Palladium(II) Chloride1


[7647-10-1]  · Cl2Pd  · Palladium(II) Chloride  · (MW 177.32)

(used as an oxidizing agent and to a lesser extent as a source of Pd0 complexes)

Physical Data: mp 678 °C (dec).

Solubility: slightly sol H2O; sol H2O in the presence of chloride ion; sol aqueous HCl; sol PhCN, forming Pd(PhCN)2Cl2; insol organic solvents.

Form Supplied in: commercially available as a rust-colored stable powder or crystalline solid.

Handling, Storage, and Precautions: air stable; not hygroscopic.

General Considerations.

Many of the reactions described below can be accomplished using derivatives of palladium chloride such as Potassium Tetrachloropalladate(II), Disodium Tetrachloropalladate(II), Bis(benzonitrile)dichloropalladium(II), dichlorobis(acetonitrile)palladium, and dichlorobis(triphenylphosphine)palladium. The physical properties of these alternative reagents are described under their separate entries, but their chemistry is included in this article.

Synthetic applications of PdCl2 and its derivatives can be classified into three types: use as oxidizing agents, use as PdII catalysts, and use as a source of Pd0 catalysts. Characteristic features of these applications are briefly summarized below.

Use as Oxidizing Agents.

PdCl2 and Palladium(II) Acetate are representative PdII salts used for various oxidation reactions, but their uses are different. For example, oxidative reactions of aromatic compounds are possible only with Pd(OAc)2; PdCl2 and its derivatives cannot be used. Oxidation reactions of various substrates with PdCl2 are stoichiometric and Pd0 is formed after the oxidation. Sometimes, but not always, Pd0 can be reoxidized in situ to PdII with proper reoxidizing agents. In such a case, the oxidation reaction can be carried out with a catalytic amount of PdCl2. Examples of reoxidants include CuCl2, CuCl, Cu(OAc)2, MnO2, HNO3, benzoquinone, alkyl nitrites, H2O2, and organic peroxides. Since solubility of PdCl2 in water and organic solvents is small, the more soluble Dilithium Tetrachloropalladate(II), Na2PdCl4, K2PdCl4, and Pd(PhCN)2Cl2 are sometimes used for similar purposes.

Use as PdII Catalyst.

Pd(PhCN)2Cl2 is used as a homogeneous PdII catalyst for some non-oxidative reactions such as rearrangement reactions.

Use as Source of Pd0 Catalyst.

PdII salts are reduced to Pd0 catalysts with various reducing agents. Although Pd(OAc)2 is more convenient for this purpose than PdCl2 and its derivatives, PdCl2 derivatives are used in many cases. Typically, Pd(Ph3P)2Cl2 is reduced to form a Pd0 phosphine complex.


Oxidative Reactions of Alkenes.2

Oxidative reactions of alkenes can be classified into two types: oxidative substitution and oxidative addition, as shown in eq 1. Here X- and Y- represent nucleophiles such as HO-, RO-, RCO2-, R2N- and CO, as well as soft carbon nucleophiles such as active methylene compounds.

Reaction with Water.2a,b

Oxidation of ethylene to acetaldehyde under oxygen atmosphere is an industrial process called the Wacker process. PdCl2 and Copper(II) Chloride in aqueous HCl are used as the catalysts. As shown by eq 2, the Wacker process comprises three unit reactions; CuCl2 is a unique reoxidant of Pd0.

Higher terminal alkenes are also oxidized in organic solvents containing water; DMF is most widely used as the solvent.3 On a laboratory scale the oxidation can be carried out easily in a way similar to the hydrogenation of alkenes under atmospheric pressure. Instead of Pd black and hydrogen, the oxidation is carried out with PdCl2 and the copper salt under an oxygen atmosphere at room temperature using a similar apparatus. Since the reaction proceeds under mild neutral conditions, many functional groups such as esters, acetals, THP ethers, alcohols, halogens, and amines are tolerated. The ketones obtained by the oxidation are sometimes chlorinated with CuCl2 to give chloro ketones as byproducts. For this reason, nonchlorinating Copper(I) Chloride is recommended as the reoxidizing agent. This is easily preoxidized to the CuII state with oxygen.4 In a laboratory synthesis, a stoichiometric amount of 1,4-Benzoquinone is conveniently used as the reoxidant.

