Palladium hydrido complexes

Oxidation of C2D4 leads to CD3CDO, and therefore, most probably in the transition state, hydrido palladium complex containing coordinated vinyl alcohol molecule is formed [equation (6.112)]. 

Recently, Y. Yamamoto reported a palladium-catalyzed hydroalkoxylation of methylene cyclopropanes (Scheme 6-25) [105]. Curiously, the catalysis proceeds under very specific conditions, i.e. only a 1 2 mixture of [Pd(PPh3)4] and P(o-tolyl)3 leads to an active system. Other combinations using Pd(0 or II) precursors with P(o-tolyl)3 or l,3-bis(diphenylphosphino)propane, the use of [Pd(PPh3)4] without P(o-tolyl)3 or with other phosphine ligands were all inefficient for the hydroalkoxylation. The authors assumed a mechanism in which oxidative addition of the alcohol to a Pd(0) center yields a hydrido(alkoxo) complex which is subsequently involved in hydropal-ladation of methylenecyclopropane. 

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. 

An excess of ligand, including CO, will often inhibit isomerisation. HCo(CO)4, an unstable hydrido-carbonyl complex, belongs to the examples of catalysts also active in an atmosphere of CO. This is the only homogeneous catalyst being commercially applied, albeit primarily for its hydroformylation activity. Higher alkenes are available as their terminal isomers or as mixtures of internal isomers and the latter, the cheaper product, is mainly converted to aldehydes/alcohols by hydroformylation technology. Later we will see that the isomerisation reaction also plays a pivotal role in this system. Since 1990 several catalysts based on rhodium, platinum and palladium have been discovered that will also hydroformylate internal products to terminal aldehydes. 

Recently, the oxidative addition of C2-S bond to Pd has been described. Methyl levamisolium triflate reacts with [Pd(dba)2] to give the cationic palladium complex 35 bearing a chiral bidentate imidazolidin-2-ylidene ligand [120]. The oxidative addition of the levamisolium cation to triruthenium or triosmium carbonyl compounds proceeds also readily to yield the carbene complexes [121], The oxidative addition of imidazolium salts is not limited to or d transition metals but has also been observed in main group chemistry. The reaction of a 1,3-dimesitylimidazolium salt with an anionic gallium(I) heterocycle proceeds under formation of the gaUium(III) hydrido complex 36 (Fig. 12) [122]. 

Because of its low acidity, hydrogen cyanide seldom adds to nonactivated multiple bonds. Catalytic processes, however, may be applied to achieve such additions. Metal catalysts, mainly nickel and palladium complexes, and [Co(CO)4]2 are used to catalyze the addition of HCN to alkenes known as hydrocyanation.l67 l74 Most studies usually apply nickel triarylphosphites with a Lewis acid promoter. The mechanism involves the insertion of the alkene into the Ni—H bond of a hydrido nickel cyanide complex to form a cr-alkylnickel complex173-176 (Scheme 6.3). The addition of DCN to deuterium-labeled compound 17 was shown to take place... 

The details of the cupric salt reaction with the palladium adduct are not clear. Exchange to form a cupric alkyl is one possibility or complex formation,"probably with chloride bridges between the palladium adduct and cupric chloride, may occur with subsequent anion shift from palladium to carbon or perhaps an Sn2 displacement of the complex metal group by an anion may occur. Rearrangements producing 1,3 and 1,4 substituted products from linear olefins have also been observed. For example, 1-butene produced several percent of 1,3- and 1,4-chloro acetates and diacetates under the reaction conditions used 16>. "Hydrido-palladium acetate or chloride" -complexes would seem to be likely intermediates in these arrangements. 

The final step of the catalytic cycle, base-assisted reductive elimination, has been addressed by Deeth et al. [14]. In their calculations, the authors investigated palladium complexes with the chelating diaminomethane H2N(CH2)NH2 and di-phosphinomethane H2P(CH2)PH2 ligands. Within this system, they found that the postulated hydrido-olefin complex, which is usually formed by p-H elimination of the y9-agostic insertion product, is in fact not a stable minimum structure in this particular case (eq. (11)) [14]. 

In the reaction, an organic halide first forms an organopalladium halide complex with the catalyst, by oxidative addition. This complex then adds to an olefin and the adduct decomposes, by elimination of a hydrido-palladium halide, to form a new olefin in which a vinylic position is substituted by the organic moiety of the substrate halide. 

