Bis(allyl)di-m-chlorodipalladium1

[12012-95-2]  · C6H10Cl2Pd2  · Bis(allyl)di-m-chlorodipalladium  · (MW 365.89)

(prototype substrate for p-allylpalladium reactions,1 catalyst precursor for allylic alkylations,1 cross-coupling reactions,1 diene dimerization,2 addition of nucleophiles to dienes,3 conversion of allyl amines to dienes,4 siloxycyclopropane cleavage,5 and decomposition of diazo compounds6)

Physical Data: mp 160 °C.

Solubility: sol benzene, CHCl3, acetone, methanol.

Form Supplied in: pale yellow crystals; widely available.

Analysis of Reagent Purity: IR;7 MS;8 13C NMR;9 1H NMR.10

Preparative Methods: numerous routes are available.1 Preferred methods are: reaction of propene with Palladium(II) Trifluoroacetate, followed by counterion exchange with chloride;11 reaction of Potassium Tetrachloropalladate(II) with Allyl Chloride in water;12 reaction of Na2PdCl4, allyl chloride, and CO in water.13

Prototype Substrate for p-Allylpalladium Reactions.

Examples of the addition of nucleophiles to bis(allyl)di-m-chlorodipalladium (1) and the homologous unsymmetrical di-m-chlorobis(crotyl)dipalladium (2) are shown below. The latter is included to illustrate aspects of the regiochemistry of nucleophilic attack. The addition of ligands, typically phosphines, is generally required to enhance the electrophilicity of these reagents.14

Carbon nucleophiles of pKa 10-20 have been most studied and often work best with these substrates (eq 1),1,15 but enamines,1 cyclopentadiene anions,1q enolates (eq 2),1,16 organotin, -thallium, -zinc, -aluminum, -lithium, and -zirconiums,1q as well as Grignards (eq 3)1,17 and borates,18 also add to p-allylpalladium complexes. Heteroatom nucleophiles also add successfully; these include amines (eq 4),1,19 amides,1q azides,1q magnesium amides,1q sulfonamides,1q alcohols,1q acids (eq 5),1,20 nitrites,1q sulfinic acids,1q thiols,1q phosphines,1q and phosphites.1q Limited use of transition metal nucleophiles is also known.21

The use of Hexamethylphosphoric Triamide and Triethylamine as ligands in place of the usual phosphines in the addition of ester enolates to (1) results in the formation of cyclopropanes via the unusual initial attack at the central carbon atom (eq 6).22 The process is very limited with respect to nucleophile, however, as only branched ester enolates produce a cyclopropane. N,N,N,N-Tetramethylethylenediamine and Carbon Monoxide as ligands also promote central carbon attack by ester enolates.23

The attack by nucleophiles on p-allylpalladium complexes is generally a stereospecific process, but whether the reaction occurs with retention or inversion with respect to the metal is a complex issue, and strongly depends on the nature of the nucleophile employed.24

Insertion Reactions.

CO can be inserted into p-allylpalladium dimers to prepare various b,g-unsaturated carboxylic acid derivatives (eq 7).25,26 This process is greatly facilitated by the presence of carboxylate anions. The bridged acetato complex (3) works equally as well as (2) in this reaction. Similarly, isocyanides,27,28 CO2,29 and SO21q can also be inserted into these complexes.

p-Allylpalladium dimers can also react with alkenes (eq 8),30 dienes,31 and allenes1 by an insertion process.

Reduction.

A variety of metal hydride reducing agents can be employed to reduce p-allylpalladium dimers. These include Sodium Borohydride, Sodium Cyanoborohydride, Lithium Triethylborohydride, Lithium Aluminum Hydride, Lithium Tri-t-butoxyaluminum Hydride, polymethylhydrosiloxane, and R3SiH (eq 9).1q,32 Formic acids and formate salts also reduce p-allylpalladium complexes to the corresponding alkenes by the liberation of CO2 and the in situ formation of a Pd hydride.

The use of stoichiometrically generated p-allylpalladium dimers such as (1), (2), and (3) has largely been obviated by the discovery that the crucial allyl intermediate can be generated catalytically from PdL4 and an allyl-X compound, where X is a wide variety of leaving groups (see Tetrakis(triphenylphosphine)palladium(0) for example).1

Use as a Catalyst Precursor. Allylic alkylations.

The allylpalladium chloride dimer (1) is a particularly useful catalyst precursor for a variety of Pd-mediated processes and possesses a number of significant advantages over PdL4 complexes. For example, the dimer (1) can be employed in conjunction with a ligand L and be subjected to designed in situ reduction to yield a Pd0-L catalyst. A catalyst generated in this fashion allows rapid access to a great spectrum of Pd0-L species by obviating the need for individual syntheses of each PdL4 complex. In addition, the ratio of Pd/L, which is often crucial in defining catalyst performance, can be simply varied. Such variation is obviously not possible with a PdL4 complex.

