Investigation of the catalytic activity of a Pd/biphenyl-based phosphine system in the Ullmann homocoupling of aryl bromides

We described Ullmann homocoupling promoted by a Pd/biphenyl-based phosphine system using DMF as solvent. Using Hammett equation it is found that the rate determining step of the reaction depends on the electronic nature of substituents of aryl bromides. Increase the rate of reaction with decreasing the electron donating of the substituent from NH₂ to H suggesting an oxidative addition step as the rate determining step. Decrease the rate of reaction with increasing the electron-withdrawing ability of the substituent from H to NO₂ indicating a reductive elimination step as the rate determining step.     

Palladium-catalyzed regioselective Heck arylation of electron-rich olefins in a molecular solvent-ionic liquid cocktail

We have recently established that highly regioselective Heck arylation of electron-rich olefins can be accomplished with aryl halides without using any halide scavengers in imidazolium ionic liquid solvents. The results presented in this paper show that the benchmark electron-rich olefins vinyl ethers can be readily arylated by aryl bromides in a molecular solvent-ionic liquid cocktail with no compromise on regioselectivity. By introducing a small amount of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]) to DMSO, the arylation reactions of the vinyl ethers 1a-d by the bromides 2a-j took place to afford essentially only the α arylated products. The enamide 1e underwent similar regioselective arylation in the solvent cocktail. In the absence of the ionic liquid, lower regioselectivities were observed. In comparison with the chemistry we have reported so far, the current method reduces considerably the reliance on the volume of ionic liquids used, providing a simpler and more practical synthetic pathway for preparing arylated vinyl ethers and aryl methyl ketones.     

Recyclable ligand-free mesoporous heterogeneous Pd catalysts for Heck coupling

Ligand-free Pd-MCM41 catalysts are highly active in the Heck coupling of bromoarenes including deactivated bromo derivatives to give coupled products in high yields with high selectivities without the need to exclude air or moisture. The catalyst samples exhibit unprecedented stability among heterogeneous catalysts and can be reused at least 20 times to achieve complete conversion without any additional activation treatment.     

Highly regioselective Heck reactions of heteroaryl halides with electron-rich olefins in ionic liquid

Palladium-catalyzed Heck reactions of the heteroaryl halides, halopyridines, bromoquinoline and bromothiophenes, with the electron-rich olefins vinyl ethers and allyl alcohol were shown to give essentially only the branched olefins in an imidazolium ionic liquid, whereas in molecular solvents a mixture of regioisomers was formed. The method obviates the need for aryl triflates and stoichiometric inorganic salt additives, providing an easy entry to functionalized heteroaromatics incorporating acetyl and 2-allyl alcohol functionalities.     

Suzuki and Heck coupling reactions mediated by palladium complexes bearing trans-spanning diphosphines

Palladium complexes of three trans-spanning diphosphines are examined for effecting C-C coupling reactions. Ten aryl halides of varying electron density were screened in Suzuki coupling reactions with phenylboronic acid and in Heck reactions with styrene. The results are discussed in terms of the unique flexibility and shape of the meta-terphenyl backbone upon which the diphosphine ligand is built.     

Efficient Heck cross-coupling of 3-iodo-benzopyrones with olefins under microwave irradiation without phosphine

An efficient and effective microwave-assisted Heck cross-coupling of different terminal olefins with various 3-iodo-benzopyrones including sterically hindered, electron-rich, electron-neutral, and electron-deficient is developed. It proceeded faster and generally gave good to excellent yields under microwave irradiation, phosphine-free, and air condition. The reaction could render this method particularly attractive for the efficient preparation of biologically and medicinally interesting molecules.     