The reaction is a unique method for the one-step synthesis of ketones from alkenes, and allows alkenes to be regarded as masked ketones which are stable to acids, bases, and nucleophiles. Particularly useful is the oxidation of terminal alkenes, which provides methyl ketones (eq 3).5 As a typical application, the allylation of a ketone, followed by the oxidation, affords a 1,4-diketone. A cyclopentenone can then be prepared by an aldol condensation (eq 4).5 The annulation method has widespread uses in the synthesis of natural products such as pentalenene,6 muscone,7 and coriolin.8 1,5-Diketones are prepared by 3-butenylation of a ketone followed by the oxidation. This process has been used to prepare cyclohexenones (eq 5).5

Simple internal alkenes are difficult to oxidize. However, the regioselective oxidation of internal alkenes takes place in the presence of suitably disposed oxygen functional groups by neighboring group participation. For example, a,b-unsaturated esters are oxidized to b-keto esters using Na2PdCl4 as catalyst and t-Butyl Hydroperoxide as the reoxidant (eq 6).9 Allylic ethers are oxidized to b-alkoxy ketones which can be converted to a,b-unsaturated ketones for use in annulation reactions (eq 7).10 Cyclohexene and cyclopentene can not be oxidized under the usual conditions, but are oxidized to cyclohexanone and cyclopentanone under different conditions. For example, chloride-free PdII salts, prepared from Pd(OAc)2 and HClO4, H2SO4, or HBF4, are active catalysts (eq 8).11 For additional examples of the Wacker process, see Palladium(II) Chloride-Copper(I) Chloride and Palladium(II) Chloride-Copper(II) Chloride.

Reaction with Alcohols and Phenols.2c

The reaction of alcohols with terminal alkenes affords acetals of ketones (eq 9).12 An elegant application of the reaction was a brevicomin synthesis (eq 10).13

Alkenes with an electron-withdrawing group such as styrene, Acrylonitrile, and acrylate are converted to acetals of the aldehydes rather than the ketones. The reaction of styrene with ethylene glycol affords the cyclic acetal (eq 11).12a 3,3-Dimethoxypropionitrile is produced commercially using methyl nitrite as the reoxidant. The nitrite can be regenerated easily by the oxidation of NO with oxygen (eq 12).14

The intramolecular reaction of phenols or enols affords furans and pyrans (eq 13).15

Reaction with Carboxylic Acids.2c

The intramolecular reaction of carboxylic acids with alkenes affords unsaturated lactones (eq 14).16

Reaction with Amines and Amides.2c

Reaction of amines with alkenes proceeds most smoothly as an intramolecular version. Amides can be used in the intramolecular reaction to afford various heterocyclic compounds. In the example shown in eq 15, it should be noticed that the PdII species is regenerated by the b-elimination of OH, rather than the b-hydrogen. For this reason the reaction proceeds catalytically without a Pd0 reoxidant.17

Reaction with Carbon Nucleophiles.

The cyclooctadiene (cod) complex of PdCl2, which is insoluble in organic solvents, reacts in ether with malonate or acetoacetate under mild heterogeneous conditions; facile carbon-carbon bond formation takes place to give a new complex in a quantitative yield. Further intramolecular reaction of the complex with a base affords the cyclopropane derivative. Attack of a second malonate on the complex yields the [3.3.0] system (eq 16).18 Carbopalladation of the double bond of N-vinylcarbamate with acetoacetate at -78 °C, and subsequent carbonylation of the Pd-carbon bond, proceeds smoothly to yield the carbocarbonylation product in 92% yield (eq 17).19

p-Allypalladium Complex Formation.20

p-Allylpalladium complexes are prepared by the reaction of alkenes with PdCl2 or its soluble forms under various conditions (eq 18).21 These p-allylpalladium chloride complexes react with carbon nucleophiles in DMSO as a coordinating solvent to form carbon-carbon bonds.22 Thus p-allylpalladium complexes are clearly different in chemical reactivity from other organometallic reagents, which normally react with electrophiles (eq 19).

