Trimer, palladium acetate

While it is evident that aUyhc acetoxylation and related reactions proceed via two different mechanisms, mainly depending on the structure of the alkenes, it is less clear how to choose reaction conditions in order to favor one route or the other. There is some evidence from early smdies that the use of polar solvents such as DMF will promote aUyl acetate formation. It also appears that excess acetate promotes the formation of products compatible with the rr-allyl route. This is also suggested by recent factorial experiments with variation of carboxylate concentration. Since trimeric palladium acetate will induce rr-allyl formation from a series of monoolefins, it might be assumed that high concentration of palladium acetate could be used for creating conditions that favor a 77-aUyl route. " Another possibility is adding strong acids, which can increase the electrophilicity of the catalyst, but this can drive the reaction toward homoallylic acetates and other isomerized products. "... 

Ne+ ion irradiation of a 0.13 nm thick film produces a metallic silvery film. A plot of the infrared COO vibrations as a function of fluence in Figure 10 shows that the intensity decreases with approximately the same functional dependence as in the He ion irradiation, but at a dose that is 17 times lower. In addition, a new band appears at 1616 cm-1, peaking at a dose of — 1.7x1012 ions/cm2, then decreasing rapidly to the same level as the original acetate bands. This may represent the formation of some monodentate acetate species as the palladium acetate trimers are cleaved. In situ infrared spectra of the He ion-irradiated films show a similar band of much smaller relative intensity. 

While palladium acetate exists as a trimer with acetate bridges in both solution and the solid state, the metal-metal distance is too long to invoke a Pd-—Pd bond.108,109 The reaction of [Pd2Cl2(dppm)2] (9) with metalloanions yields trimers and tetramers (see... 

Palladium acetate has a trimeric ring structure (17) which is necessarily cleaved if a complex is formed. Both the a-olefin I and the internal olefin II follow a similar path in reacting with Pd3(OAc)6 to form trinuclear 1,2,3- and 2,3,4-/i3-hexenylpalladium complexes, Pd3-(C6Hii)2(OAc)4 (IVa and IVb, respectively) (Reaction 1). At 25°C in acetic acid this reaction is complete within 6 hrs, and if oxygen is absent, a second reaction follows in which the corresponding dimeric 1,2,3- and 2,3,4-/i3-hexenyl complexes Pd2(C6Hii)2(OAc)2 (Va and Vb, respectively) appear (Reaction 2). 

A study of the olefin oxidation catalyst system, palladium acetate-MOAc (M = Li or Na), has shown that in the absence of acetate ion, Pd acetate-acetic acid exists as the trimeric species [Pd3(OAc)6].32 Reaction with MOAc is not instantaneous, and u.v.-visible spectra indicate an initial equilibrium involving trimer - dimer (9). When M = Na conversion into dimer is complete at 0.2M-NaOAc. Further addition of... 

Co30(OAc)fi(HOAc)3. Similarly, copper(II) acetate371 exists as a dimer and palladium(II) acetate372 as a trimer in acetic acid. 

The more electrophilic reagent Pd(OAc)2 is another usefril reagent in organopalladium chemistry. It is monomeric in benzene at 80 °C, but is trimeric at room temperature in benzene. For even greater reactivity, Pd(02CCF3)2 can be used. Both the acetate and trifluoroacetate are soluble in organic solvents. Reaction of palladium acetate with acetylacetone produces Pd(acac)2. This acetylacetonate and especially the hexafluoroacetylacetonate, Pd(hfac)2, are useful as volatile sources of palladium in metalorganic chemical vapor deposition. 

Similar complexes are formed in the reaction of allylpalladium acetylacetonate (acac) and allene Palladium acetate, however, leads to a binuclear complex containing a trimer of allene ... 

The main path of the palladium-catalyzed reaction of butadiene is the dimerization. However, the trimerization to form /j-1, 3,6,10-dodeca-tetraene takes place with certain palladium complexes in the absence of a phosphine ligand. Medema and van Helden observed, while studying the insertion reaction of butadiene to 7r-allylpalladium chloride and acetate (32, 37), that the reaction of butadiene in benzene solution at 50°C using 7r-allylpalladium acetate as a catalyst yielded w-1,3,6,10-dodecatetraene (27) with a selectivity of 79% at a conversion of 30% based on butadiene in 22 hours. 

In high polarity solvents, such as acetonitrile, dimethyl sulfoxide, methyl acetate, and methanol, the branched dimer, 4-methyl azelate precursor, is formed in high yield. In methanol, a methoxy dimer (CH302CCgH] 4(0013)0020113) is also formed in moderate yield. Heavies contain both an acyclic methyl, 4-pentadienoate trimeric product and high molecular weight methyl, 4-pentadienoate homopolymer. Polymerization in the absence of air(48) appears to be catalyzed by traces of the tertiary phosphine which is used to prepare the palladium dimerization catalyst. 