Hydrogen-bond-directed catalysis: faster, regioselective and cleaner Heck arylation of electron-rich olefins in alcohols

A general method for the regioselective Heck reaction of electron-rich olefins is presented. Fast, highly regioselective Pd-catalysed alpha-arylation of electron-rich olefins, vinyl ethers (1 a-d), hydroxyl vinyl ethers (1 e, f), enamides (1 g, h) and a substituted vinyl ether (1i) has been accomplished with a diverse range of aryl bromides (2 a-r), for the first time, in cheap, green and easily available alcohols with no need for any halide scavengers or salt additives. The reaction proceeds with high efficiency, leading exclusively to the alpha-products, in 2-propanol and particularly in ethylene glycol. In the latter, the arylation can be catalysed at a palladium loading of 0.1 mol% and finish in as short a time as 0.5 h. The remarkable performance of the alcohol solvents in promoting alpha regiocontrol is attributed to their hydrogen-bond-donating capability, which is believed to facilitate the dissociation of halide anions from PdII, and hence, the generation of a key ionic PdII-olefin intermediate responsible for the alpha product. This belief is further strengthened by the study of a benchmark arylation reaction in 21 different solvents. The study revealed that exclusive alpha-regioselective arylation takes place in almost all of the protic solvents, and there is a rough correlation between the alpha-arylation rates and the solvent parameter E(T)N. The method is simpler, cleaner and more general than those established thus far.     

Palladium catalyzed Suzuki coupling reactions using cobalt-containing bulky phosphine ligands

Three bulky mono-dentate alkyne-bridged dicobalt-phosphine complexes [(μ-PPh₂CH₂PPh₂)Co₂(CO)₄](μ,η-PhCCPPh₂) 4a, [(μ-PPh₂CH₂PPh₂)Co₂(CO)₄](μ,η-Me₃CCCPPh₂) 5a and [(μ-PPh₂CH₂PPh₂)Co₂(CO)₄](μ,η-Me₃SiCCPPh₂) 6a were prepared from the reactions of the bis(diphenylphosphino)methylene (dppm) bridged dicobalt complex Co₂(CO)₆(μ-Ph₂PCH₂PPh₂) with PhCCPPh₂1, Me₃CCCPPh₂2, and Me₃SiCCPPh₂3, respectively. These three metal-containing compounds 4a, 5a and 6a were employed as ligands, replacing conventional organic phosphines, in the Suzuki cross-coupling reactions and have been proved to be effective, authentic mono-dentate phosphine ligands.     

Palladium-catalyzed Heck coupling reactions using different fluorinated phosphine ligands in compressed carbon dioxide and conventional organic solvents

Palladium-catalyzed Heck reaction of iodobenzene and styrene was investigated in compressed CO₂ using different fluorinated phosphine compounds as ligands at a temperature of 70 °C. The reaction mixture is a single phase at 12 MPa but biphasic at 8 MPa, a little higher than the critical pressure of pure CO₂ under the reaction conditions used. Although the solubility of fluorinated ligands is very high in dense CO₂, they have marginal improvements in Heck conversion in this medium compared with a non-fluorinated ligand of triphenylphosphine. The activity of palladium complexes strongly depends on the kind of phosphine compound used, in the order of bis(pentafluorophenyl)phenylphosphine (III)>triphenylphosphine (I), tris(pentafluorophenyl)phosphine (IV)>diphenyl(pentafluorophenyl)phosphine (II), tris(p-fluorophenyl)phosphine (V)>tris(p-trifluoromethyl phenyl)phosphine (VI), 1,2-bis[bis(pentafluorophenyl)phosphino]ethane (VII), for the homogeneous reaction at 12 MPa. This order of effectiveness of these ligands is different from those obtained in conventional organic solvents. Hexane, toluene, ethanol, and N-methylpyrrolidone (NMP) showed maximum conversions with the ligands VI, IV, V and VII, respectively. The conversion in CO₂ with the ligand III is comparable with those in polar solvents of ethanol and NMP, and larger than those in hexane and toluene in the presence of the best ligands. The dense CO₂ may affect the specific activity of palladium complex catalysts and/or the reactivity of reacting species. Small quantities of fluorinated products were observed to form at high pressure of CO₂ and this is direct evidence of P–C bond cleavage during Heck reaction in dense CO₂. The activity of palladium complexes with those ligands is higher in more polar solvent.     

