Mechanistic Implications of the Observation of Kinetic Resolution in a Palladium-Catalyzed Enantioselective Allylic Alkylation

Preferential rotation in substrate—palladium intermediates in a catalyzed asymmetric allylic alkylation is proposed to be responsible for both the observed kinetic resolution of the racemic allylic acetate starting material as well as the high selectivity found in the enantiodiscriminating product-forming step [Eq. (a)].     

3β-羟基-24-降胆-5-烯-23-酸的合成

以妊娠双烯醇酮醋酸酯为起始原料, 经过6步反应以28.2%的总收率, 合成了具有抗炎活性的海洋天然产物3β-羟基-24-降胆-5-烯-23-酸. 关键反应为钯催化的烯丙基烷基化.     

Rate and mechanism of the reversible formation of cationic (eta3-allyl)-palladium complexes in the oxidative addition of allylic acetate to palladium(0) complexes ligated by diphosphanes

The oxidative addition of the allylic acetate, CH2=CH-CH2-OAc, to the palladium(o) complex [Pd0(P,P)], generated from the reaction of [Pd(dba)2, with one equivalent of P,P (P,P = dppb = 1,4-bis(diphenylphosphanyl)butane, and P,P = dppf = 1,1'-bis(diphenylphosphanyl)ferrocene), gives a cationic (eta3-allyl)palladium(II) complex, [(eta3-C3H5)Pd(P,P)+]. with AcO as the counter anion. This reaction is reversible and proceeds through two successive equilibria. The overall equilibrium constants have been determined in DMF. Compared with PPh3, the overall equilibrium lies more in favor of the cationic (eta3-allyl)palladium(II) complex when bidentate P,P ligands are considered in the order: dppb > dppf > PPh3. The reaction proceeds via a neutral intermediate complex [(eta2-CH=CH-CHCH2-OAc)Pd0(P,P)], which has been kinetically detected. The rate constants of the successive steps have been determined in DMF by UV spectroscopy and conductivity measurements. The overall complexation step of the Pd0 by the allylic acetate C=C bond is faster than the oxidative addition/ionization step which gives the cationic (eta3-allyl)palladium(II) complex.     

5-Vinylpyrimidines. Synthesis via organopalladium intermediates

Reactions of 1,3-dimethyl-5-iodouracil or 2,4-dimethoxy-5-iodopyrimidine with vinyl acetate in the presence of a catalytic amount of diacetato-bis (triphenylphosphine) palladium (II) resulted in good yields of the corresponding 5-vinylpyrimidines. The reactions are viewed as resulting from regioselective addition of an initially formed 5-pyrimidinyl palladium species to the double bond of vinyl acetate followed by elimination of a palladium acetate with regeneration of the double bond and formation of the 5-vinylpyrimidine product.     

Expeditious synthesis of 3,4-dihydroisocoumarins and phthalides using the Heck-Matsuda reaction

Several 3,4-dihydroisocoumarins and phthalides were synthesized by an effective Heck-Matsuda reaction involving an ortho carboxybenzenediazonium salt with a series of styrenes bearing electron donating and electron withdrawing groups, methylvinyl ketone, and methyl acrylate. The reaction was carried out in an open-flask with 1% mol of palladium acetate in aqueous ethanol at ∼80 °C, giving the correspondent 3-aryl-3,4-dihydroisocoumarins and phthalides with good overall yields. The electronic nature of the group attached to the olefin is a key feature for the regioselectivity of the cyclization step.     

Prochlorperazine bismethanesulfonate as a new reagent for the spectrophotometric determination of palladium

A simple, rapid, and selective method for the determination of palladium is described. The orange-red palladium(II)-prochlorperazine bismethanesulfonate complex in the presence of hydrochloric acid-sodium acetate buffer exhibits maximum absorbance at 480 nm with a molar absorptivity of 4.32 × 10³ liters mol^?1 cm^?1. The sensitivity of the reaction is 24.62 ng cm^?2. The system obeys Beer's law over the concentration range 0.4–20 ppm of palladium with an optimum concentration range of 1–19 ppm. The apparent stability constant of the complex is found to be log K = 5.3 ± 0.1 at 27 °C. The effects of pH, time, temperature, order of addition of reactants, reagent concentration, and interferences from various ions are reported. The proposed method offers the opportunity to carry out the determination at room temperature without the need for an extraction step. The method is also found to be suitable for the determination of palladium in jewelry alloy.     

Characterization of a Polymer-Bound Palladium Acetate Catalyst and Studies on the Selective Hydrogenation of Dienes and Alkynes

Abstract

Palladium(II) acetate has been anchored onto a copolymer support containing pyridyl and carboxyl groups. XPS studies showed the Pd 3d binding energies for the recovered catalyst to be less by 1 eV after being used in hydrogenation studies. However, x-ray studies and a chemical test based on KCN treatment failed to reveal any palladium oxide or palladium metal formation in the recovered catalyst. It is presumed that an acetate ligand is lost during hydrogenation, which could be the reason for the lowering of the palladium 3d binding energies in the recovered catalyst. Results of investigations of the hydrogenation of olefins and selectivity of the catalyst toward the hydrogenation of dienes and alkynes are presented. The loss of palladium due to leaching under the reaction conditions employed was found to be very low (<1%/cycle).     

