Catalytic cycle, allyl acetates

The in situ regeneration of Pd(II) from Pd(0) should not be counted as being an easy process, and the appropriate solvents, reaction conditions, and oxidants should be selected to carry out smooth catalytic reactions. In many cases, an efficient catalytic cycle is not easy to achieve, and stoichiometric reactions are tolerable only for the synthesis of rather expensive organic compounds in limited quantities. This is a serious limitation of synthetic applications of oxidation reactions involving Pd(II). However it should be pointed out that some Pd(II)-promoted reactions have been developed as commercial processes, in which supported Pd catalysts are used. For example, vinyl acetate, allyl acetate and 1,4-diacetoxy-2-butene are commercially produced by oxidative acetoxylation of ethylene, propylene and butadiene in gas or liquid phases using Pd supported on silica. It is likely that Pd(OAc)2 is generated on the surface of the catalyst by the oxidation of Pd with AcOH and 02, and reacts with alkenes. 

A chemo- and highly regio-selective Pd-catalysed allylic oxidation reaction that proceeds via a novel mechanism where two different ligands interact serially with palladium to promote different steps of the catalytic cycle has been reported. Initial formation of a dimeric 7r-allylpalladium acetate complex has been proposed.41... 

Frequently, Pd(PPh3)4 (or another Pd" complex) is used as a catalyst for the displacement of aUyUc acetates or halides by nucleophiles. A general catalytic cycle is depicted in Scheme 30. If chloride ions are present, no ( 7r-allyl)PdL2 forms. Instead, a ((( -allyl)PdL2Cl intermediate is formed. Thus, different precursors (such as aUylic chlorides or aUylic acetates) or different reaction conditions can lead to different reactivities, regioselectivities, and enantioselectivities. 

From a mechanistic point of view the first steps of the catalytic cycle should be similar to the telomerization of butadiene itself (Scheme 2). The catalytic precursor generates the Pd(0) species A that reacts to the bis-(ri -allyl) complex C. The C,C bond formation between two C4 units is followed by insertion of carbon dioxide into a Pd,C bond affording the carboxylate intermediate D. Different pathways have been discussed to describe the multiple product formation (refer to ). Interestingly, a bis-(carboxylato) complex may be prepared directly from the reaction of lactone 1, palladium acetate and P(i-Pr)3. This complex was structurally characterized by Behr and co-workers and shows good activity as catalyst. Reviewing the literature, there are some remarkable facts and open questions of theoretical and technical interest ... 

A wide variety of nucleophiles add to an -rf-allyl ligand. Desirable nucleophiles typically include stabilized carbanions such as CH(COOR)2 or 1° and II0 amines. Unstabilized nucleophiles such as MeMgBr or MeLi often attack the metal first and then combine with the n-allyl by reductive elimination. The Tsuji-Trost reaction, which is typified by the addition of stabilized carbanions to T 3—allyl ligands complexed to palladium followed by loss of the resulting substituted alk-ene, comprises an extremely useful method of constructing new C-C bonds, and many applications of this reaction have appeared in the literature.61 Equation 8.43 illustrates an example of a Pd-catalyzed addition of a stabilized enolate to an allyl acetate.62 The initial step in the catalytic cycle is oxidative addition of the allyl acetate to the Pd(0) complex, followed by nq1 to nq3—allyl isomerization, and then attack by the nucleophile to a terminal position of the T 3—allyl ligand. We will discuss the Tsuji-Trost reaction, especially in regard to its utility in chiral synthesis,63 more extensively in Chapter 12. 

The generally accepted mechanism of palladium-catalyzed allylic substitutions is shown in Scheme 1. An allylic substrate 1, typically an acetate or a carbonate, reacts with the catalyst, which enters the catalytic cycle at the Pd(0) oxidation level. Both Pd(0) and Pd(II) complexes can be used as precatalysts, because Pd(II) is easily reduced in situ to the active Pd(0) form. Presumably, the reaction is initiated by formation of a Ji-complex which eliminates X to produce an (ri -allyl)palladium(II) complex. The product of this oxidative addition can... 

One binuclear complex may be involved in the catalytic cycle for butadiene oligomerization. Allylpalladium acetate reacts with butadiene to form an acetate-bridged allyl complex. Heptadiene is displaced from this intermediate when it is treated with additional butadiene, and a binuclear, acetate-bridged complex of the 2,6,10-dode-catriene-l,12-diyl ligand is claimed to be formed ... 

