Palladium-oxygen bond

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],... 

The general trends of this oxidation are consistent with the mechanism depicted in Scheme 6. This involves the complexation of the alkene to the metal followed by its insertion into the palladium-oxygen bond, forming the five-membered pseudocyclic intermediate which decomposes to give the methyl ketone and the palladium- t-butoxy complex. The decomposition of (84a) is similar to that of the rhodium dioxametallacycles previously shown in Scheme 3.42... 

The most generally accepted mechanistic speculations center about a y3-acetoxyalkylpalladium intermediate (VI) formed by an insertion reaction of ethylene into a palladium-oxygen bond (Reaction 3). It is proposed that this intermediate would then decompose via a palladium-assisted hydride transfer to vinyl acetate (Reaction 4) (36). While Reaction 4 would appear more awkward than a simple )3-hydrogen... 

Nucleophilic attack on alkene-palladium(ll) complexes became commercially important when the Wacker process was developed. In this process, ethylene is converted to acetaldehyde (Scheme 6.4). This involves coordination of ethylene to palladium. The oxygen atom, which comes from water, may then become attached to carbon in one of two different ways. Direct nucleophilic attack on one carbon atom of the t -ethylene complex 6.1 by water can form an t -complex 63. Alternatively, ligand exchange at palladium, with water replacing chloride, can be followed by insertion of the coordinated ethylene into the palladium-oxygen bond to give the same t -complex 63, via the hydroxy complex 6.2. Which of these two variants of the mechanism... 

Sulphur dioxide inserts into palladium-oxygen bonds. When a solution of palladium chloride in an alcohol is treated with sulphur dioxide, a sulphinato-complex is produced, presumably by insertion into a palladium-alkoxide bond. The reaction of sulphur dioxide with molecular oxygen complexes of platinum and of iridium, producing sulphato-complexes, involves insertion of sulphur dioxide into one of the metal-oxygen links, as shown in Scheme 5. ... 

Shaw concluded that hydrogenation of 3-alkyl-4-aminomethylene isoxazol-5-ones (184) in the presence of palladium catalyst resulted in the saturation of either the endocyclic double carbon-nitrogen bond or the exocyclic double C—C bond with the retention of the heterocyclic nitrogen-oxygen bond. Recent data reported by Kochetkov et al. on the properties, and in particular on hydrogenation, of isoxazolid-5-ones - indicate, however, that Shaw had probably ob-... 

A more expected difference between platinum oxide and palladium-on-carbon was found in the hydrogenolysis of 5-phenyI-2-(3,4-dimethoxybenzyI)-2-oxazoline. Cleavage occurred at the benzyl-oxygen bond over both catalysts, but over platinum, the less substituted phenyl group was saturated as well (78). 

Relative differences between S 2p3/2 and O 1 s ionization potentials show a characteristic separation for oxygen-bound and sulphur-bound sulphoxides. It is clearly shown in Table 20 that sulphur-bound complexes have (O 1 s-S 2p3/2) relative shifts of 365.0 eV, while oxygen-bound complexes have relative shifts of 365.8 eV. Infrared and X-ray crystallographic results also show that most neutral platinum and palladium dialkyl sulphoxide complexes contain metal-sulphur rather than metal-oxygen bonds, while first-row transition metals favour oxygen-bonded sulphoxide. 

Oxidative addition involving carbon-to-oxygen bonds is of relevance to the catalysis with palladium complexes. The most reactive carbon-oxygen bond is that between allylic fragments and carboxylates. The reaction starts with a palladium zero complex and the product is a ir-allylic palladium(II) carboxylate Figure 2.16. 

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,... 

Oxygen-oxygen bond in peroxides and hydroperoxides is cleaved very easily by catalytic hydrogenation over platinum oxide [661,662], over palladium [75, 662, 663, 664] or over Raney nickel [66S. Hydroperoxides yield alcohols... 

Following our first report on the palladium-catalyzed reaction of vinyl triflates with olefins (Heck-type reaction), oxidative insertion of palladium(O) into the carbon-oxygen bond of easily available vinyl triflates ... 

The Tsuji-Trost-type reaction is applicable to bifunctional vinyl epoxide 144 and 1,3-diketone using a palladium catalyst as demonstrated by Koizumi, who obtained polymer 145 (Equation (67)). The reaction proceeds at 0 °C to a reflux temperature of THE. The resulting polymer 145 is isolated in a quantitative yield. The molecular weight of 145 is ca. 3000 (PDI = 2.0-2.7) when 5 mol% of Pd(PPh3)4 is employed as a catalyst. Use of Pd2(dba)3 with several bidentate phosphines such as dppe, dppp, dppb, and dppf is also effective for the polymerization reaction. Propargyl carbonate 146 also reacts with bisphenols in the presence of a palladium catalyst to afford polyethers 147 via carbon-oxygen bond formation at s - and r/) -carbon atoms (Equation (68)). 

The formation of oxygen heterocycles through carbon-oxygen bond formation was also reported. Substituted 2-(o-halophenyl)-ethanols were converted to dihydrobenzofuranes using palladium and Buchwald s bulky biaryl-type ligands (3.43.). The reaction was also efficient in the formation of six and seven membered oxygen heterocycles.53... 

The palladium catalysed conversion of alkenes to enols, also known as the Wacker reaction, has also been used in the formation of oxygen heterocycles. In the example shown in 3.68. the subsequent formation of two carbon-oxygen bonds leads to the desired dioxabicyclo[3.2.1]octane derivative. The first Wacker reaction gives selectively a six membered ring formation (other possible routes would lead to even larger rings), while in the second Wacker reaction the selective formation of the five membered ring is observed.86... 

C-M bond addition, for C-C bond formation, 10, 403-491 iridium additions, 10, 456 nickel additions, 10, 463 niobium additions, 10, 427 osmium additions, 10, 445 palladium additions, 10, 468 rhodium additions, 10, 455 ruthenium additions, 10, 444 Sc and Y additions, 10, 405 tantalum additions, 10, 429 titanium additions, 10, 421 vanadium additions, 10, 426 zirconium additions, 10, 424 Carbon-oxygen bond formation via alkyne hydration, 10, 678 for aryl and alkenyl ethers, 10, 650 via cobalt-mediated propargylic etherification, 10, 665 Cu-mediated, with borons, 9, 219 cycloetherification, 10, 673 etherification, 10, 669, 10, 685 via hydro- and alkylative alkoxylation, 10, 683 via inter- andd intramolecular hydroalkoxylation, 10, 672 via metal vinylidenes, 10, 676 via SnI and S Z processes, 10, 684 via transition metal rc-arene complexes, 10, 685 via transition metal-mediated etherification, overview,... 

---