Palladium complexes insertion reactions

Metal-Halogen Compounds. An unusual example of the addition of a metal halide to a conjugated diene has been reported. The complex formed from palladium chloride and butadiene has been shown to be a dimer of 1-chloromethyl-7r-allylpalladium chloride, (85). Whether this is a true insertion reaction or some type of ionic reaction has not been determined, but its close analogy with the olefin-palladium chloride insertion reaction mentioned above would suggest an insertion mechanism for the diene reaction also. 

The C-0 bond in vinyl esters can also be cleaved promoted by Ru(0) complex to give vinylruthenium complex (Eq. 23) [64]. Another example of the vinyl-O bond cleavage has been recently reported in the treatment of vinyl crotonate with a (perfluorophenyl)palladium complex. The reaction course has been accounted for by insertion- 3-elimination processes (Eq. 24) [57]. 

Organometallic Compounds. Mononuclear carbon monoxide complexes of palladium are relatively uncommon because of palladium s high labihty, tendency to be reduced, and competing migratory insertion reactions in the presence of a Pd—C bond (201). A variety of multinuclear compounds... 

A large amount of the work on palladium isocyanide complexes has been mentioned earlier, in discussions on insertion reactions 30,74,108,169,170) and on addition reactions of coordinated isocyanides 25, 33, 34, 49) the reactions of [Pd(CNBu )2] with oxygen 107) and with various olefins 29, 110) were noted. 

NMR monitoring of the reaction of the palladium complex with 1-octyne suggested that the alkyne inserts into the Pd-H bond. Further heating produced a mixture of the two regioisomeric alkenylphosphine oxides, the anti-Markovnikov adduct being the favored product (54 46, 65% yield). 

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]. 

Palladium(II) complexes possessing bidentate ligands are known to efficiently catalyze the copolymerization of olefins with carbon monoxide to form polyketones.594-596 Sulfur dioxide is an attractive monomer for catalytic copolymerizations with olefins since S02, like CO, is known to undergo facile insertion reactions into a variety of transition metal-alkyl bonds. Indeed, Drent has patented alternating copolymerization of ethylene with S02 using various palladium(II) complexes.597 In 1998, Sen and coworkers also reported that [(dppp)PdMe(NCMe)]BF4 was an effective catalyst for the copolymerization of S02 with ethylene, propylene, and cyclopentene.598 There is a report of the insertion reactions of S02 into PdII-methyl bonds and the attempted spectroscopic detection of the copolymerization of ethylene and S02.599... 

The indazoline products can also be made directly from the palladium complexes 78 by heating them with the isonitrile in toluene at 120CC.162 They are also formed in dicobalt octacarbonyl-catalyzed reactions of azo-arenes with isocyanides but in this case an alternative reaction pathway leading to indazolo[2,l- ]indazoles (79) is observed (Scheme 96).163 Products of the latter type are formed from sterically hindered isocyanides hence it is likely that in these cases a double metallation is favored over isocyanide insertion into a monometallated species (Scheme 97). 

In that complex, it may be that water reacts with the coordinated C2H4 to produce a cr-bonded CH2CH2OH group rather than an insertion reaction involving an OH group. The aldehyde is formed as H+ is lost, and the palladium is produced as shown in Eq. (22.31). The palladium chloride catalyst can be recovered (the price of palladium is almost 500/oz as this is written) by the reaction with CuCl2. 

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. 

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. 

The reactions catalyzed by cationic palladium complexes are believed to proceed via a different mechanism (Scheme 67).273 Initially, a cationic silylpalladium(n) species is generated by cr-bond metathesis of the Br-Pd+ with a silylstannane. Subsequently, the alkyne and alkene moieties of the 1,6-diyne successively insert into the Pd-Si bond to form a cationic alkylpalladium(n), which then undergoes bond metathesis with silylstannane to liberate the product and regenerate the active catalyst species, S/-Pd+. 

To circumvent some of the above-mentioned drawbacks of sulfur-based mercury chemodosimeters, a system based on the alkyne oxymercuration of 58 has been developed (Fig. 22) [146]. 58 shows high selectivity, a limit of detection of ca. 8 ppm, resistance against strong oxidants, and a positive reaction even in the presence of cysteine, which is known to form stable mercury complexes and is used for the extraction of mercury from tissue samples. Another metal that is well-known for its catalytic ability is palladium, catalyzing different reactions depending on its oxidation state. Since this metal is toxic, assessment of the maximum allowable concentration of Pd in consumer products such as pharmaceuticals requires highly sensitive and selective detection schemes. For this purpose, indicator 60 was conceived to undergo allylic oxidative insertion to the fluorescein... 

As indicated under section 2.2. the overall result is the same as that of an insertion reaction, the difference being that insertion gives rise to a yw-addition and nucleophilic attack to an anri-addition. Sometimes the two reaction types are called inner sphere and outer sphere attack. There is ample proof for the anti fashion the organic fragment can be freed from the complex by treatment with protic acids and the organic product can be analysed [19], Appropriately substituted alkenes will show the syn or anti fashion of the addition. The addition reaction of this type is the key-step in the Wacker-type processes catalysed by palladium. 

The study of alkene insertions in complexes containing diphosphine ligands turned out to be more complicated than the study of the CO insertion reactions [13], When one attempts to carry out insertion reactions on acetylpalladium complexes decarbonylation takes place. When the reaction is carried out under a pressure of CO the observed rate of alkene insertion depends on the CO pressure due to the competition between CO and ethene coordination. Also, after insertion of the alkene into the acetyl species (3-elimination occurs, except for norbomene or norbomadiene as the alkene. In this instance, as was already reported by Sen [8,27] a syn addition takes place and in this strained skeleton no (3-elimination can take place. Therefore most studies on the alkene insertion and isolation of the intermediates concern the insertion of norbomenes [21,32], The main product observed for norbomene insertion into an acetyl palladium bond is the exo species (see Figure 12.8). 

Quantitative data for the difference in complexation of ethene and CO to hydrocarbylpalladium(dppp)+ were reported by Brookhart and co-workers [15,33], The equilibrium between CO and ethene coordination amounts to about 104 at 25 °C. Multiplied by the concentrations of the two gases and the two individual rate constants for the insertion they calculated that the ratio of CO insertion versus ethene insertion is about 105 in an alkyl-palladium intermediate under Curtin-Hammett conditions, that is to say fast exchange of coordinated CO and ethene ligands compared to insertion reactions. Figure 12.9 summarises this. 

The nucleophilic attack of the water or hydroxide species takes place in an anti fashion i.e. the oxygen attacks from outside the palladium complex and the reaction is not an insertion of ethene into the palladium oxygen bond. This has been demonstrated in a model reaction by Backvall [4], The reaction studied was the Wacker reaction of dideuterio-ethene (cis and trans) in the presence of excess of LiCl, which is needed to form 2-chloroethanol as the product instead of ethanal. The latter product would not reveal the stereochemistry of the attack Note that all of the mechanistic work has been carried out, necessarily, on systems deviating in one aspect or another from the real catalytic one. The outcome depends strongly on the concentration of chloride ions [5],... 

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