Based on this reaction, allylic alkylation of alkenes is possible. Active methylene compounds, such as malonates and b-keto esters, can be introduced to a steroid skeleton by the reaction of the steroidal p-allylpalladium complex in DMSO (eq 20).23 The reaction of carbon nucleophiles also proceeds in the presence of an excess of Triphenylphosphine (eq 21).24

ortho-Palladation of Aromatic Compounds and Cyclopalladation of Allyl and Homoallyl Compounds.25

Azobenzene,26 N,N-dimethylbenzylamine,27 and related aromatic compounds react with Na2PdCl4 in ethanol to form stable ortho-palladation complexes. These carbon-palladium s-bonded complexes are useful for the preparation of ortho-substituted aromatic compounds by the facile insertion of alkenes, alkynes, and CO. For example, insertion of CO to the azobenzene complex affords 2-aryl-3-indazolone (eq 22),28 and facile insertion of styrene to the benzylamine complex yields a stilbene derivative (eq 23).1a,29

The cyclopalladation of allylic or homoallylic amines and sulfides proceeds due to the chelating effect of N and S atoms, and has been used for functionalization of alkenes. For example, i-propyl 3-butenyl sulfide is carbopalladated with methyl cyclopentanecarboxylate and Li2PdCl4. Reduction of the chelated complex with Sodium Cyanoborohydride affords the alkylated keto ester in 96% yield (eq 24).30 Functionalization of 3-N,N-dimethylaminocyclopentene for the synthesis of a prostaglandin skeleton has been carried out via a N-chelated palladium complex as an intermediate. In the first step, malonate was introduced regio- and stereoselectively by carbopalladation (eq 25).31 Elimination of a b-hydrogen generated a new cyclopentene, and its oxypalladation with 2-chloroethanol, followed by insertion of 1-octen-3-one and b-elimination, afforded the final product.

Oxidative Carbonylation.32

Oxidative Carbonylation of Alkenes.

Oxidative carbonylation of alkenes with PdCl2 in benzene affords b-chloroacyl chlorides (eq 26).33 Oxidative carbonylation of alkenes in alcohol affords a,b-unsaturated esters and b-alkoxy esters by monocarbonylation and succinate derivatives by dicarbonylation (eq 27).34

Intramolecular oxycarbonylation and aminocarbonylation are also known. As an example, frenolicin has been synthesized using oxycarbonylation at 1.1 atm of CO as a key step (eq 28).35 The intramolecular aminopalladation of a carbamate group and subsequent carbonylation of the substituted 3-hydroxy-4-pentenylamine proceeds smoothly in AcOH (eq 29).36

Oxidative Carbonylation of Alkynes.

Terminal alkynes are carbonylated to give acetylenecarboxylates using PdCl2 and CuCl2 as catalysts (eq 30).37 The acetylenecarboxylate in a b-lactam has been prepared by this procedure and then converted to a b-keto ester (eq 31).38

Oxidative dicarbonylation of acetylene with Pd(PhCN)2Cl2 in benzene affords the chlorides of maleic, fumaric, and muconic acids (eq 32).39 Methyl muconate is obtained by passing acetylene and oxygen through MeOH containing thiourea and a catalytic amount of PdCl2.40 The oxidative dicarbonylation of alkynes produces maleate derivatives as a main product using PdCl2 and CuCl2 as catalysts under oxygen in alcohol.41

Oxidative Carbonylation of Alcohols.

Oxalates and carbonates are formed by the oxidative carbonylation of alcohols. The reaction can be made catalytic by using PdCl2 and CuCl2 under oxygen in the alcohol.42 Either oxalate or carbonate is obtained chemoselectively under different conditions (eq 33). Alkyl oxalates are produced commercially using alkyl nitrites as reoxidants (eq 34).43

Reactions via Transmetallation of Organometallic Reagents.

Transmetalation of organometallic compounds of Hg, B, Sn, Si, Tl, etc., with PdCl2 produces the reactive organopalladium species, which undergoes insertion and coupling reactions. Aryl- or alkenylpalladium complexes, generated in situ from aryl- or alkenylmercury compounds, undergo insertion reactions with alkenes;44,45 an example is shown in eq 35.46 The arylmercury compound with 1,3-cyclohexadiene and Li2PdCl4 generates a p-allylpalladium intermediate, which then attacks the amide group intramolecularly to yield the cyclized product (eq 35).46 CO insertion produces ketones and esters.47 The ortho-thallation of benzoic acid and subsequent transmetalation with PdII generates a reactive arylpalladium complex, which reacts with butadiene to give an isocoumarin (eq 36).48

a,b-Unsaturated esters are obtained by the carbonylation of alkenylboranes49 and alkenyl- or arylpentafluorosilicates (eq 37).50 Conjugated dienes and diaryls are formed by the coupling of alkenyl- and arylstannanes. The homocoupling of the vinylstannane of benzoquinone is catalyzed by PdCl2(PhCN)2 with benzoquinone as the reoxidant (eq 38).51

Miscellaneous Oxidation Reactions.

