Palladium-carbon bonds, insertion

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 use of 1,6-diene systems usually does not result in cyclization reactions with palladium ) salts. For example, with 1,6-heptadiene a /i-elimination takes place from the cqjr-intermediate to give diene 22 as the major product (equation 10)27. However, more recently Trost and Burgess21 have shown that with a 4,4-bis(phenylsulfonyl) derivative of 1,6-heptadiene (23) an insertion takes place to give a 5-membered ring product (24, equation 11). The final step of the latter reaction is oxidative cleavage of the palladium-carbon bond by CuCl2 to produce a carbon-chlorine bond. 

Alternating insertions. The reaction proceeds via a perfectly alternating sequence of carbon monoxide and alkene insertions in palladium-carbon bonds (Figure 12.1). Several workers have shown the successive, stepwise insertion of alkenes and CO in an alternating fashion. In catalytic studies this was demonstrated by Sen, Nozaki, and Drent etc. In particular the work of Brookhart [15,22] and Vrieze/van Leeuwen [12,13,14,20,23,32] is relevant for stepwise mechanistic studies. The analysis of final polymers shows that also in the final product a perfect alternation is obtained. It is surprising that in spite of the thermodynamic advantage of alkene insertion versus CO insertion nevertheless exactly 50% of CO is built in. 

The reaction starts with the oxidative addition of an aryl halide (Cl, Br or I) to palladium zero. The next step is the insertion of an alkene into the palladium carbon bond just formed. The third step is (3-hydride elimination giving the organic product and a palladium hydrido halide. The latter reductively eliminates HX, which reacts with base to give a salt (Figure 13.15). 

Interception of the palladium-carbon bond by inserting another molecule such as an alkene or a carbon monoxide molecule is a very useful tool. In... 

Palladium salts will attack C-H bonds in functionalised aromatics such as acetoaniline to form palladium-carbon bonds that subsequently undergo insertion of alkenes [31], (3-Hydride elimination gave styryl derivatives and palladium hydride, which requires re-oxidation of palladium by benzoquinone. The reaction can be regarded as a combined Murai reaction (C-H activation, if electrophilic) and a Heck reaction (arylalkene formation), notably without the production of salts as the cross-coupling reactions do. An example is shown in Figure 19.15. 

The first step in the cycle, analogous to the cross-coupling reactions, is the oxidative addition of an aryl (vinyl) halide or sulfonate onto the low oxidation state metal, usually palladium(O). The second step is the coordination of the olefin followed by its insertion into the palladium-carbon bond (carbopalladation). In most cases palladium is preferentially attached to the sterically less hindered end of the carbon-carbon double bond. The product is released from the palladium in a / -hydrogen elimination and the active form of the catalyst is regenerated by the loss of HX in a reductive elimination step. To facilitate the process an equivalent amount of base is usually added to the reaction mixture. 

Carbon monoxide, a common ligand in organometallic chemistry, is known to insert into palladium-carbon bonds readily. This feature of the metal is frequently utilized when palladium catalyzed reactions are run in the presence of CO. The products of such reactions, also known as carbonylative couplings, incorporate a carbonyl group between the coupling partners. 

The catalytic process (Figure 2-4) usually begins with the oxidative addition of an aryl halide or sulfonate onto the active form of the catalyst. In the presence of carbon monoxide the formed palladium-carbon bond breaks up with the concomitant insertion of a CO unit to give an acylpalladium complex. Such complexes might also be formed by the oxidative addition of acyl halides onto palladium. 

The insertion of acetylene derivatives might also be utilised in the preparation of six membered rings. A characteristic distinction between such processes and olefin insertion is the fact, that the intermediate formed by the insertion of an acetylene into the palladium-carbon bond is unable to undergo /2-hydride elimination, therefore the concluding step of these processes is usually reductive elimination. 

The sequence of events is (i) the insertion of the alkene into the palladium-carboxylate bond followed by (ii) CO insertion into die newly generated palladium-carbon bond followed by (iii) the reaction with solvent to give a palladium hydride that undergoes reductive elimination to palladium(O) (Scheme 12). 

The second step is insertion or transmetallation. An insertion reaction occurs when the palladium-carbon bond adds across a it bond to give a new organopal-ladium species. The types of it bonds normally reactive include alkenes, dienes, alkynes, carbon monoxide, and sometimes carbonyl it bonds. By far the most common reactions use alkenes and alkynes for the insertion reaction. This step results in a new carbon-carbon bond. 

This phosphine complex, however, is not reduced by alcohol to zero-valent palladium and oxalate ester, nor is it formed by insertion of carbon monoxide into the palladium-carbon bond of the related aikuxycarbonyl species. 

The insertion of CO into palladium carbon bonds is a common step in many palladium-catalyzed carbonylation reactions and polymerizations. This reaction takes place under moderate CO pressure (1-3 atm). From the range of compounds that can be carbonylated, it can be inferred that CO will insert into alkyl, aryl, and alkynic bonds (equation 13). One of the few types of Pd-C bonds inert to CO insertion is the Pd-acyl bond, thus only single carbonylations are normally observed. However, a few examples of double carbonylation have been reported. In the case of palladium-catalyzed formation of PhCOCONEt2 from Phi, CO, and NHEt2, reductive elimination from a bisacyl complex has been established as the mechanism, rather than CO insertion into a Pd-acyl bond. 

