Palladium complexes elimination

Addition of several organomercury compounds (methyl, aryl, and benzyl) to conjugated dienes in the presence of Pd(II) salts generates the ir-allylpalladium complex 422, which is subjected to further transformations. A secondary amine reacts to give the tertiary allylic amine 423 in a modest yield along with diene 424 and reduced product 425[382,383]. Even the unconjugated diene 426 is converted into the 7r-allyllic palladium complex 427 by the reaction of PhHgCI via the elimination and reverse readdition of H—Pd—Cl[383]. 

It was found [99JCS(PI )3713] that, in all cases, the formation of the deiodinated products 38 and 39 was accompanied by formation of the diynes 40 which were isolated in 60-90% yield. The authors believed that the mechanism of deiodination may be represented as an interaction ofbis(triphenylphosphine)phenylethynyl-palladium(II) hydride with the 4-iodopyrazole, giving rise to the bisftriphenylphos-phine)phenylethynyl palladium(II) iodide complex which, due to the reductive elimination of 1 -iodoalkyne and subsequent addition of alk-1 -yne, converts into the initial palladium complex. Furthermore, the interaction of 1-iodoalkynes with the initial alkyne in the presence of Cul and EtsN (the Cadiot-Chodkiewicz reaction) results in the formation of the observed disubstituted butadiynes 40 (Scheme 51). 

In the direct coupling reaction (Scheme 30), it is presumed that a coordinatively unsaturated 14-electron palladium(o) complex such as bis(triphenylphosphine)palladium(o) serves as the catalytically active species. An oxidative addition of the organic electrophile, RX, to the palladium catalyst generates a 16-electron palladium(n) complex A, which then participates in a transmetalation with the organotin reagent (see A—>B). After facile trans- cis isomerization (see B— C), a reductive elimination releases the primary organic product D and regenerates the catalytically active palladium ) complex. 

Some of the details of the mechanism may differ for various catalytic systems. There have been kinetic studies on two of the amination systems discussed here. The results of a study of the kinetics of amination of bromobenzene using Pd2(dba)3, BINAP, and sodium r-amyloxide in toluene were consistent with the oxidative addition occurring after addition of the amine at Pd. The reductive elimination is associated with deprotonation of the animated palladium complex.166... 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

In one mechanism, Pd° generated by reduction of Pd2+ with formic acid forms hydridoformatopalladium complex 102, which reacts with isoprene to form formato(methylbutenyl)palladium complex 103. Then, insertion of the second molecule of isoprene takes place. Finally, reductive elimination and evolution of carbon dioxide give the dimers ... 

As mentioned above nonconjugated dienes give stable complexes where the two double bonds can form a chelate complex. A common pathway in palladium-catalyzed oxidation of nonconjugated dienes is that, after a first nucleophilic addition to one of the double bonds, the second double bond inserts into the palladium-carbon bond. The new (cr-alkyl)palladium complex produced can then undergo a /(-elimination or an oxidative cleavage reaction (Scheme 2). An early example of this type of reaction, although not catalytic, was reported by Tsuji and Takahashi (equation 2)12. 

A proposed mechanism of the bis(allene) cyclization involves the formation of the allyl(stannyl)palladium species 6, which undergoes carbocyclization to give vinyl(stannyl)palladium intermediate 7 (Scheme 36). Reductive elimination and cr-bond metathesis may lead to the formation of the m-pentane derivative and the bicyclic product, respectively. The cyclization of allenic aldehydes catalyzed by a palladium complex was also reported.163... 

In order to account for the high regioselectivities observed in the rhodium-catalyzed hydroboration of styrenes, Hayashi proposed a modified mechanism which proceeds through 73-benzyl-rhodium complex 22 as a key intermediate (Scheme 7). Reductive elimination from this 73-benzyl-rhodium complex 22 produces the secondary alkylborane regioselectively.12 A related 73-benzyl-palladium complex was recently isolated by Hartwig in studies of hydroamination.75... 

The Pd-catalyzed amination of / -rm-butylphenyl bromide with pyrrole in the presence of Pd(OAc)2, dppf and one equivalent of NaOr-Bu led to the Af-arylation product 88. A simplified version of the mechanism commences with the oxidative addition of p-te/t-butylphenyl bromide to Pd(0), giving rise to the palladium complex 89. Ligand exchange with pyrrole followed by deprotonation by the base (NaOr-Bu) results in amido complex 90. Reductive elimination of 90 then gives the amination product 88 with concomitant regeneration of Pd(0) catalyst. If the amine had a (3-hydride in amido complex 90, a (3-hydride elimination would be a competing pathway, although reductive elimination is faster than P-hydride elimination in most cases. 