The trimerization of cyclopentadiene (6) is catalyzed by a homogeneous bifunctional palladium-acid catalyst system.7 The reaction gives trimers 7 and 8 as a 1 1 mixture in 70% yield with bis(acetylacetonato)palladium(II) [Pd(acac)2] or with bis(benzylideneacetone)-palladium(O) as the palladium component of the catalyst. As the phosphorus component, phosphanes like trimethyl-, triethyl-, or triphenylphosphane, and triisopropylphosphite or tris(2-methylphcnyl)phosphite, are suitable. A third component, an organic acid with 3 < pK < 5, is necessary in at least equimolar amounts, in the reaction with cyclopentadiene (6), as catalytic amounts are insufficient. Acids that can be used are acetic acid, chloroacetic acid, benzoic acid, and 2,2-dimethylpropanoic acid. Stronger acids, e.g. trichloroacetic acid, result in the formation of poly(cyclopentadiene). The new catalyst system is able to almost completely suppress the competing Diels-Alder reaction, thus preventing the formation of dimeric cyclopentadiene, even at reaction temperatures between 100 and 130°C. 

In the absence of added acetate, molecular weight measurements showed the palladium(II) acetate existed in the form of trimer, Pd3(OAc)6. 

Finally a kinetic study of the oxidation of ethylene by palladium (II) acetate gave a rate-[NaOAc] profile similar to Figure 1 which could also be interpreted as conversion of less reactive trimer to more reactive dimer. However at [NaOAc] > 0.2M the decrease in rate with increase in [NaOAc] is much greater than that shown in Figure 1 and corresponds to a 1/[NaOAc] term in the rate expression for reaction of dimer. This difference in rate expression between exchange and olefin oxidation could have very interesting mechanistic implications. For instance, the added acetate inhibition term could result from the need for a vacant coordination site on the Pd (II) before hydride elimination can occur. The scheme is shown in Equations 31 and 32. 

The above approaches can be extended to 1,2-carboranes in which the BH vertices are substituted with alkyl groups <1996101235, 1996JA70>. The 9- and 12-boron vertices are the furthest from the carbon atoms in 1,2-carborane 7. Methyl substitution of 7 at these positions is accomplished by iodine substitution to form 9,12-diiodo-l,2-carborane 13 then treatment with alkyl Grignard reagent in the presence of a palladium catalyst affords 9,12-Me2-l,2-carborane 14, which can be deprotonated at the carbon vertices in the same manner as the parent 7. Using the same strategy to prepare cyclic trimer 5, dimethyl 14 is deprotonated and treated with mercuric acetate to form 5-hexamethyl-[9]-mercuracarborand-3 15 in 60% yield (Equation 1). 

The complex [Pt 4(CH3COO)8] (1), which is a tetrameter of plat-inum(II) acetate, was first found by Skapski and co-workers in a study aiming at the isolation of pure platinum(II) acetate (24-26). They revealed that pure platinum acetate obtained using the silver acetate method (27) has an unusual tetrameric structure (Fig. 1). The structure is totally different from the trimeric structure known for palladium(II) acetate, [PdgCCHgCOOle] (28). 

Palladium(II) acetate is usually prepared by the reaction of Pd sponge in hot glacial acetic acid with ititric acid as oxidant. A slight excess of Pd sponge must be used to make sure that all the HNO3 is consumed. Several other paUadium(II) carboxylates can be obtained similarly. The crystal-structure determination of the acetate reveals a trimeric stmcture with bridging acetates in the sohd state (1). Thus, the compound is better formulated as [Pd(/u-02CMe)2]3. 

The vinyl ester exchange has also been studied in the chloride-free palladium(II) acetate system, using vinyl propionate as substrate. Of the three Pd(II) species present in this system (Section II, A, 2), the dimer is most reactive, with the trimer Pd3(OAc)g next, and the monomer unreac-tive. The rate expression for exchange catalyzed by the dimer is (214). 

It has been concluded on the basis of spectroscopic evidence that the trimer formed on irradiation of a mixture of norbornadiene and (Ph3P)2Ni(CO)2 has the exo,trans, exo,trans,exo-structure (338) the exo,trans,exo-dimeT, also formed in the reaction, is the precursor of (338), The exo-cyclopropanation of norbomene and of norbornadiene by reaction with diazomethane in the presence of palladium(ii) acetate has been described. ... 

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