A variety of organosilyl-hydridopalladium complexes have been assumed to be generated in situ from hydrosilanes through oxidative addition of the silicon-hydrogen bonds onto palladium complexes. However, few studies have been reported on the isolation and characterization of silyl-hydrido complexes because of their instability. 

Palladium-catalyzed hydrostannation of alkynes proceeds regio- and stereospecifically to afford the synthetically useful ( )-vinylstannanes. This reaction implies oxidative addition of RsSn—H to Pd(0) to generate a Pd(ll) hydrido stannyl intermediate, which then undergoes cis addition of the Pd—Sn bond to the alkyne bond, followed by reductive elimination of the ( )-vinylstannane. The supposed cis-PdCII) hydrido trialkylstannyl intermediates had so far remained elusive. Very recently, cis-PdCll) hydrido trialkylstannyl complexes have been synthesized for the first time. ... 

With ruthenium catalysts the same products are formed as in palladium catalysis, i.e. the Cg-6-lactone and traces of esters (compare Equation 6). Ruthenium(II)-hydrido-phosphine complexes such as RuH(OAc)-(PPh3)3, RuH2(PPh3)4, RuH(OAc)(CO)(PPh3)2 or RuH (C0)(PPh3)3 can be used as catalyst precursors. When triisopropyl phosphine is added as ligand yields up to 7 5l> are obtained. Of course, the yields are rather low, however, this is the first proof that both rhodium and ruthenium are active catalyst metals in diene/C02 chemistry. 

Cobalt, nickel, iron, ruthenium, and rhodium carbonyls as well as palladium complexes are catalysts for hydrocarboxylation reactions and therefore reactions of olefins and acetylenes with CO and water, and also other carbonylation reactions. Analogously to hydroformylation reactions, better catalytic properties are shown by metal hydrido carbonyls having strong acidic properties. As in hydroformylation reactions, phosphine-carbonyl complexes of these metals are particularly active. Solvents for such reactions are alcohols, ketones, esters, pyridine, and acidic aqueous solutions. Stoichiometric carbonylation reaction by means of [Ni(CO)4] proceeds at atmospheric pressure at 308-353 K. In the presence of catalytic amounts of nickel carbonyl, this reaction is carried out at 390-490 K and 3 MPa. In the case of carbonylation which utilizes catalytic amounts of cobalt carbonyl, higher temperatures (up to 530 K) and higher pressures (3-90 MPa) are applied. Alkoxylcarbonylation reactions generally proceed under more drastic conditions than corresponding hydrocarboxylation reactions. 

The proposed catalytic cycle for the above-described conjugate reduction is outlined in Scheme 17. Initial coordination of the nucleophilic Pd(0)-phosphine complex to the electron-deficient olefin to form complex I is a reversible process that occurs rapidly at room temperature. Oxidative addition of the sihcon hydride moiety to complex I would result in the hydrido olefin complex II. Migratory insertion of the hydride ligand into the electrophilic /S-carbon of the coordinated olefin can result in the palladium enolate intermediate in. Reductive elimination of the silicon moiety and the enolate completes the catalytic cycle and forms the silyl enol ether IV. The latter is prone to acid-catalyzed hydrolysis to produce the saturated ketone. "" ... 

A thorough study on the complexation of 1-alkynes and internal alkynes to Pd(0) has been reported. The reaction of alkynes with the Pd(0) complex Pd(dPr pe)(C2H4) leads to Pd(dPr pe)(77 -alkyne) derivatives, as shown in Scheme 46. Pd(0) complexes of terminal alkynes were believed to be unstable due to facile oxidative addition of the C-H bond of the alkyne to give an alkynyl(hydrido)palladium(n) complex that could undergo further alkyne... 

In the earliest publications [33], it was proposed that the initiation step in both hydroxycarbonylation and polymerization reactions involved the reaction of the alcohol with the palladium complex to give the catalytically active palladium-methoxy complexes. After chain growing reactions, the termination mechanism was supposed to proceed via protonolysis of the alkyl-palladium complex to give the keto-ester (KE) product (methyl propanoate or polymer) and regenerate the active catalyst (Scheme 1.4). In addition, hydrido palladium species are smoothly formed from palladium(II) salts in methanol (not shown). 

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