含有酯基二茂铁亚胺环钯化合物催化Suzuki和Heck反应

合成并表征了系列含有酯基二茂铁亚胺环钯化合物.结果表明,该系列环钯催化剂在纯水或有机溶剂中均能高效催化Heck和Suzuki偶联反应.     

Synthesis of 1,2-disubstituted acetylenes via copper-catalyzed Suzuki coupling of organoboronic acids with 1,1-dibromo-1-alkenes

An efficient synthesis of 1,2-disubstituted acetylenes has been described. Reactions of organoboronic acids with 1,1-dibromo-1-alkenes in the presence of a catalytic amount of CuI (10 mol %), 8-hydroxyquinoline (10 mol %), and using potassium phosphate as base in C₂H₅OH afforded the desired 1,2-disubstituted acetylenes in good to excellent yields. It is important to note that the formation of 1,3-diynes, which are derived from the homocoupling of 1,1-dibromo-1-alkenes, can be almost completely suppressed under these optimized conditions.     

Calculation of the polymerization process of some 1,1-disubstituted cyclopropanes: the first propagation step

A theoretical study of the first steps of the polymerization propagation process of 1,1 cyano, fluoro and methyl disubstituted cyclopropanes and 1-cyano 1-amino cycloprane is presented. The thermodynamic and kinetic feasibility of the reactions are discussed through 6-31+G(d,p) Hartree–Fock calculations. The anionic initiation and propagation process follows an S_N2 like reaction path mainly governed by the differences in electron affinities between the incoming and outgoing anions, further modulated by the nature of the substituents. Main conclusions are that the π electron-withdrawing groups maintain the barrier heights low, making the process feasible from a kinetic viewpoint. The σ electron-withdrawing groups give more exothermic reactions but their reaction barriers are very high.     

Heck coupling reaction of iodobenzene and styrene using supercritical water in the absence of a catalyst

Heck coupling reaction of iodobenzene and styrene proceeds rapidly and selectively in supercritical water even without any catalyst in the presence of base. Both the choice of base and the reaction conditions had a significant effect on the conversion and the selectivity of the coupling products. The addition of a relatively mild base such as potassium acetate facilitated the cross-coupling reaction, while the hydrolysis of phenyl halide was favored in the presence of a strong base. The conversion and the yields of coupling products increased with increasing temperature, reaching a maximum at 650 K near the critical temperature of water, and then decreased as the temperature was further increased. Water density had a significant influence on the reaction rate, showing nearly 30% augmentation with a slight increase in density from 0.45 to 0.56 g cm(-3), but had less effect on the product selectivity. Two possibilities of the role of water responsible for the noncatalytic Heck coupling reaction in supercritical water, that is, ion and water-catalyzed mechanisms have been considered.     

Heck-type olefination and Suzuki coupling reactions using highly efficient oxacalix[4]arene wrapped nanopalladium catalyst

A simple one-pot method is used for the synthesis of water dispersible and stable palladium nanoparticles (PdNps) where oxacalix[4]arene dihydrazide (OXDH) is used as both a reducing and capping agent. The OXDH-PdNps have been characterized by UV–Visible spectroscopy, Fourier transform infrared (FT-IR), transmission electron microscopy (TEM), energy‐dispersive X‐ray spectroscopy (EDX), powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The OXDH-PdNps are spherical in shape with an average size of 3–4?nm and well crystallized in a face centered cubic structure. The capping of the OXDH ligand on the nanoparticles surface was evaluated using FT-IR. The OXDH-PdNps have been used in carbon–carbon coupling reactions, namely, the Suzuki–Miyaura and Mizoroki–Heck reactions. Both of the reactions are carried out under phosphine-free conditions to provide better yields. The nanocatalyst can be easily recovered and reused for six consecutive catalytic cycles without any significant loss in its catalytic activity.     

