Acyl palladium complex

A similar involvement of palladium hydride, palladium alkyl, and palladium acyl complexes as intermediates in the catalytic cycle of the Pd-catalyzed hydroxycarbonylation of alkenes was reported for the aqueous-phase analogs. The cationic hydride PdH(TPPTS)3]+ was formed via the reduction of the Pd11 complex with CO and H20 to [Pd(TPPTS)3] and subsequent protonation in the acidic medium. The reaction of the hydride complex with ethene produced two new compounds, [Pd(Et)(TPPTS)3]+ and Pd(Et)(solvent)(TPPTS)2]+. The sample containing the mixture of palladium alkyl complexes reacted readily with CO to afford trans-[Pd(C(Q)Et)(TPPTS)2]+.665... 

Palladium acyl complexes such as [Pd(COR)(MeCN)L2] BF4 or [Pd(COR)ClL2]/ AICI3 (L = PPh3) catalyze the isomerization of isobutyryl chloride to an equilibrium mixture with -butyryl chloride. The reaction probably proceeds via a palladium hydride intermediate which was not observed (Scheme 10). The turnover is slow up to 12.9 in 7 days. " Palladium cyclopropenylidene pyridine complexes (21) catalyze the isomerization of quadricyclane to norbomadiene at... 

Alkenes may also insert into the Pd-C bonds of some palladium acyl complexes, " " an important reaction that has been successfully applied to the synthesis of polyketones of general formula [-CH2CH2C(0)-] via the alternating co-polymerization of CO and ethylene (Scheme... 

Despite all the synthetic developments, relatively little detailed mechanistic work has been performed on Sonogashira carbonylations until the present. The generally accepted mechanism is shown in Scheme 5.25. The typical reaction begins with the oxidative addition of ArX to a palladium(0) complex to form an aryl palladium(II) intermediate. The subsequent insertion of CO leads to the respective palladium acyl complex. Transmetallation, and finally reductive elimination, releases the product and a new catalytic cycle can be started. Notably, all species passing through the cycle are believed to be in a reversible equilibrium. 

Theoretical studies on a model diphosphine complex [PdEt(GO)(H2PGH2GH2PH2)Pd]" underestimate the activation energy for insertion somewhat (48 kj moP ). The product of GO insertion is predicted to adopt an 77 -acyl conformation following insertion, on account of its three-coordinate nature, although experimentally a further ligand coordinates as discussed above. The activation barrier for a second GO insertion into the palladium-acyl complex is calculated to be 109 kj mol and the reaction is strongly endothermic by 80 kj moP This provides an explanation for the lack of observable double GO insertion in palladium catalyzed GO/ethylene co-polymerization. 

The carbonylation was explained by the following mechanism. Formation of dimeric 7r-allylic complex 20 from two moles of butadiene and the halide-free palladium species is followed by carbon monoxide insertion at the allylic position to give an acyl palladium complex which then collapses to give 3,8-nonadienoate by the attack of alcohol with regeneration of the zero-valent palladium phosphine complex. When halide ion is coordinated to palladium, the formation of the above dimeric 7r-allylic complex 20 is not possible, and only monomeric 7r-allylic complex 74 is formed. Carbon monoxide insertion then gives 3-pentenoate (72). 

Carbon monoxide rapidly inserts into the carbon—zirconium bond of alkyl- and alkenyl-zirconocene chlorides at low temperature with retention of configuration at carbon to give acylzirconocene chlorides 17 (Scheme 3.5). Acylzirconocene chlorides have found utility in synthesis, as described elsewhere in this volume [17]. Lewis acid catalyzed additions to enones, aldehydes, and imines, yielding a-keto allylic alcohols, a-hydroxy ketones, and a-amino ketones, respectively [18], and palladium-catalyzed addition to alkyl/aryl halides and a,[5-ynones [19] are examples. The acyl complex 18 formed by the insertion of carbon monoxide into dialkyl, alkylaryl, or diaryl zirconocenes may rearrange to a r 2-ketone complex 19 either thermally (particularly when R1 = R2 = Ph) or on addition of a Lewis acid [5,20,21]. The rearrangement proceeds through the less stable... 

The hydride cycle starts with palladium hydride and a fast migratory insertion of ethene. Insertion of CO is also a last reaction, enhanced by the preferential coordination of CO with respect to ethene. The product is formed by reaction of propanoylpalladium with methanol which coordinates to palladium cis to the acyl group. A palladium zero complex may form that regenerates palladium hydride via an oxidative addition with a proton. 