Formates. The decarboxylation reaction of metal formates is a fairly general route for the synthesis of metal hydrides and it has been applied to many transition metals. As an example, allyl palladium formates, which are believed to be intermediates in the catalytic reductive cleavage of allylic acetates and carbonates with formic acid to give monoolefins (Scheme 6.32), have been synthesized. In fact the complexes undergo decarboxylation and the reductive elimination of the allyl hydrido fragments, supporting the catalytic cycle proposed [105]. 

Among very rare examples of catalytic transformations involving polyene and polyenyl ligands is Pd-catalyzed oxidation of diene derivatives by the use of acetic acid and quinone with unique stereochemical control being achieved by judicious choice of the reaction condition (Scheme 8.65) [121]. Thus, the oxidation carried out with high Cl concentration afforded cis diacetate product, while trans adduct was obtained in the absence of Cl ion. The initial step of the catalytic cycle would be the exo attack of OAc at the Pd-bound diene, giving rise to r -allyl intermediate with OAc and Pd positioned trans to each other. This then underwent either exo or endo attack of the second OAc in the presence or absence of Pd-bound Cl ligand, respectively. The endo attack may have proceeded in a manner similar to Scheme 8.50. The hnal step of the catalysis would be oxidation of Pd(0), formed by the OAc attack at the r -allyl terminus, with benzoquinone as an oxidant. 

Two crucial requirements for any catalytic reactions are (i) that the overall catalytic processes be thermodynamically favorable (i.e., AAG<0) and (ii) that all steps in a given catalytic cycle be kinetically accessible (i.e., of reasonably low activation energies). Moreover, so long as these two requirements are met, one or more of the microsteps in a catalytic cycle can be thermodynamically unfavorable. This is an obvious principle that nonetheless is frequently misunderstood. For example, the stoichiometric oxidative addition reaction of allyl acetate with Pd(0) complexes does not normally give the desired allylpalladium derivative in significant yields, and it may well be thermodynamically unfavorable. And yet, the Tsuji-Trost reaction of allyl acetate with malonates is normally facile. It is very important not to rule out any potentially feasible catalytic processes simply because some microsteps are or appear to be thermodynamically unfavorable. 

The authors suggested that the real catalyst was the neutral intermediate I79b, detected as a cluster at m/z = 611, corresponding to I79b. The oxidative addition of ailylic acetate Sib to the above intermediate afforded the formation of a Pd(IV) ji-allyl complex 184, which was not detected directly by ESI-MS. However, the authors postulate that this complex may be related to intermediates I81b,I83 and 182, which were all observed in the ESI mass spectra. Reductive elimination from 184 and interaction with S4 close the catalytic cycle. 

In 1982 Jiro Tsuji and co-workers published a conununication launching allyl carbonates as substrates in the Pd(0)-catalyzed allylation of nucleophiles. It was followed three years later by a full paper. The overall reaction and catalytic cycle are shown in Scheme 1. One of the advantages over acetates is that addition of external base is not required. Indeed, the pronucleophile Nu-H reacts with the in situ generated alkoxide to form the actual nucleophile. The maximal concentration of base at any moment in the reaction medium depends on the relative rates of the individual steps of the cycle, but cannot be higher than the maximal quantity of the catalytic species PdL2. Therefore, the reaction takes place formaUy in... 

Carboxylic acids have been prepared from carbonylation of aUyl bromide over palladium catalysts supported by polyphenol, polyvinylpyrrolidone,polyacrylamide (PAA), modified poly(2,6-dimethyl-l,4-phenylene oxide), and polysulfone- It is unclear under these reducing conditions if gel form metal cluster is involved as the active catalyst. Palladium acetate immobilized on the clay montmoriUonite has proved to be an effective catalyst for the carbonylation of secondary aUylic alcohols, affording a,/3-unsaturated carboxylic acids in moderate yields. However, triphenylphosphine was needed for the acliviva-tion of the catalyst. Palladium catalyst bound to a platinum cluster has been used up to three times for allylic alkylation without a significant loss of its activity. Preliminary study indicated that the Pd—Pt bond remains intact during the catalytic cycle. 

Terminal alkenes can be transformed into predominately linear and -configured allylic acetates using 1,4-benzoquinone in the presence of catalytic quantities of Pd(OAc)2 and a mixture of DMSO and acetic acid as solvent (eq 74). Wacker-type oxidation products are not observed, perhaps as a result of the stabilization, by DMSO, of a charged intermediate in the catalytic cycle. ... 

SCHEME 6 Proposed catalytic cycle for the Heck-Matsuda reaction with allyl acetates. 

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