Catalytic activity of Pd dithiolate complexes with large-bite-angle diphosphines in Heck coupling reactions

Palladium(II) complexes of aryl dithiolates and wide-bite-angle diphosphines Xantphos and dppf have been developed as efficient catalysts in Suzuki and Suzuki carbonylation reactions. The catalytic activity of these highly stable, discrete and charged complexes was investigated in Heck coupling reactions of styrene and a variety of aryl bromides. Under optimized reaction conditions these palladium complexes showed excellent activity with high turnover number (6 × 10⁶) and high turnover frequency (4 × 10⁵ h⁻¹). The effect of bite angle of diphosphines on the catalytic activity of the complexes [Pd₂(P^∩P)₂(SC₁₂H₈S)]₂(OTf)₄ followed the trend P^∩P = Xantphos > dppf > dppe as the order of their bite angles. The catalyst could be reused, and after three cycles the formation of significant amount of Pd nanoparticles was noticed, which were characterized using powder X-ray diffraction, energy-dispersive X-ray analysis and transmission electron microscopy. The high catalytic activity has been attributed to the Pd nanoparticles.     

Heck reactions in monoglyme and diglyme using BmimCl as pre‐ligand

Heck reactions between iodobenzene and methyl acrylate were carried out in monoglyme and diglyme as solvents, using different ligands and palladium sources, and good to high yields were obtained in the production of (E)‐substituted olefins. The ionic liquid BmimCl was successfully utilized as pre‐ligand to substitute triphenylphosphine. Copyright © 2007 John Wiley & Sons, Ltd.     

Palladacycles bearing pendant benzamidinate ligands as catalysts for the Suzuki and Heck coupling reactions

Three pendant benzamidines [Ph-C(NC₆H₅)-{NH(CH₂)₂NMe₂}] (1), [Ph-C(NC₆H₅)-{NH(CH₂Py)}] (2) and [Ph-C(NC₆H₅)-{NH(o-C₆H₄)(oxazoline)}] (3) are described. Reactions of 1, 2 or 3 with one molar equivalent of Pd(OAc)₂ in THF give the palladacyclic complexes [Ph-C{-NH(η¹-C₆H₄)}{N(CH₂)₂NMe₂}]Pd(OAc) (4), [Ph-C{-NH(η¹-C₆H₄)}{N (CH₂Py)}]Pd(OAc) (5) and [Ph-C{-NH(η¹-C₆H₄)}{N(o-C₆H₄)(oxazoline)}]Pd(OAc) (6), respectively. Treatment of 4, 5 or 6 with excess of LiCl in chloroform affords [Ph-C{-NH(η¹-C₆H₄)}{N(CH₂)₂NMe₂}]PdCl (7), [Ph-C{-NH(η¹-C₆H₄)}{N(CH₂Py)}]PdCl (8) and [Ph-C{-NH(η¹-C₆H₄)}{N(o-C₆H₄)(oxazoline)}]PdCl (9). The crystal and molecular structures are reported for compounds 1, 3, 5, 6 and 7. The application of these palladacyclic complexes to the Suzuki and Heck coupling reactions was examined.     

New Heck coupling strategies for the arylation of secondary and tertiary amides via palladium-catalyzed intramolecular cyclization

A new synthetic protocol has been developed for the arylation of secondary and N-alkylated amide Heck precursors by the implementation of the palladium-catalyzed intramolecular Heck reaction strategies.     

Heck arylation of acrolein acetals using the 9-bromoanthracene: A case of study

The influence of several parameters on the selectivity of the palladium catalysed Heck coupling of 9-bromoanthracene with acrolein acetals was studied. While the ester is the quasi exclusive product when only a base (i.e. NaOAc, K₂CO₃, etc.) is added in the medium, the presence of halide abstracting agent such as thallium or silver salts decreases noticeably the selectivity towards the ester. On the other hand, the addition of n-Bu₄NOAc yields to the formation of the aldehyde with up to 74% selectivity. The presence of water was found to play a significant role not only on the rate but also on the selectivity of the reaction. A comprehensive mechanism is proposed outlining the influence of each additive, particularly on the selectivity of the reaction.