The resting state of the propanoate catalysts may well be an acyl complex [60,61], while the attack of alcohol at the acylpalladium complex is considered to be the rate-determining step. It is probably more precise to say that fast preequilibria exist between the acyl complex and other complexes en route to it and that the highest barrier is formed by the reaction of alcohol and acylpalladium complex. The precise course of the reaction is still not known presumably deprotonation of the coordinating alcohol and the migratory elimination are concerted processes, accelerated by the steric bulk of the bidentate ligand. Toth and Elsevier showed that the reaction of an acetylpalladium complex and sodium methoxide is very fast and occurs already at low temperature to give methyl acetate and a palladium(I) hydride dimer [46]. 

The factors that control the strictly alternating copolymer chain with no detectable errors (e. g., microstructures involving double insertion of ethene) have been the object of detailed studies since the discovery of the first Pd" catalysts for the alternating alkene/CO copolymerisation [11]. Sen was the first to demonstrate that double carbonylation is thermodynamically unfavorable and to suggest that the higher binding affinity of Pd" for CO relative to ethene inhibits multiple ethene insertions, even in the presence of very low concentrations of CO [12]. Therefore, once a palladium alkyl is formed, CO coordination ensures that the next monomer will be a CO molecule to generate the acyl complex. 

Displacement of the chelate carbonyl from palladium by ethene has never been observed in model studies, which accounts for the virtual absence of double ethene insertions in actual copolymerisation reactions. Indeed, (5-chelate opening is actually brought about by CO to generate a six-membered metallacycle (y-chelate), while p-chelates of catalytically active systems generally react with CO to yield carbonyl acyl complexes, even at very low temperature. For the systems investigated by Bianchini [5e, f], the activation barriers for the conversion of the P-chelates... 

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry has contributed remarkably to unravelling the termination and initiation steps of the styrene/CO copolymerisation catalysed by the highly active bis-chelated complex [Pd(bipy)2](Pp5)2 in TFE [40]. Chain-end group analysis of the material produced in the absence of BQ showed that the termination by P-H elimination is accompanied by three different initiators two palladium alkyls from Pd-H formed by reaction of the precursor with CO and water (a and b) and a palladium carboalkoxy species formed by reaction of the precursor with the fluorinated alcohol and CO (c) (Chart 7.4). The suppression of the chain-transfer by alcoholysis was proposed to be responsible for the enhanced stability of the palladium acyl intermediates and hence for the high molecular weight of the copolymers produced. 

In the process of carbonyl insertion the 1,1 migratory insertion of the coordinated CO ligand into the metal-carbon bond results in the formation of a metal-acyl complex (Figure 1-7). This process, as nearly all elementary steps discussed so far, is reversible, but even when using atmospheric CO pressure the equilibrium is mostly shifted towards insertion. In the process of insertion a vacant coordination site is also produced on the metal, where further reagents might be attached. Of the metals covered in this book palladium is by far the most frequently utilized in such transformations. 

If the reaction mixture also contains a nucleophile, then the acyl-palladium complex might undergo displacement of the metal, which usually leads to the formation of a carboxylic acid derivative. The side product in this process is a palladium(II) complex that undergoes reductive elimination to regenerate the catalytically active palladium(O) complex. 

The oxidative addition is quite general with alkyl, allyl, benzyl, vinyl, and aryl halides as well as with acyl halides to afford the palladium (II) complex VII. The frans-bis( triphenylphosphine )alkylpalladium halides can also be carbonylated in an insertion reaction to give the corresponding acyl complexes, the stereochemistry of which (17, 18) proceeds with retention of configuration at the carbon bonded to palladium. The acyl complex also can be formed from the addition of the corresponding acid halide to tetrakis (triphenylphosphine) palladium (0). 

The facts that the phenylacetyl complex Villa also undergoes a reaction with phenylacetyl chloride under identical experimental conditions to yield dibenzyl ketone, XVIII (218% with respect to Villa), and that XVII was also obtained from the reaction of phenylacetyl chloride with dichlorobis(triphenylphosphine)palladium(0), XIX, implicate XIX in the catalytic cycle. The palladium (II) complex XIX does indeed catalyze the conversion of an acyl halide to the corresponding symmetrical ketone (123% with respect to XIX). The palladium (IV) structures are only... 

The metal hydride mechanism has been written particularly for hydrocarboxylation reactions with a palladium catalyst.67,68 In the reactions of propene in the presence of (PhaP dCk, the acyl complex (18) was isolated from the reaction mixture, and also shown to be a catalyst for the reaction. 

Initially the Pd(0) complex oxidatively adds to enol triflate 6 to form a vinyl-Pd(II) species. Carbon monoxide then inserts into the new Pd—C o-bond to yield a palladium(ll)-acyl complex which captures methanol. The methanolysis step is formally a reductive elimination reaction in which the Pd(0) catalyst is regenerated to propagate the catalytic cycle (Scheme 6.8).7... 

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