Palladium hydride bond

Two competing chain-transfer mechanisms in copolymerization of CO and ethene catalyzed by Pd11 acetate/dppp complexes were found. One involves termination via an isomerization into the enolate followed by protonation with methanol the rate of this reaction should be independent of the concentration of the protic species. The second chain-transfer mechanism comprises termination via methanolysis of the acylpalladium species, and subsequent initiation by insertion of ethene into the palladium hydride bond.501... 

The calculated activation barrier for migratory insertion of the substrate into the palladium-hydride bond was determined to be 4.2 kcal/mol for the pathway 8a to 9a. For isomer 8c, a large thermodynamic insertion barrier of AEins = +15.2 kcal/mol exists, so the activation barrier transforming 8c to 9c was not examined further. 

This reaction constitutes a special type of process in which a hydrogen atom and a nucleophile are added across the diene with fonnation of a carbon-hydrogen bond in the 1-position and a carbon-Nu bond in the 4-position. Some examples of such reactions are hydrosilylation [12-18], hydrostannation [19,20] amination [21,22], and addition of active methylene compounds [21 a,23,24], These reactions are initiated by an oxidative addition of H-Nu to the palladium(0) catalyst, which produces a palladium hydride species 1 where the nucleophile is coordinated to the metal (Scheme 8-1). The mechanism commonly accepted for these reactions involves insertion of the double bond into the palladium-hydride bond (hydride addition to the diene), which gives a (jr-allyl)palladium intermediate. Now depending on the nature of the nucleophile (Nu) the attack on the jr-allyl complex may occur either by external trans-aVtBck (path A) or via a cw-migration from palladium to carbon (path B). 

In an interesting extension. Oh has recently shown that palladium complexes can be used to catalyze the cascade cyclization of 23 to furan derivatives [56]. This reaction presumably involves the initial generation of a palladium-hydride bond under the reaction conditions, which can mediate sequential insertion of the alkyne units to... 

Addition of element-element compounds to alkynes has been reviewed.Other insertions of alkynes into palladium-hydride bonds have been identified in Drent s palladium-catalyzed alkoxycarbonylation of alkynes palladium(ii)-alkenyl complexes have been invoked to account for the observed H/D exchange when conducted in GH3OD and to identify the pathway (i.e., through migratory insertion into Pd-H and formation of acyl species by carbonylation) of the overall reaction. ... 

The mechanisms of the hydroxycarbonylation and methoxycarbonylation reactions are closely related and both mechanisms can be discussed in parallel (see Section 9.3.6).631 This last reaction has been extensively studied. Two possibilities have been proposed. The first starts the cycle with a hydrido-metal complex.670 In this cycle, an alkene inserts into a Pd—H bond, and then migratory insertion of CO into an alkyl-metal bond produces an acyl-metal complex. Alcoholysis of the acyl-metal species reproduces the palladium hydride and yields the ester. In the second mechanism the crucial intermediate is a carbalkoxymetal complex. Here, the insertion of the alkene into a Pd—C bond of the carbalkoxymetal species is followed by alcoholysis to produce the ester and the alkoxymetal complex. The insertion of CO into the alkoxymetal species reproduces the carbalkoxymetal complex.630 Both proposed cycles have been depicted in Scheme 11. 

Reactivity patterns of this in situ method also support the hypothesis that Pd oxidation occurs via the acidic phosphonium. This initial reaction would involve initial formation of a palladium hydride. Such hydride intermediates are usually very reactive due to the weakness of the Pd-H bond, which in turn imparts instability. 

Arylation of cycloalkenes.1 Aryl halides undergo Heck coupling with cy-cloalkenes in the presence of a palladium catalyst. The reaction involves addition of an arylpalladium intermediate to the double bond followed by elimination of a palladium hydride. 

The mechanism of this reaction was considered on the basis of hydropalladation (Scheme 14). To minimize steric repulsions, the palladium hydride complex approaches the C=CH2 moiety of the allene in the anti-Markovnikov mode from the opposite side of the substituent. This addition gives a 7t—allyl palladium complex with the (Z)-configuration,18 which is converted to the (Z)-product by C-P bond formation, with regeneration of the Pd(0) catalyst. 

For unsaturated lactones containing an endocyclic double bond also the two previously described mechanisms are presumably involved and the regio-selectivity of the cyclocarbonylation is governed by the presence of bulky substituents on the substrate. Inoue and his group have observed that the catalyst precursor needs to be the cationic complex [Pd(PhCN)2(dppb)]+ and not a neutral Pd(0) or Pd(II) complex [ 148,149]. It is suggested that the mechanism involves a cationic palladium-hydride that coordinates to the triple bond then a hydride transfer occurs through a czs-addition. Alper et al. have shown that addition of dihydrogen to the palladium(O) precursor Pd2(dba)3/dppb affords an active system, in our opinion a palladium-hydride species, that coordinates the alkyne [150]. 

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). 

The mechanism of the reaction in Figure 15.4 involves coordination of palladium to the alkene and nucleophilic attack of oxygen at the internal carbon atom to form the flve-membered ring. Palladium is bonded to the exocyclic carbon atom. (3-hydride elimination gives the exocyclic methylene,... 

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. 

Yamamoto has proposed a mechanism for the palladium-catalyzed cyclization/hydrosilylation of enynes that accounts for the selective delivery of the silane to the more substituted C=C bond. Initial conversion of [(77 -C3H5)Pd(GOD)] [PF6] to a cationic palladium hydride species followed by complexation of the diyne could form the cationic diynylpalladium hydride intermediate Ib (Scheme 2). Hydrometallation of the less-substituted alkyne would form the palladium alkenyl alkyne complex Ilb that could undergo intramolecular carbometallation to form the palladium dienyl complex Illb. Silylative cleavage of the Pd-G bond, perhaps via cr-bond metathesis, would then release the silylated diene with regeneration of a palladium hydride species (Scheme 2). 

The other strategy requires dienes or enynes containing allylic or propargylic ester fragments. The main pathway in this case involves the formation of allylpalladium intermediates, which perform carbapalladation of double or triple bonds with subsequent acylpalladation forming two cycles, and termination by palladium hydride elimination or other usual trapping pathways (Scheme for example, in the following examples in Scheme 22. ... 

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. 

If two hydrogen atoms on a (3-carbon atom are available for elimination, ( )-alkenes are strongly preferred. If only one hydrogen atom is present the product stereochemistry will be predictable on the basis of a syn addition of the organopalladium group to the double bond followed by a syn elimination of a palladium hydride group, provided the reaction is conducted under the proper conditions as described in Section 4.3.5.1.2.i. Yields of substituted alkenes from these reactions generally decline as the number and size of the substituents on the vinyl carbons increase. 

Vinyl substitution of primary or secondary allylic alcohols with aryl halides usually produces 3-aryl aldehydes or ketones, respectively. The reaction is believed to involve an addition of the intermediate arylpalladium halide to die double bond, placing the aryl group mainly on the more distant carbon from the hydroxy group, followed by palladium hydride elimination, a reverse readdition and another elimination with a hydrogen atom on the carbon bearing the hydroxy group. The product is probably a ir-com-plex of the enol which ultimately either dissociates or collapses to a a-complex with palladium on the... 

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