When CO inserts into a palladium-carbon bond, the resulting acyl can be hydrolyzed to an ester if methanol or another alcohol is present in the reaction. This overall reaction... 

Cyclopalladated sulfur-containing < 1995JOC1005> and oxygen-containing complexes <2003OM3967,2005CEJ3268> have also been synthesized. The insertion of phenylacetylene 122 into the palladium-carbon bond of complex 121 yielded the palladacycle 123 (Equation 43). 

Transformation of the chloroacetate from cyclohexa-1,3-diene to amide 72 followed by a Pd(0)-catalyzed reaction afforded products 73 [89] and 74 (Scheme 8-25) [90]. Product formation is dependent on the substitution pattern. Both reactions proceed via a similar intermediate. When = Me and = H, jS-elimination cannot occur and a cyclization takes place instead, via insertion of the double bond into the intermediate palladium-carbon bond. 

A common pathway in palladium-catalyzed oxidation reactions is that the 7r-olefin complex formed reacts with a nucleophile, either external or coordinated, and the new organometallic intermediate may then undergo a number of different reactions (Scheme l) (i) an intramolecular hydride shift leads to ketone formation (ii) a )6-elimination results in the formation of a vinyl functionalized olefin (iii) an oxidative cleavage of the palladium-carbon bond produces a 1,2-functionalized olefin and (iv) an insertion reaction, exemplified by insertion of an olefin, leads to formation of a new palladium-carbon bond, which may be cleaved according to one of the previous processes ()6-elimination or oxidative cleavage). In all cases palladium has removed 2 electrons from the organic molecule, which becomes oxidized. These electrons, which end up on Pd(0), are in turn transferred to the oxidant and Pd(II) is regenerated, in this way a palladium(II)-catalyzed oxidation is realized. 

Depending on the catalyst system and the reaction conditions, especially at elevated CO pressure it is possible to obtain selectively double carbonylation reactions to 1-keto carboxylic derivatives [25]. Recent mechanistic investigations have shown that double CO insertion into the palladium-carbon bond does not occur directly instead, the terminal step of double carbonylation is generally a coupling reaction between metal-bonded acyl, alkoxycarbonyl or amidocarbonyl groups and CO. 

Carbon monoxide insertion in a palladium-carbon bond is a fairly common reaction [21]. Under polymerization conditions, CO insertion is thought to be rapid and reversible. Olefin insertion in a palladium-carbon bond is a less common reaction, but recent studies involving cationic palladium-diphosphine and -bipyridyl complexes have shown that olefin insertion also, particularly in palladium-acyl bonds, appears to be a facile reaction [22], Nevertheless, it is likely that olefin insertion is the slowest (rate-determining) and irreversible step vide infra) in polyketone formation. 

A variety of palladium-catalyzed organic reactions involve the oxidative addition process. A typical example is seen in the catalytic arylation and alkenylation of olefins (eq (60)) [85]. Aryl- and alkenylpalladium(ll) complexes (9) formed by oxidative addition undergo olefin insertion into the palladium-carbon bond to give an alkylpal-ladium species (10), which provides arylated and alkenylated olefins via p-hydrogen elimination. The hydridopalladium species 11 thus generated is reduced to a Pd(0) species upon its interaction with a base and carries the sequence of reactions... 

While wanning the catalysis mixture to 55 C (Step D, Scheme 1) leads to no other observable reaction intermediates, the generation of intermediate 8 would allow the series of steps shown in Scheme 1. Insertion of the coordinated CO into the palladium-carbon bond would lead to the overall coupling of acid chloride, imine and carbon monoxide in conq>lex 10. The subsequent loss of HCl from 10, either via direct deprotonation or P-H elimination, would form the a-amide substituted ketene 11. The latter is known to be in rapid equilibrium with its cyclic mesoionic l,3-oxazolium-5-oxide tautomeric 12 (14). These steps would lead to the liberation of the Pd(0) catalyst, which can return to the catalytic cycle. 

Aryl-, alkenyl- and alkynylpalladium species readily undergo carbonylation reactions because carbon monoxide as a loosely bonded ligand can reversibly insert into any palladium-carbon bond [110]. Thus, 2-allyl-l-iodocyclopentene (148), under palladium catalysis, reacts with carbon monoxide in two modes, depending on the excess of carbon monoxide and the catalyst cocktail (Scheme 3-39) [110a]. With a slight excess (1.1 atm of CO) in the presence of [Pd(PPh3)4] in tetrahydrofuran, 148 cyclized with one CO insertion to yield 3-methylenebicyclo[3.3.0]oct-l(5)-en-2-one (152), and under 40 atm of CO with [Pd(PPh ,)2Cl2] in benzene/acetonitrile/methanol, methyl 2- 3 -(2 -oxobicyclo[3.3.0]oct-1 (5 )-enyl) acetate 149 after two CO insertions (Scheme 3-39). 

The presumed mechanism of the formation of the 6-lactone is shown in Figure 26. Two molecules of butadiene combine to a Cs-chain and form a palladium-bis-n -allyl complex which is in equilibrium with a ri. ql-complex. Carbon dioxide inserts into the palladium-carbon bond yielding a carboxylate complex. The oxygen of the carboxylate group and the allyl group react and form the 6-lactone by a cycli-zation step. 

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