Pd-catalyzed Heck reactions of aryl bromides with 1,2-diarylethenes (original) (raw)
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Catalytic activity of Pd(II) and Pd(II)/DAB-R systems for the Heck arylation of olefins
Journal of Organometallic Chemistry, 2003
Palladium-catalyzed reactions of aryl bromides with various olefins involving Pd(II)/diazabutadiene (DAB-R) systems have been investigated. The scope of a coupling process using Pd(II) sources and an a-diimine as ligand in the presence of Cs 2 CO 3 as base was tested using various substrates. The Pd(OAc) 2 /DAB-Cy (1, DAB-Cy 0/1,4-dicyclohexyl-diazabutadiene) system presents the highest activity with respect to electron-neutral and electron-deficient aryl bromides in coupling with electron rich olefins. The synthesis and X-ray characterization of a Pd(II)-diazabutadiene ligand is reported. Extensive optimization experiments showed that another Pd(II) source, Pd(acac) 2 (acac 0/acetylacetonate), proved to activate aryl bromides at high temperatures, low catalyst loadings when the appropriate concentration of n Bu 4 NBr additive was employed. The effect of the DAB-Cy ligand is important at very low catalyst loadings and high temperatures. Pd(acac) 2 and Pd(acac) 2 /DAB-Cy precatalysts were very effective for the arylation of various olefins with aryl bromides with respect to reaction rate, catalyst loadings, and functional group tolerance. #
Ligand and base-free Heck reaction with heteroaryl halides
Tetrahedron Letters, 2011
Pd(CH 3 CN) 2 Cl 2 -catalyzed Heck reaction of different heteroaryl halides with olefins is carried out in the absence of both the ligand and base to obtain the corresponding coupling products in good yields. tional group tolerance, mild reaction conditions, and performance with a variety of aryl halides and olefins make the Heck reaction very attractive in the field of synthetic organic chemistry. Significant advances have been made in the past two decades on the Heck reaction using different phosphine ligands 3 , palladacylces 4 , and non-phosphine Pd-catalysts, such as N-heterocyclic 5 , carbocyclic carbenes 6 , and N, O, S-donor 7 atoms containing Pd-catalysts. However, the industrial application of this reaction is limited owing to the high cost of the ligands and catalysts. Although the first protocol of the Heck reaction of aryl iodides under ligand-free conditions has been known for years 9 , Pd(OAc) 2 in combination with a suitable base has been traditionally used as an effective catalyst for the coupling of aryl bromides with terminal olefins. 10 It has been shown that the palladium catalyst, irrespective of the nature of its precursor, is rapidly reduced to Pd(0) at high temperature, which has a strong tendency to form colloids, 11 and the palladium nanoparticles that are present in these colloids catalyze the Heck reaction. However, the ligand-free approaches are usually limited to aryl iodides and aryl bromides.
Highly efficient Heck olefin arylation in the presence of iminophosphine–palladium(0) complexes
Journal of Molecular Catalysis A: Chemical, 2005
The Heck coupling of aryl bromides with olefins such as styrene or butyl acrylate is efficiently catalysed by the iminophosphine-palladium(0) complex [Pd(dmfu)(P-N)] (dmfu = dimethyl fumarate; P-N = 2-(PPh 2 )C 6 H 4 -1-CH NC 6 H 4 OMe-4) (1) in polar solvents. With activated aryl bromides such as 4-bromoacetophenone turnover numbers of up to 20,000 can be achieved at 140 • C in 2 h. The presence of electron-donating groups leads to decreased reaction rates, nevertheless, high substrate conversions can be obtained in reasonable reaction times. Kinetic studies indicate that complex 1 is only a precursor of the actual catalytic species. Experiments aimed to demonstrate the intervention of metallic palladium did not lead to conclusive findings.
Accounts of Chemical Research, 2011
T he Pd-catalyzed MizorokiÀHeck reaction of olefins with aryl halides, more often simply called the Heck reaction, was recently recognized with the 2010 Nobel Prize in chemistry. Although highly selective with electron-deficient olefins, which generally yield the linear β-arylated product exclusively, the Heck reaction is less satisfactory with electron-rich olefins. This substrate typically generates a mixture of both rand β-arylated regioisomeric products, hampering wider application of the reaction in chemical synthesis. Pioneering studies by a number of researchers revealed that high r-regioselectivity could be obtained under PdÀdiphosphine catalysis either through (i) the substitution of aryl triflates for halides or (ii) the addition of stoichiometric silver or thallium salts when aryl halides are used. Under these conditions, the arylation is believed to proceed via an ionic pathway. However, silver introduces added cost, thallium salts are toxic, and triflates are generally commercially unavailable, base sensitive, and thermally labile. Believing that the ionic pathway would be promoted in an ionic medium, in the early 2000s, we attempted the Pd-catalyzed arylation of the benchmark electron-rich olefin butyl vinyl ether with aryl bromides in an imidazolium ionic liquid. We were delighted to observe that highly regioselective r-arylation could readily be accomplished, with no need for silver additives, thallium additives, or aryl triflates. A range of other electron-rich olefins has since been shown to be viable as well. The high r-selectivity probably results from the high ionic strength of the medium, which facilitates the dissociation of halide anions from the [L 2 Pd(Ar)X] intermediate, channeling the arylation into the ionic pathway. Hydrogen bonding interactions may also play a role, however. We subsequently discovered that the r-arylation can indeed be significantly accelerated by a hydrogen bond donor salt, in both ionic liquids and common solvents. Evidence shows that the concentration of the cationic Pd(II)Àolefin species along the ionic pathway is increased as a result of hydrogen bonding between the hydrogen bond donor and the halide anion. More recently, we reported that cheaper and greener alcohols allow the Heck arylation of electron-rich olefins to proceed in a much faster, productive, and totally r-regioselective manner, circumventing the need for an ionic medium or hydrogen bond donor salt. In particular, aryl chlorides with diverse properties have been demonstrated to be viable substrates for the first time. Significantly, it appears that ethylene glycol facilitates both the oxidative addition of ArCl to Pd(0) and the subsequent dissociation of chloride from Pd(II). A closely related reaction, acylation of aryl halides with aldehydes, was also developed. Proceeding via the intermediacy of an electron-rich enamine, this PdÀpyrrolidine cooperative catalysis affords alkyl aryl ketones in a straightforward manner, extending the Heck reaction from olefins to aldehydes.