Primary propargylic formates decarboxylate in the presence of Pd(acac)2 and Bu3P at room temperature to give mainly allenic products (Eq. 9.115) [91]. Initial formation of a propargylic palladium complex, which rearranges to the more stable allenylpalladium species, accounts for this transformation. Under similar conditions, a terminal allenyl formate afforded a 99 1 mixture of allene and acetylene product (Eq. 9.116) [91]. However, a mixture of enyne elimination products was formed when a secondary propargylic carbonate was treated with a palladium catalyst (Eq. 9.117). 

Often Lewis acids are added to the system as a cocatalyst. It could be envisaged that Lewis acids enhance the cationic nature of the nickel species and increase the rate of reductive elimination. Indeed, the Lewis acidity mainly determines the activity of the catalyst. It may influence the regioselectivity of the catalyst in such a way as to give more linear product, but this seems not to be the case. Lewis acids are particularly important in the addition of the second molecule of HCN to molecules 2 and 4. Stoichiometrically, Lewis acids (boron compounds, triethyl aluminium) accelerate reductive elimination of RCN (R=CH2Si(CH3)3) from palladium complexes P2Pd(R)(CN) (P2= e g. dppp) [7], This may involve complexation of the Lewis acid to the cyanide anion, thus decreasing the electron density at the metal and accelerating the reductive elimination. 

The last possibility for ester formation (20, Figure 12.15) comprises the reductive elimination of esters from acyl-alkoxy-palladium complexes 17, formed by deprotonation of the alcohol adducts 16. Clearly, it requires cis coordination of the alkoxide and acyl fragment. Since monodentates have a preference for ester formation, it was thought that this mechanism was very unlikely. 

It is important to note that in methanol as the solvent the reaction is much slower and also the molecular weight is much lower. Apparently a major part of the palladium complex occurs in an inactive state and the termination reaction is relatively accelerated by methanol. This suggests that ester formation is the dominant chain transfer mechanism in methanol, although P-hydride elimination will still occur. 

The styrene/CO polymers formed with palladium complexes of diimine ligands indeed contain ester and alkene end groups [65,66,67], Slightly more ester end groups than alkene groups are formed, showing that in addition to P-hydride elimination some termination via methanolysis of acylpalladium chain ends occurs. 

Sonogashira has proposed a catalytic cycle (Figure 4) which shows 1) the reduction of the palladium complex, 2) coordination of the aryl halide and acetylene with the palladium (0) complex and 3) the reductive elimination of the substituted aryl acetylene and regeneration of the active catalyst.(10)... 

Although several Lewis Acids were evaluated, including titanium(lV) chloride, aluminum(lll) chloride and tin(lV) chloride, ferric(lll) chloride proved to be the most effective co-catalyst. We believe that in the presence of a Lewis Acid, the rate of j3-palladium hydride elimination (H-Pd-X) from the -allyl carbomethoxy palladium complex 4 can be enhanced. A good leaving group such as iodide attached to -allyl carbomethoxy palladium complex 4 would facilitate iodopalladium hydride elimination to selectively form methyl, -pentadienoate (Equation 11.). 

Trost and Tanoury found an interesting skeletal reorganization of enynes using a palladium catalyst.In this reaction, the second product is derived from a metathesis reaction (Equation (5)). It was speculated that the reaction would proceed by oxidative cyclization of enynes with the palladium complex followed by reductive elimination and then ring opening. To confirm this reaction mechanism, they obtained a compound having a cyclobutene ring, which was considered to be formed by the reductive elimination (Equation (6)). 

The effect of having bulky groups (e.g., /-butyl) in the phosphine ligand of platinum and palladium complexes has been studied in some detail by Shaw and his co-workers (9-12, 30, 31, 88). They have used a number of phosphines containing aromatic groups, e.g., di-/-butylphen-ylphosphine intramolecular ring closure occurs with the elimination of HC1 (9, 10), e.g.,... 

An acyl-palladium complex might undergo a series of follow up reactions. Subsequent transmetalation and reductive elimination lead to the formation of a carbonyl compound. This process is also coined carbonylative coupling, referring to the cross-coupling reaction, which would take place in the absence of carbon monoxide under similar conditions (for more details see Chapter 2.4.). 

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