Allylstannanes as electrofugal partners in allylic alkylation

Unsymmetrical allyl-allyl couplings occur between allylstannanes and allyl acetates catalyzed by palladium(O) and a novel direct coupling of an allyl acetate in the presence of a distannane and a palladium(O) catalyst is also possible.     

The mechanism of reactions of organopalladium salts with vinylcyclopropanes

Cyclopropylmercuric chloride, lithium chloropalladite and methyl acrylate react at 0–25°C to give an 81% yield of methyl sorbate. A π-allylic palladium intermediate is proposed since vinylcyclopropane, carbomethoxymercuric acetate, and lithium chloropalladate give the same product. The corresponding reactions with styrene and cyclopropylmercuric chloride or vinylcyclopropane and phenylmercuric chloride also give the same, isolable π-allylic palladium complex. Deuterium labeling experiments support the occurrence of a common intermediate in the two reactions. 1,1-Dicarboethoxy-2-vinylcyclopropane reacts similarly with “phenylpalladium chloride” but the π-allylic product has the palladium attached at the benzyl carbon rather than the next carbon away. The reaction with “phenylpalladium acetate” in place of the chloride yields only dienes. Studies with the deuterated vinylcyclopropane diester suggest that the mode of elimination from the initial palladium adduct is strongly influenced by the anion present. The reaction of “cyclopropylpalladium chloride” with alkenes appears to be a general method for preparing, selectively, internal π-allylic palladium complexes.     

Selective preparation of non-symmetrically substituted divinylbenzenes by palladium-catalysed arylations of alkenes with bromobenzoic acid derivatives

Palladium-catalysed arylation of alkenes with the three bromobenzoic acids or their acyl chlorides provides an efficient and selective method for the preparation of non-symmetrically substituted divinylbenzene derivatives. In the presence of palladium acetate and a phosphorus ligand the free acids react as aryl bromides, with the exception of 2-bromobenzoic acid. If palladium acetate is used alone as catalyst, all three bromobenzoyl chlorides react only as aroyl chlorides. Using two different alkenes a given non-symmetrically substituted divinylbenzene can be prepared by four different routes, allowing for an optimum choice of synthesis path. Substituent effects in the aromatic derivatives and the reactivity of the alkenes in arylation are the principal features to be taken into account. The reaction pathway can generally be chosen to give excellent yields in short reaction times at low palladium concentrations.     

A mild and efficient palladium–triethylsilane system for reduction of olefins and carbon–carbon double bond isomerization

The versatility of palladium(II) acetate and palladium on activated charcoal catalysts with triethylsilane has been investigated in the hydrogenation and the isomerization of carbon–carbon double bond of 1‐alkenes. The reduction of 1‐alkenes was carried out in the presence of triethylsilane, ethanol and a catalytic amount of palladium(II) acetate or palladium on activated charcoal, at room temperature. This facile and efficient method affords high yields for hydrogenation of unsaturated alkenes to the corresponding alkanes. Then the carbon–carbon double bond isomerization of 1‐alkenes was tested using the same catalysts in the absence of solvent. The system palladium(II) acetate‐triethylsilane was found to be more effective compared with palladium on an activated charcoal–triethylsilane system at room temperature, while comparable results were obtained at 50 °C for both catalysts. Copyright © 2006 John Wiley & Sons, Ltd.     

Synthesis of resveratrol using a direct decarbonylative Heck approach from resorcylic acid

The phytoalexin resveratrol has been made using a decarbonylative Heck reaction. The acid chloride derived from 3,5-dihydroxybenzoic acid was coupled with 4-acetoxystyrene in the presence of palladium acetate and N,N-bis-(2,6-diisopropylphenyl)dihydroimidazolium chloride to give the substituted stilbene in 73% yield as the key step.     

N-heterocyclic carbenes: Reaction to give anilines

The attempted synthesis of bis(NHC)palladium complexes via the direct reaction of an imidazolium salt with palladium acetate results in the formation of a mixed NHC/aniline complex. The route by which the aniline is formed has not been completely elucidated, but it does originally derive from an imidazolium salt.     