The use of (1) as a catalyst precursor in a classic p-allyl alkylation is shown in eq 10 as a key step in the preparation of a carbanucleoside, carbovir.33

Significant use of (1) as a Pd0 catalyst precursor has come in the area of asymmetric p-allylpalladium alkylations.34 A large number of optically active ligands, L*, have been screened in the palladium-catalyzed alkylation step. Particularly promising L* ligands for this reaction include: phosphinoaryldihydrooxazoles (eq 11);35 ferrocenylphosphines appended with a side chain designed to interact with the incoming nucleophile (eq 12);36 phosphinooxazoles;37 thienyloxazoles;38 bis-oxazoles;39 5-aza-semicorrins;40 and BINAP.41

The chloro dimer (1) has also been employed in Pd-catalyzed asymmetric hydrosilation of alkenes (eq 13).42

Cross-Coupling Reactions.

The transition metal catalyzed cross-coupling reaction of an organometallic nucleophile and an organic halide constitutes an important carbon-carbon bond forming process. Organometallic compounds of zinc, aluminum, boron, zirconium, magnesium, mercury, copper, tin, silicon, and palladium have all been employed as coupling partners. The dimer (1) has served as an excellent catalyst for this process, as illustrated below. The coupling of organotin (eq 14)43,44 and -zirconium (eq 15)45 reagents with allyl halides has employed a combination of (1) and maleic anhydride to effect cross coupling. The fluoride promoted, Pd-catalyzed, cross coupling of organosilanes and organohalides appears to be a particularly mild and selective way to achieve this reaction. This process allows coupling of alkenylsilanes with alkenyl and allyl halides (eq 16),46,47 aryl- and heteroarylsilanes with aryl and heteroaryl iodides (eq 17),48,49 alkyl silicates with aryl halides (eq 18),50 and ethynyl- and allylsilanes with vinyl, aryl, and allyl halides.51

In the presence of CO these fluoride-promoted Pd-catalyzed cross-coupling reactions provide unsymmetrical ketones (eq 19),52,53 including polyaryl ketones.54

The title reagent can also catalyze the stannylation (eq 20)55,56 and silylation57,58 of organic halides via cross coupling.

Diene Dimerization.

The reductive dimerization of isoprene to its head-to-tail dimer has been achieved in high yield using di-m-acetatobis(allyl)dipalladium (4) as the catalyst in combination with phosphine ligands.2 To achieve optimal selectivity for the desired dimers, a large number of phosphines have been examined in this reaction. Use of the complex (4) with the phosphine allows rapid access to each of the desired catalysts, as well as maximum flexibility with respect to Pd/L ratio (eq 21).2

The organopalladium intermediates in the diene dimerization can also be intercepted by reagents such as ROH,2 R3SiSiR3,59 and CO2 (eq 22).60

Codimerization of dienes and alkenes is also possible (eq 23). In this case, (1) and Ph3P-BF3 are employed as cocatalysts.61 Complex (1) has also been used in the polymerization of butadiene.62

Addition of Nucleophiles to Dienes.

The use of (1) with bidentate phosphine ligands and NaOMe to effect in situ Pd reduction allows the selective addition of nucleophiles to 1,3-dienes, including isoprene, without significant competing diene dimerization (eq 24).3

Arenesulfinic acids have also been reported to add to 1,3-dienes in the presence of (1) and Ph3P to yield allylic sulfones (eq 25).63

Preparation of Dienes from Allylic Amines.

Treatment of allylic amines with a catalyst generated from the reaction of (1) with Ph2P(CH2)4PPh2 (dppb) and NaClO4 gave the corresponding diene (eq 26).4 Interestingly, the catalyst generated in this fashion was found to be superior to Pd(dppb)2.

Siloxycyclopropane Cleavage.

Siloxycyclopropanes undergo C-C bond cleavage and coupling to aryl triflates (eq 27)5a,b and acid chlorides (eq 28)5a,c in the presence of (1) and a phosphine or phosphite ligand. In this fashion, the siloxycyclopropanes serve as synthetically useful homoenolate anions.

Decomposition of Diazo Compounds.

The title reagent decomposes ethyl diazoacetate to the corresponding carbene under mild conditions.6a If the decomposition is conducted in the presence of an alkene, a cyclopropane is produced (eq 29).6a

Diazo ester (5) undergoes ring enlargement via a carbene intermediate with (1) as the catalyst (eq 30).6b

Diazo ester (6) follows different decomposition pathways depending on the transition metal catalyst employed.6c Rh2(OAc)4 yielded (7), while (1) gave (8) (eq 31).


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Stephen A. Godleski

Kodak Research Laboratories, Rochester, NY, USA



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