Some oxidative reactions can be carried out only with Pd(OAc)2, but not with PdCl2. However, Pd(OAc)2 can be generated in situ by the reaction of PdCl2 with AcOK or AcONa. The oxidative coupling of aromatic rings is a typical example of a Pd(OAc)2-promoted reaction. The following coupling reaction proceeds by Pd(OAc)2 generated in situ from PdCl2 (eq 39).52

The following oxidative rearrangement of a propargylic ester proceeds with a catalytic amount of PdBr2 under oxygen. Interestingly, the reoxidation of Pd0 takes place with oxygen without addition of other reoxidants (eq 40).53

Catalytic Reactions with PdII.

Exchange Reactions of Vinyl Ethers and Esters.54

Vinyl ethers are activated by PdII. Exchange with other alcohols to give mixtures of acetals and vinyl ethers is catalyzed by PdCl2 (eq 41).55 This reaction was used as the key step in the total synthesis of rhizobitoxine (eq 42).56

The exchange reaction of the acid component of vinyl esters with other acids is catalyzed by PdCl2 (eq 43).54 Thus various vinyl esters are prepared from easily available Vinyl Acetate. As an example, vinyl itaconate is prepared by the reaction of vinyl acetate with itaconic monomethyl ester (eq 44).57 N-Vinyllactams and cyclic imides are prepared by the exchange reaction of lactams and imides with vinyl acetate (eq 45).58

PdII-Catalyzed Rearrangement Reactions.

Cope rearrangements are accelerated by catalytic amounts of Pd(PhCN)2Cl2, such that they proceed at room temperature in benzene or CH2Cl2 (eq 46).59 Successful PdII catalysis appears to require that atoms 2 and 5 of the substituted 1,5-hexadienes have one H and one nonhydrogen substituent.60 Oxy-Cope rearrangements proceed at room temperature using Pd(PhCN)2Cl2 catalysis (eq 47).61

The Pd(PhCN)2Cl2 catalyzed Claisen rearrangement of allyl vinyl ethers has been studied to a lesser extent. The Claisen rearrangement shown in eq 48 proceeds smoothly even at room temperature to give the syn product with high diastereoselectivity.62 The Claisen rearrangement of 2-(allylthio)pyrimidin-4-(3H)-one affords the N-1 allylation product as a main product rather than the N-3 allylation product (eq 49).63

The rearrangement of allylic esters, a useful reaction, is catalyzed efficiently by PdII.64 The allylic rearrangement shown in eq 50, used in a prostaglandin synthesis, proceeds in one direction irreversibly, yielding the thermodynamically more stable product possibly due to steric reasons.65 The diacetate of a 1,5-diene-3,4-diol is isomerized to the more stable conjugated diene with complete transfer of chirality (eq 51).66 The PdII-catalyzed allylic rearrangement has been explained by an oxypalladation or cyclization-induced rearrangement. It is mechanistically different from rearrangements catalyzed by Pd0 complexes, which proceed by formation of p-allylpalladium intermediates.

Skeletal rearrangements of some strained compounds, such as bulvalene to bicyclo[4.2.2]deca-2,4,7,9-tetraene,67 cubane to cuneane,68 hexamethyl Dewar benzene to hexamethylbenzene (eq 52),69 and quadricyclane to norbornadiene (eq 53),70 are catalyzed by derivatives of PdCl2.

Intramolecular Reactions of Alkynes with Carboxylic Acids, Alcohols, and Amines.

