. Product formation is examined as a function of laser fluence over 50 2500 mJ cm -2 . Employed dopant concentrations are e2 wt %, corresponding to absorption coefficients e1000 cm -1 . At low fluences, ArH-type products are formed via one-photon photolysis of the dopants. However, close to the swelling onset, product formation increases sharply. In parallel, at least in the NapX-doped systems, formation of biaryl species (Nap2 and perylene) is observed. It is argued that these changes do not result from differences in the excitation/fragmentation of the dopants. They must, instead, reflect differences in the reactivity of the photoproduced radicals. In particular, the increase in ArH product can be ascribed to the enhanced reactivity of the radicals due to the high attained temperatures and heat diffusion to the sublayers. The formation of biaryl species strongly indicates a very low polymer viscosity (10 3 Pa s), consistent with transient polymer melting. In fact, biaryl formation provides a direct probe for assessing melting upon laser irradiation. In all, the observations are well accounted for by a thermal model of the laser interaction with PMMA and illustrate the factors that may control reactivity in the UV ablation of polymer substrates.

Efficient Stille cross-coupling reaction using aryl chlorides or bromides in water.

An efficient Stille cross-coupling reaction using a variety of aryl halides in neat water has been developed. Employing palladium-phosphinous acid catalyst [(t-Bu)(2)P(OH)](2)PdCl(2) allows formation of biaryls from aryl chlorides and bromides in good to high yields. Functional groups such as ketones and nitriles are tolerated, and organic cosolvents are not required. The air stability and solubility in water of the palladium complexes used in this study facilitate operation of the coupling reaction and product isolation. The feasibility of catalyst recycling has also been demonstrated.

Biaryl formation: palladium catalyzed cross-coupling reactions between hypervalent silicon reagents and aryl halides

Abstract Quaternary onium triorgano(difluoro)silicates R 1 2 R 2 SiF 2 − Q + were prepared by reaction of the corresponding quaternary onium hydrogendifluoride (Q + H 2 F 2 − ) with a (triorgano)silane or a silyl enol ether. These silicates were isolated in very high yield and purity >99% by excluding halogenated solvents from the synthetic protocol. Ph 3 SiF 2 − Bu 4 N + (TBAT) was used as phenylating agent of aromatic halides in DMSO, at 120 °C and in the presence of 2.5% M equivalents of allyl palladium chloride dimer (APC) as a catalyst.

Oxidative Dimerization: Pd(II) Catalysis in the Presence of Oxygen Using Aqueous Media.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Oxidative dimerization: Pd(II) catalysis in the presence of oxygen using aqueous media

Reported herein is a method for the formation of symmetric biaryls and dienes via oxidative dimerization of aryl and alkenyl boronic acids. These conditions utilized Pd(II) catalysts under an oxygen atmosphere with water as the solvent. The use of phase transfer catalysts promoted efficient and mild syntheses of a wide range of materials.

Homogeneous Catalysis for the Production of Fine Chemicals. Palladium- and Nickel-Catalysed Aromatic Carbon–Carbon Bond Formation

In this article we describe our recent efforts in the area of palladium- and nickel-catalysed aromatic substitution reactions. Main focus is on low cost and low waste production methods. The use of aromatic carboxylic anhydrides in the Heck reaction leads to a waste-free protocol. In addition these reactions are easy to work up as no ligands or bases are used. For Heck reactions where substrates or products do not tolerate high temperatures we found that use of a bulky phosphoramidite (13b) as ligand for palladium leads to a very fast reaction at low temperatures. Recycle of palladium in ligand-free Heck and Suzuki reactions is easily accomplished by treating the palladium black that precipitates at the end of the reaction on a carrier material with a small excess of I2 prior to its re-use in the next run. Use of aryl chlorides in the palladium- and nickel-catalysed formation of biaryls can be accomplished by using the nickel-catalysed coupling with arylzinc chlorides. Better still, it was possible to make use of the arylgrignard and use a catalytic amount of ZnCl2. Whereas the strength of these aromatic substitution reactions lies in their broad tolerance of functional groups, one exception was the Sonogashira reaction on 3-bromoaniline. The problem was solved by making use of in situ catalytic protection of the NH2 group with benzaldehyde.

