Palladium hydride species

With Pd(0) generated in situ, the oxidative addition of aryl bromide 102 to Pd(0) proceeds to form Pd(II) intermediate 104. Migratory insertion of 104 then occurs to furnish the cyclized indoline intermediate 105. Subsequent reductive elimination of 105 takes place in a cis fashion, giving rise to exo-cyclic olefin 107, which then tautomerizes spontaneously to the thermodynamically more stable indole 103. The reductive elimination by-product as a palladium hydride species 106 reacts with base, regenerating Pd(0) to close the catalytic cycle. 

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]. 

There is a lot of evidence and general agreement that the catalytic cycle of the new, fast catalysts starts with a palladium hydride species [45,56], with perhaps one exception [57],... 

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). 

As was the case with 271, the treatment of palladacycle 277 with water also results in the liberation of H2, the mechanistic features of which were studied theoretically, using the somewhat simplified model complex PdMe2 (H2C=N-NH)3BH, 105 and proceeding from the assumption that an unstable palladium-hydride species is involved. It was thus concluded that reduction of H20 to H2 involves two proton-transfers to palladium, each proceeding with protonation of the pendant pyrazolyl donor, which then behaves as an intramolecular nucleophile (Scheme 20). 

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). 

The addition of a noii-stabilized carbon nucleophile and another nucleophile to a conjugated diene has similarities to the addition of H-Nu (cf. Section 8.2.1). The formation of RPdX from oxidative addition of RX and Pd(0) corresponds to the genei-ation of a palladium hydride species in the H-Nu addition (Scheme 8-3). 

Another reaction of this type is the nonoxidative dimerization of olefins. They very likely proceed via palladium-hydride species which may be formed by a small amount of oxidation of the olefin. Often Pd(II)-olefin 7T complexes are used. Kawamoto et al. (155) have recently reported the dimerization of styrene and vinyl compounds using the styrene Pd(II) tt complex. Also, it has been reported (254) that phosphine complexes, Pd(PPh3)4 or (PPh3)aPdX2 (X = Ng or NCO), have been employed to give a novel dimerization of malonotrile ... 

A second mechanism, the alcoholysis of the palladium-acyl bond, gives an ester end-group and a palladium hydride species (eq. (7)), which is again an initiator for the next polymer chain. 

These observations are summarized in a mechanistic proposal shown in Scheme 1. The palladium hydrido species (compare Eq. 2) is able to add equally to the double bonds in positions 9 and 12. The alkene inserts into the Pd H bond, yielding the corresponding alkyl complexes. After reaction with hydrogen the monounsaturated fatty acids (C18 1) are set free, and the palladium hydride species is formed again, thus starting a new catalytic cycle. 

Scheme 3 Formation of palladium hydride species after oxidative addition of imidazolium salts to a Pd(0) center...

PSiP-pincer ligands and their analogs developed in our laboratory [20-27]. The unique structural and electronic nature of the PSiP-ligands enables these catalytic reactions to proceed through the generation and reaction of palladium hydride species as a key intermediate. Some mechanistic aspects of the reactions are also described based on the synthesis and reaction of q -(Si-H)Pd(0) complex as an equivalent to the palladium hydride species [28-33]. 

Hydride elimination is the step of the Mizoroki-Heck reaction yielding the product (Figure 3.1, step 4). For this process to occur, the insertion complex must be able to rotate to a position where a /3-hydrogen is aligned syn to the palladium(ll) centre. The elimination will then result in formation of a reconstituted alkene and a palladium hydride species. The j8-H-ehmination is reversible (see Figures 3.5 and 3.6) and the preferred formation of the thermodynamically more stable tram-products is thus explained [12]. There is today no precise knowledge of how palladium(ll) is reduced back to catalytically... 

Functionalized benzenes preferentially induced ortho-para substitution with electron-donating groups and meta substitution with electron-withdrawing groups (see above). Additionally, the order of reactivity found with aromatics was similar to that of electrophilic aromatic substitution. These observations implicated an electrophihc metalation of the arene as the key step. Hence, Fujiwara et al. [4b] believed that a solvated arylpalladium species is formed from a homogeneous solution of an arene and a palladium(ll) salt in a polar solvent via an electrophilic aromatic substitution reaction (Figure 9.2). The alkene then coordinates to the unstable arylpalladium species, followed by an insertion into the aryl-palladium bond. The arylethyl-palladium intermediate then rapidly undergoes )8-hydride elimination to form the alkenylated arene and a palladium hydride species, which then presumably decomposes into an acid and free palladium metal. Later on, the formation of the arylpalladium species proposed in this mechanism was confirmed by the isolation of diphenyltripalladium(ll) complexes obtained by the C-H activation reaction of benzene with palladium acetate dialkylsulfide systems [19]. 

Insertion of styrene does not always lead to branched alkyl species. Insertion of styrene into a cationic palladium hydride species may give purely linear alkyl species, but perhaps not for steric reasons [67]. An early transition state for this process may involve the interaction of the LUMO for PdH, which has the highest coefficient on palladium, with the HOMO of styrene, which has the highest coefficient on the terminal carbon atom. 

Similar to /S-elimination of Y-Pd-X from alkylpalladium complexes to give olefins, elimination of Y-Pd-X species from 7r-aIIyIpaIIadium complexes proceeds to afford 1,3-dienes (Scheme 1). A variety of atoms and functional groups represented as Y are known in this elimination process among them elimination of palladium hydride species (H-Pd-X) is the most common. 

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