Addition of carboxylic acids, alcohols, and amines to alkynes via oxypalladation and aminopalladation proceeds with catalysis by PdII salts. Intramolecular additions are particularly facile.71 Unsaturated g-lactones are obtained by the treatment of 3-alkynoic acid and 4-alkynoic acid with Pd(PhCN)2Cl2 in THF in the presence of Et3N (eq 54), and d-lactones are obtained from 5-alkynoic acids.72 5-Hydroxyalkynes are converted to the cyclic enol ethers (eq 55).71 The oxypalladation is a trans addition. Thus stereoselective enol ether formation by reaction of the alkynoic alcohol with Pd(PhCN)2Cl2, followed by reduction with Ammonium Formate, has been applied to the synthesis of prostacyclin (eq 56).73 Intramolecular addition of amines affords cyclic imines. 3-Alkynylamines are cyclized to 1-pyrrolines while 5-alkynylamines are converted to 2,3,4,5-tetrahydropyridines (eq 57).74

Simple alkynes cannot be hydrated with a palladium catalyst, but triple bonds are hydrated regioselectively to yield ketones with participation of suitably located carbonyl or hydroxy groups. 1,5-Diketones are prepared by the participation of a 5-keto group (eq 58).75 4-Hydroxyalkynes are converted to 4-hydroxy ketones and then oxidized to 1,4-diketones (eq 59).71

Cyclopentenone formation by the isomerization of 3-acetoxy-1,4-enynes is catalyzed by Pd(PhCN)2Cl2 (eq 60).76

Generation of Carbenes from Diazo Compounds.

Both PdCl2 and Pd(OAc)2 are used for carbene generation from azo compounds.77 The cyclopentenone carboxylates have been prepared by intramolecular insertions of the carbenes generated from a-diazo-b-keto esters (eq 61).78

Generation of Pd0 catalysts.

Pd0 catalysts can be generated in situ from PdII in the presence or absence of phosphine ligands. Tetrakis(triphenylphosphine)palladium(0) is a commercially available Pd0 complex used frequently as a catalyst, but it is air unstable. Therefore in situ generation of Pd0(Ph3P)n catalysts by the reduction of PdII in the presence of Ph3P is convenient to use. In many cases the in situ reduction to Pd0 takes place without addition of reducing agents. Alkenes, alcohols, CO, and phosphines, present in the reaction medium, behave as the reducing agent and react with PdII to give Pd0. Generation of Pd0 by reduction of Pd(OAc)2 with phosphines has been reported.79 Similarly, PdCl2 and its derivatives have been converted to Pd0 species with phosphines and bases.

PdCl2 itself is used for the carbonylation of an aryl iodide in the presence of a base (eq 62).80 More frequently, Bis(benzonitrile)dichloropalladium(II) is used for various Pd0-catalyzed reactions. The coupling reaction of an acyl chloride with a disilane is catalyzed by Pd0, generated from Pd(PhCN)2Cl2 and Ph3P (eq 63).81 The intermolecular coupling of a vinylenedistannane with two alkenyl iodides has been carried out using Pd(PhCN)2Cl2 without addition of Ph3P in a total synthesis of rapamycin (eq 64).82

Dichlorobis(triphenylphosphine)palladium is used for Pd0-catalyzed reactions without adding a reducing agent. For example, the coupling of terminal alkynes with halides is carried out with Pd(Ph3P)2Cl2 and Copper(I) Iodide in the presence of Triethylamine without addition of a reducing agent. Hexaethynylbenzene is prepared by the coupling of hexabromobenzene with trimethylsilylacetylene (eq 65).83 Similarly, the carbonylation of cinnamyl acetate, to give naphthyl acetate, is carried out in the presence of Et3N (eq 66).84

In some cases, Pd(Ph3P)2Cl2 is reduced to Pd0 in situ with reducing agents such as metal hydrides, and used for Pd0 catalyzed reactions. For example, Pd(Ph3P)2Cl2 is reduced with Diisobutylaluminum Hydride and used for coupling reactions (eq 67).85

The carbonylation of alkenes in alcohols to give saturated esters proceeds smoothly with PdCl2 or Pd(Ph3P)2Cl2 as a catalyst (eq 68).86 Alkynes are carbonylated efficiently to give a,b-unsaturated esters with the same catalyst in the presence of Iodomethane (eq 69).87 In some reactions the Pd0 species generated from PdCl2-Ph3P and Pd(OAc)2-Ph3P show different reactivities. For example, in the carbonylation of 1,3-Butadiene, 3-pentenoate is obtained with PdCl2-Ph3P, while 3,8-nonadienoate is obtained with Pd(OAc)2-Ph3P. The presence of chloride anion in the coordination sphere of palladium gives different catalytic activity (eq 70).88

Related Reagents.

Palladium(II) Chloride-Copper(I) Chloride; Palladium(II) Chloride-Copper(II) Chloride; Palladium(II) Chloride-Silver(I) Acetate.

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Jiro Tsuji

Okayama University of Science, Japan

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