Acetylation of N -Heteroaryl Bromides via PdCl 2 /( o -tolyl) 3 P Catalyzed Heck Reactions
Synthesis, 2008
A new user-friendly and convenient method for the acetylation of N-heteroaryl bromides is described. This process is based on the palladium-catalyzed olefination of an N-heteroaryl bromide with butyl vinyl ether, followed by acid hydrolysis of the intermediate heteroaryl vinyl ether in situ. Isopropanol at 85°C, in the presence of K 3 PO 4 •3H 2 O (2 equiv), PdCl 2 (2 mol%) and (otolyl) 3 P (4 mol%), provided the best conditions, giving yields of Nheteroaryl bromides up to 75%.
Examination of Ligand Effects in the Heck Arylation Reaction
Tetrahedron, 2000
AbstractÐThe Heck arylation reaction between aryl bromide and iodide with methyl acrylate was evaluated. A new class of ligands, R±N(CH 2 CH 2 PPh 2 ) 2 , is compared with conventional monodentate and bidentate ligands, and was found to confer greater catalyst stability. The nitrogen substituent (R) has a noticeable effect on the rate of the turnover. We believe this is the ®rst report of the application of such hemilabile ligands in Heck arylation chemistry. q
Recent Developments and New Perspectives in the Heck Reaction
Accounts of Chemical Research, 1995
More than two decades ago, Mizorokil and Heck2 independently discovered the palladium-catalyzed arylation and vinylation of olefins. This methodology, known worldwide as the Heck reaction (Scheme 1),3 is attractive from a synthetic point of view because high chemoselectivity and mild reaction conditions are associated with low toxicity and cost of the reagent^.^ During the past few years, the experience obtained in several palladium-based reactions helped in understanding the rules which govern the coordinationinsertion of unsaturated systems on palladium(I1) complexes. In fact, this process has a central role in determining the reaction rate, stereoselectivity, and regioselectivity of intra-and intermolecular Heck reactions, hydrosilylations, oligomerizations of olefins, and olefin-CO copolymerization^.^ The purpose of this account is to review and critically evaluate the results on the Heck reaction recently obtained by several research groups. These results can be explained on the basis of a common mechanistic hypothesis for the coordination-insertion process of unsaturated systems on palladium(I1) complexes (Scheme 2, step b). Heck Reaction Mechanism Oxidative Addition (Scheme 2, Step a). Except for the case of aryl iodides, the presence of ligands is necessary in order to effect oxidative addition of RX to the palladium(0) complex at a reasonable temperature (Scheme 2, step a).6 The catalytically active species is a 14-electron complex, LzP~(O),~ and effective ligands in the Heck reaction are monodentate3 and bidentate phosphines8 and 1 ,lo-phenanthroline de-rivative~.~ Coordination-Insertion (Scheme 2, Step b). The geometry of the complex is a central point, and
Organic Letters, 2003
r,-Unsaturated acetals afford, in the presence of the LIC−KOR superbase, 1-alkoxybuta-1,3-dienes. These substrates cross couple with aryl derivatives in the presence of Pd catalyst (Heck conditions) in a regio-and stereoselective mode. With dialkyl acetals, the reaction affords arylated dienes; on the other hand, in the case of 1,3-dioxane derivatives, the final outcome of the process formally corresponds to the direct γ-arylation reaction of the starting r,-unsaturated material. Metal-catalyzed coupling reactions are very efficient and reliable procedures for the construction of new carboncarbon bonds. 1 In particular, the Heck reaction has been used extensively over the past three decades for the elaboration of alkenes. 2 The reaction is a straightforward way to achieve substituted alkenes, dienes, and other unsaturated structures, many of which are important intermediates for the preparation of dyes, UV screens, and drugs. 3 Although arylation of two-carbon vinyl fragments might sometimes result in scarcely regioselective reactions, 4,5 highly regioselective R-arylation and R-vinylation procedures have been reported under specific experimental conditions. 6 In particular, 100% regioselective R-functionalization of vinyl ethers can be achieved by favoring the coordination of the Pd complex to