Oxygen, reaction with palladium

Oxygen Interactions and Reactions on Palladium(lOO) Coadsorption Studies with C2H4, H2O, and CH3OH... 

C. Palladium-catalyzed reactions with oxygen nucleophiles. 

Kostic el al. discovered that Pd11 complexes, when attached to tryptophan residues, can rapidly cleave peptides in acetone solutions to which a stoichiometric amount of water is added, for hydrolysis.436 The indole tautomer in which a hydrogen has moved from the nitrogen to C(3) is named indolenine. Its palladium(II) complexes that are coordinated via the nitrogen atom have been characterized by X-ray crystallography and spectroscopic methods.451 Binuclear dimeric complexes between palladium(II) and indole-3-acetate involve cyclopalladation.452 Bidentate coordination to palladium(II) through the N(l) and the C(2) atoms occurs in binuclear complexes.453 Reactions of palladium(II) complexes with indole-3-acetamide and its derivatives produced new complexes of unusual structure. Various NMR, UV, IR, and mass spectral analyses have revealed bidentate coordination via the indole carbon C(3) and the amide oxygen.437... 

Monoanions derived from nitroalkanes are more prone to alkylate on oxygen rather than on carbon in reactions with alkyl halides, as discussed in Section 5.1. Methods to circumvent O-alkylation of nitro compounds are presented in Sections 5.1 and 5.4, in which alkylation of the a.a-dianions of primary nitro compounds and radial reactions are described. Palladium-catalyzed alkylation of nitro compounds offers another useful method for C-alkylation of nitro compounds. Tsuj i and Trost have developed the carbon-carbon bond forming reactions using 7t-allyl Pd complexes. Various nucleophiles such as the anions derived from diethyl malonate or ethyl acetoacetate are employed for this transformation, as shown in Scheme 5.7. This process is now one of the most important tools for synthesis of complex compounds.6811-1 Nitro compounds can participate in palladium-catalyzed alkylation, both as alkylating agents (see Section 7.1.2) and nucleophiles. This section summarizes the C-alkylation of nitro compounds using transition metals. 

Gold forms a continuous series of solid solutions with palladium, and there is no evidence for the existence of a miscibility gap. Also, the catalytic properties of the component metals are very different, and for these reasons the Pd-Au alloys have been popular in studies of the electronic factor in catalysis. The well-known paper by Couper and Eley (127) remains the most clearly defined example of a correlation between catalytic activity and the filling of d-band vacancies. The apparent activation energy for the ortho-parahydrogen conversion over Pd-Au wires wras constant on Pd and the Pd-rich alloys, but increased abruptly at 60% Au, at which composition d-band vacancies were considered to be just filled. Subsequently, Eley, with various collaborators, has studied a number of other reactions over the same alloy wires, e.g., formic acid decomposition 128), CO oxidation 129), and N20 decomposition ISO). These results, and the extent to which they support the d-band theory, have been reviewed by Eley (1). We shall confine our attention here to the chemisorption of oxygen and the decomposition of formic acid, winch have been studied on Pd-Au alloy films. 

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. 

Trickle bed oxidation. Dilute aqueous ethanol (about 2-3%) is oxidized to acetic acid by the action of pure oxygen at 10 atm in a trickle bed reactor packed with palladium-alumina catalyst pellets and kept at 30°C. According to Sato et al., Proc. First Pacific Chem. Eng. Congress, Kyoto, p. 197,1972, the reaction proceeds as follows ... 

The oxygen atom has also been used to generate other functionalities, such as the aldehyde moiety in Kibayashi s syntheses of (—)-coniine (197) and its enantiomer (Scheme 1.43) (253). Here, reaction of tetrahydropyridine N-oxide 93 with a silylated chiral allyl ether dipolarophile 198 delivered the adduct 199 with the desired bridgehead stereochemistry via the inside alkoxy effect . Desilylation and hydrogenolytic N—O bond rupture with palladium(II) chloride provided the diol 200... 

Abstract Palladium-catalyzed oxidation reactions are among the most diverse methods available for the selective oxidation of organic molecules, and benzoquinone is one of the most widely used terminal oxidants for these reactions. Over the past decade, however, numerous reactions have been reported that utilize molecular oxygen as the sole oxidant. This chapter outlines the fundamental reactivity of benzoquinone and molecular oxygen with palladium(O) and their catalyst reoxidation mechanisms. The chemical similarities... 

A study of Pd(PPhs)4,4, reported recently by Roth and coworkers, reveals yet another mechanistic pathway for Pd° oxygenation [122]. Complete dissociation of one PPha ligand from Pd occurs in solution to produce a three-coordinate palladium(O) species, 26. Kinetic studies reveal that 26 reacts with dioxygen via parallel associative and dissociative pathways (Scheme 5). The latter dissociative pathway results in the formation of the two-coordinate complex 27, which undergoes very rapid reaction with dioxygen in a manner directly analogous to that of the well-defined two-coordinate NHC complexes 23 and 24. 

Oxygen-containing solvents such as water, alcohols or ethers are such poor donors that few complexes with palladium(II) have been isolated. The most important class of complexes of this type consists of those containing water, which are formed as intermediates in the substitution reactions of palladium(II) when carried out in aqueous solution. In these reactions their formation is in competition with the second order reaction of the complex with the incoming ligand. The aqua complexes can also be formed by reaction of halo complexes with silver salts (e.g. N03, C104, BF4) in water. These complexes are acidic, being in equilibrium with hydroxo complexes in neutral or basic media. 

It is now possible to understand the curious phenomenon whereby the reaction of palladium acetate with I in vacuo first rapidly produces a metal precipitate and then slows at about 20% conversion and finally stops with much of the palladium (II) unreacted. These stages in the reaction correspond to oxidation first by Pd3(OAc)6 and then by IVa with ultimate formation of the inert species Va. A complex mixture of hexenyl acetates is formed in the oxidation of which the major constituent l-hexen-2-yl acetate (VI) is 0.68 mole fraction of the whole mixture. Overall the mixture is closely similar to that obtained in the catalytic reactions of 02 described later, suggesting that the same active palladium-containing species is involved. Much of I is isomerized to a 5 1 mixture of trans- and ds-2-hexene (85% at 6 hrs) with only 3% each of the 3-hexene isomers. This aspect of the selectivity problem in which only one shift of the double bond takes place is also reproduced in the catalytic reaction, but oxygen suppresses the rate of isomerization relative to oxidation. 

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