Oxidative Ligand Coupling of Tetraarylborates Promoted by Chlorosilane and Molecular Oxygen

Abstract Oxidative ligand coupling of organoborates is promoted by chlorosilane and molecular oxygen. When tetraarylborate compounds are treated with Ph2SiCl2 or PhSiCl3 in acetonitrile under an oxygen atmosphere, the corresponding biaryls are obtained in good yields. Use of unsymmetrical borates results in the selective formation of unsymmetrical biaryls.

Mild and efficient formation of symmetric biaryls via Pd(II) catalysts and Cu(II) oxidants

Described herein is a mild and efficient palladium-catalyzed synthesis of symmetric biaryls from organostannanes. This methodology offers products rapidly in very high yields.

Convergent Solution-Phase Combinatorial Synthesis with Multiplication of Diversity through Rigid Biaryl and Diarylacetylene Couplings

The solution-phase synthesis of iminodiacetic acid diamide libraries linked to aryl iodides and their Pd-catalyzed dimerization are detailed. Mixtures containing 64 980 components are synthesized in only 4 steps from N-BOC-iminodiacetic acid anhydride (1) and 21 readily available starting materials, demonstrating the multiplication of diversity achievable by the convergent assembly of building blocks. Both biaryl formation and sequential Stille couplings with bis(tributylstannyl)acetylene are utilized to dimerize the functionalized iminodiacetic acid diamide precursors resulting in product libraries with two sets of binding groups separated by variable length rigid linkers suitable for probing protein−protein interactions. Deconvolution libraries synthesized alongside the full mixtures allow for identification of active components.

Palladium(II)-Catalyzed Aryl Coupling of Triarylbismuthines under Air

Abstract Triarylbismuthines readily reacted with a catalytic amount of palladium(II) acetate in methanol under air to give the corresponding biaryls in high yields. The presence of oxygen was indispensable for this catalytic coupling, biaryl formation being quite slow under nitrogen. Oxygen absorption was observed during the reaction and a Pd-oxygen complex prepared separately worked effectively as a reagent for this coupling even under inert gas.

The palladium-catalyzed arylation of thiophenes study of the catalytic system

The direct arylation of activated thiophenes was successful using a Heck type reaction with a mixture of Pd(OAc) 2 and phase transfer agent as catalytic system. However, the competitive formation of biaryl resulting from an Ullmann type coupling may occur. To get a better understanding of the reaction mechanism and to find the best conditions, the reaction parameters have been studied. Several general features have been pointed out as a solid–liquid phase transfer catalysis occurring at the base interface, as the crucial role of the quaternary ammonium anion and the nature and/or the amount of the phase transfer agent that may favour thiophene arylation vs. biaryl formation.

Electrospray Ionization Mass Spectrometry Detection of Intermediates in the Palladium-Catalyzed Oxidative Self-Coupling of Areneboronic Acids.

Several intermediates of the oxidative coupling of areneboronic acids to afford biaryls have been identified by electrospray ionization mass spectrometry. Knowledge has been gained about the steps occurring after the biaryl formation and leading to the recovery of the catalytic species.

The reaction of diaryl derivatives of bis(triphenylphosphine)platinum(II) with [60]fullerene as a route to the platinum complex η2-C60Pt(PPh3)2

The reaction ofcis-Ar2Pt(PPh3)2 (Ar=p-MeC6H4 (1a) and Ar=Ph (1b)) with [60]fullerene in toluene afforded the metal-fullerene complex η2-C60Pt(PPh3)2 (2), which was isolated in the crystalline state. The reductive elimination between C60 and1a or1b also resulted in the formation of biaryls (p-MeC6H4)2 and Ph−Ph. The composition and structure of the compounds were established by1H and31P NMR spectroscopy, electronic absorption spectroscopy, and elemental analysis. The homolytic phosphorylation of2 was additionally studied by the ESR method.

Macrocyclic Triarylethylenes via Heck Endocyclization: A System Relevant to Diazonamide Synthesis

Macrocyclic Triarylethylenes via Heck Endocyclization: A System Relevant to Diazonamide Synthesis