Dilithium Tetrachloropalladate(II)1

Li2PdCl4

[15525-45-8]  · Cl4Li2Pd  · Dilithium Tetrachloropalladate(II)  · (MW 262.11) (hydrate)

[123334-21-4]

(catalyst for coupling of aryl- and vinylmercurials with alkenes2 and carbon monoxide;3 can cyclopalladate many compounds;4 can oxidize alkenes5)

Solubility: sol H2O, polar organic solvents.

Form Supplied in: commercially available as a hydrate, Li2PdCl4.xH2O; red purple solid. Drying: at 100 °C and 0.1 mmHg for 24 h.

Preparative Method: very often prepared in situ from 1 equiv of Palladium(II) Chloride and 2 equiv of Lithium Chloride in the appropriate solvent (MeOH, EtOH, MeCO2H, THF).

Handling, Storage, and Precautions: is hygroscopic and should be stored in the absence of moisture. Toxicity data are not available.

Coupling of Mercury Derivatives.

Various arylmercurials undergo transmetalation with Li2PdCl4 in methanol, ethanol, acetic acid, or THF to form arylpalladium salts. In the presence of alkenes these produce arylated alkenes (eq 1).6 The reaction is usually stoichiometric in Li2PdCl4; however, a catalytic version with Copper(II) Chloride as a cooxidant has been reported.7

Similarly, organopalladium intermediates derived in situ from 5-mercurated pyrimidine nucleosides and Li2PdCl4 in MeOH react with alkenes and disulfides to produce nucleosides substituted at the C-5 position by carbon8 or sulfur (eq 2).9

Vinylmercury(II) chlorides, acyclic alkenes, and Li2PdCl4 generate halide-bridged dimeric (p-allyl)palladium complexes in high yield.10 The analogous reaction of monocyclic alkenes affords either (p-allyl)palladium complexes or 1,4-dienes, depending on the reaction conditions employed (eq 3).2 (p-Allyl)palladium complexes can be prepared by the reaction of Li2PdCl4 with bicyclic alkenes,11 dienes,12 vinyl- and methylenecyclopropanes13, vinyl- and methylenecyclobutanes,14 and dienolsilyl ethers.15 These p-allyl complexes are electrophilic in character and undergo reaction with a variety of nucleophiles.16

Aryl- and vinylmercurials readily react with Carbon Monoxide in the presence of Li2PdCl4 at atmospheric pressure. a,b-Unsaturated acids are formed in high yields in aqueous THF or acetic acid.3 Analogous reaction in the presence of alcohols affords a,b-unsaturated esters (eq 4).17

Addition of vinyl- and arylmercurials to vinyl epoxides gives allylic alcohols.18

Cyclopalladation.

Tertiary benzylamines,19 benzalimines,20 aromatic azo compounds,21 and substituted furans22 undergo coordination with Li2PdCl4, followed by electrophilic attack of the metal on the ortho-position of the benzene/furan ring, to give a cyclopalladated five-membered ring. Cyclopalladation reactions have been extensively reviewed.4 The system Li2PdCl4/NaOAc is most commonly used, acetate acting as a base to facilitate the C-H bond cleavage. The palladation can be performed in methanol, ethanol, acetone, dioxane, and chloroform, as well as in binary water-organic mixtures. As a rule, palladation occurs at ambient temperature. However, heating is required in certain cases. These palladacycles can react with alkenes (eq 5), carbon monoxide, carbon dioxide, halogens, organolithium and Grignard reagents, acid chlorides, and alkyl chlorides.23

Allylic24 and homoallylic25 amines and sulfides undergo regiospecific carbopalladation in the presence of stabilized enolates and Li2PdCl4 to provide stable five-membered palladacycles (eq 6). This reaction has also been performed intramolecularly.26 Pd in these cycles can be replaced by carboxylate26 (CO, MeOH), hydrogen24 (H2 or Sodium Borohydride), or by a substituted vinyl group26 (Methyl Vinyl Ketone, Triethylamine).

Oxidation of Alkenes.

Alkenes can be oxidized by Li2PdCl4 to aldehydes or ketones (Wacker reaction).5 Because the reagent is expensive, the reaction is usually carried out with a cooxidant, most often CuCl2. In the presence of alcohols, acetals and vinyl ethers may be formed (eq 7).5b Intramolecular versions of oxypalladation are known to produce unsaturated lactones. Carboxylic acids with D3,4, D4,5, and D5,6 unsaturation undergo this reaction in aqueous solution.25,27,28 Cyclization of 2-hydroxychalcones gives flavones (eq 8).29

Vinylation.

N-Phenylhydroxamic acids can be vinylated in the presence of Li2PdCl4 to give derivatives that readily undergo [3,3]-sigmatropic rearrangement to form indoles (eq 9).30


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Valeri Martichonok

University of Toronto, Ontario, Canada



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