Kinetic Studies on the Palladium(II)‐Catalyzed Oxidative Cross‐Coupling of Thiophenes with Arylboron Compounds and Their Mechanistic Implications

Reaction orders for the key components in the palladium(II)‐catalyzed oxidative cross‐coupling between phenylboronic acid and ethyl thiophen‐3‐yl acetate were obtained by the method of initial rates. It turned out that the reaction rate not only depended on the concentration of palladium trifluoroacetate (reaction order: 0.97) and phenylboronic acid (reaction order: 1.26), but also on the concentration of the thiophene (reaction order: 0.55) and silver oxide (reaction order: ?1.27). NMR spectroscopy titration studies established the existence of 1:1 complexes between the silver salt and both phenylboronic acid and ethyl thiophen‐3‐yl acetate. A low inverse kinetic isotope effect (k_H/k_D=0.93) was determined upon employing the 4‐deuterated isotopomer of ethyl thiophen‐3‐yl acetate and monitoring its reaction to the 4‐phenyl‐substituted product. A Hammett analysis performed with para‐substituted 2‐phenylthiophenes gave a negative ρ value for oxidative cross‐coupling with phenylboronic acid. Based on the kinetic data and additional evidence, a mechanism is suggested that invokes transfer of the phenyl group from phenylboronic acid to a 1:1 complex of palladium trifluoroacetate and thiophene as the rate‐determining step. Proposals for the structure of relevant intermediates are made and discussed.     

Direct Synthesis of 3-Aryl Substituted Isocoumarins and Phthalides through Palladium Acetate Catalyzed C(sp2)−H Activation in Ionic Liquids

A novel Pd-catalysed oxidative coupling between benzoic acids and vinylarenes or acrylates to furnish isocoumarins and phthalides is reported. The reaction proceeds smoothly in molten tetrabutylammonium acetate via a selective C−H bond activation, with very low percentage of ligand-free palladium acetate as the catalyst, under atmospheric pressure of oxygen. Sub-stoichiometric amount of copper acetate is also required as a reoxidant for the palladium.     

Alkylation of olefins with “alkylpalladium acetates” lacking β hydrogen substituents. Observation of an intramolecular shift of an acetatopalladium group in neophylpalladium acetate

Methyl-, benzyl-, neopentyl- and (2-methyl-2-phenylpropyl)palladium acetates, prepared in situ by exchange reactions of the corresponding mercurials with palladium acetate, alkylate monosubstituted ethylene derivatives in fair to good yields. The “(2-methyl-2-phenylpropyl)palladium acetate” apparently underwent an unusual rearrangement during reaction with methyl acrylate. The palladium acetate group was partially transferred from the side-chain to the ortho position of the aromatic ring and produced methyl o-tert-butylcinnamate in 49% yield. The “normal product”, methyl 5-methyl-5-phenyl-2-hexenoate was also obtained, in 16% yield. A similar rearrangement occurred in the reaction with styrene.     

Insertion of molecular oxygen into a palladium(II) hydride bond

Insertion of molecular oxygen into a palladium(II) hydride bond to form an (eta1-hydroperoxo)palladium(II) complex is reported. The hydroperoxo palladium(II) product has been crystallographically characterized. A second-order rate law (first-order in palladium and first-order in oxygen) is observed for the reaction and a large kinetic isotope effect implicates Pd-H bond cleavage in the rate-determining step. The results of studies with radical inhibitors and light suggest that the reaction does not proceed by a radical chain mechanism.     

Kinetics and mechanism of palladium(II) acetate reduction by hydrogen on the surface of a carbon support

The kinetics of the reduction of Pd(II) compounds by dihydrogen on the surface of a carbon support has been investigated for palladium acetate as an example. A kinetic model has been constructed for this reaction. An autocatalytic mechanism is suggested, in which the key role is played by Pd(0) compounds and their hydrides. The reaction occurring on the support surface is compared with the same reaction in solutions of palladium phosphine acetate complexes, where a similar mechanism is observed. One of the most important features of the surface reaction is the relatively slow reduction of the Pd(I) compounds to Pd(0). This makes it possible to obtain materials with a high Pd(I) content of 5% and above.     

Palladium-catalyzed fluorination of carbon-hydrogen bonds

This communication describes the development of a new Pd-catalyzed method for the fluorination of carbon-hydrogen bonds. A key step of these transformations involves palladium-mediated carbon-fluorine coupling-a much sought after, but previously unprecedented, transformation. These reactions were successfully achieved under oxidative conditions using electrophilic N-fluoropyridinium reagents. Microwave irradiation in the presence of catalytic palladium acetate served as optimal conditions for the fluorination of C-H bonds in a variety of substituted 2-arylpyridine and 8-methylquinoline derivatives.     

Unexpected participation of nucleophiles in the reaction of palladium(II) acetate with divalent 3<Emphasis Type="Italic">d</Emphasis> metals

The kinetics of reactions of palladium(II) acetate with cobalt(II), nickel(II), and copper(II) acetates were studied by spectrophotometry. These reactions produce heterobimetallic complexes Pd^II(μ-OOCMe)₄M^II(OH₂)(HOOCMe)₂, where M = Co, Ni, or Cu. These reactions are very slow in carefully dehydrated (<0.01% H₂O) acetic acid, but are considerably enhanced by water or acetonitrile. Our data indicate that the activation of the kinetically inert ring structure of the initial palladium complex Pd₃(μ-OOCMe)₆ by means of the nucleophilic attack of an H₂O or acetonitrile molecule is the key step of the reaction mechanism.