Re-oxidation of Pd

As noted in Section 16.6.2.1, many reagents have been used to re-oxidize the Pd(0) product of the olefin oxidation to the starting Pd(II) species. Each oxidant reforms the Pd (II) by a different mechanism. The least imderstood is the re-oxidation of Pd(0) by the combination of CuCl in acidic medium. This process formally occurs by the transfer of one Cl atom from two CuClj to the Pd(0) to re-form PdCl. This re-oxidation could include Cu-Pd clusters possessing bridging oxo ligands. - "... 

The re-oxidation with quinone can occur by several mechanisms. The re-oxidation of Pd(0) by quinone in acid has been studied by Backvall. This work showed that the well-known quinone complexes of Pd(0), such as the Pd(COD)(benzoquinone) complex in Equation 16.128, react with acid to protonate the oxygen of the quinone. A second protonation... 

The palladium-catalysed oxidative acyloxylation/carbocyclization of allenynes (78) has been found to produce acyloxylated vinylallenes (82), where a new C-C bond, a new C-0 bond, and a new allene moiety are formed (Scheme 6). The reaction is believed to proceed via the initial Pd-coordination (79), followed by cyclization generating the 7 -complex (82) either via the Pd(II) (80a) or Pd(IV) (80b) complex. The reaction is completed by extrusion of the product (82) and re-oxidation of Pd(0) to Pd(II) by p-benzoquinone (p-BQ). Mechanistic evidence was gathered by isotopic labelling. An aerobic version was also realized using catalytic amounts of p-BQ together with a catalytic amount of cobalt salophen complex. ... 

An interesting variant involves the use of an allylic alcohol as the alkene component. In this process, re-oxidation of the catalyst is unnecessary since the cyclization occurs with /Uoxygen elimination of the incipient cr-Pd species to effect an SN2 type of ring closure. Both five- and six-membered oxacycles have been prepared in this fashion using enol, hemiacetal, and aliphatic alcohol nucleophiles.439,440 With a chiral allylic alcohol substrate, the initial 7r-complexation may be directed by the hydroxyl group,441 as demonstrated by the diastereoselective cyclization used in the synthesis of (—)-laulimalide (Equation (120)).442 Note that the oxypalladation takes place with syn-selectivity, in analogy with the cyclization of phenol nucleophiles (1vide supra). 

Nishihata et al. (2002) reported the re-dispersion of Pd in a Perovskite-type oxide. They investigated the oxidation state and the local structure of Pd by using X-ray absorption analysis. Pd occupies the -site in La2PdCo06 in the oxidized sample. For the reduced catalyst, the XAD and XANES measurements suggested the segregation of metallic Pd from the perovskite crystal. They imply that Pd also moves back and forth between the -site in the perovskite structure and sites within the lattice of Pd metal clusters dispersed on perovskite surface when the catalyst is exposed to fluctuations in the redox characteristics of the emission exhaust. 

Bennett RA, Stone P, Bowker M (1999) Pd nanoparticle enhanced re-oxidation of HOj support material imaging of spillover and a new form of SMSI. Catal Lett 59 99... 

Electron withdrawal from the coordinated alkene to an electrophilic metal center makes the coordinated alkene susceptible to attack by an external nucleophilic agent or by a ligand coordinated to the metal. A classic example using modification of the chemical nature of ethylene coordinated to a cationic metal center can be seen in palladium-catalyzed Hoechst-Wacker process [111]. The catalytic cycle can be represented by Scheme 1.37, which is comprised of the main cycle to convert the ethylene coordinated to Pd(II) into acetaldehyde and auxiliary cycles to re-oxidize the Pd(0) species to Pd(II) with Cu(I). The Cu(I) produced in the process is oxidized in turn to Cu(II) with oxygen. 

The /1-hydroxyethylpalladium species is subsequently transformed into a-hydroxyethylpalladium species by skeletal isomerization involving /I-hydrogen abstraction and transfer of the hydride hgand to the methylene carbon of vinyl alcohol formed. The a-hydroxyethylpalladium liberates acetaldehyde as shown in the scheme with generation of Pd(0). The key of success in realizing the catalytic process was to re-oxidize the Pd(0) species produced to Pd(II) with Cu(II), which is reduced in the auxiliary redox process to Cu(I) that is in turn re-oxidized by oxygen to Cu(II). 

In contrast to the processes based on the external attack of a nucleophile on the coordinated CO or olefin ligands on Pd(II) species, where re-oxidation of the Pd(0) produced to reactive Pd(II) presents a considerable problem, no such problem is involved in reaction of a Pd(0) complex with allylic substrates. As we have already discussed in Schemes 1.9 and 1.10, allylic compounds such as allylic acetates or carbonates readily oxidatively add to Pd(0) species to form 7 -allyl palladium(II) complexes that are susceptible to nucleophilic attack. The catalytic process converting allylic substrates to produce allylation products of nucleophiles has found extensive uses in organic synthesis, notably in the work of Tsuji and Trost. Employment of a chiral ligand in the catalytic allylation of nucleophiles allows catalytic asymmetric synthesis of allylation... 

Deactivation of the catalysts in presence of water could be attributed to the formation of thermally stable hydroxyl groups on the surface of oxide shells (Ce02 and Ti02) which hinder re-oxidation of metallic Pd. 

Dienes can bind to metals in an t) or r fashion. Many metal complexes contain t) -diene ligands, and reactions of -ri -diene complexes are described in Section 11.7.2. However, other metals, particularly Pd(II), bind dienes and allenes in an t) fashion. The reactivity of these species has been exploited to develop useful synthetic methods and is described in this section. Backvall has developed chemistry based on nucleophilic addition to palladium-diene complexes to generate allyl products, which subsequently react with a second nucleophile to generate free organic products from 1,4-addition of two nucleophiles across the diene. The palladium(O) byproduct is then re-oxidized to Pd(II) with quinone. 

While the industrial Wacker reaction is run with CuClj as a catalyst for re-oxidation of the Pd(0) by Oj, other oxidants have been used in the laboratory without the need for copper cocatalysts. These oxidants include benzoquinone, heteropolyphosphates, many metal ions, and oxygen alone. The rate of the oxidation of ethylene (and other olefins) is... 

Chemical oxidation of [Pd-o-C, -C(Me)=N-NPh)L] (I P(OMe),, 1) with (Cp FelPF in CH Q, molar ratio 1 1, affrnrd complex 3 which gives the cyclic voltammogram shown in Figure 1. The RE (rotating electrode) experiment reveals ttet 3 undergoes two one-... 

Complementarily, a chloropalladation of the alkyne can be set using PdCl2, and the resulting vinylpalladium chloride adds, in turn, on acrylates. Cholorodienic diesters are thus obtained provided CuCl2 is added to the medium to re-oxidize the Pd(0) and close the catalytic cycle [80]. This reaction is further discussed in the section dedicated to halodienes (Sect. 5). 

In 1999, Mikami s group reported the first example of an intermolecular asymmetric Fujiwara-Moritani reaction of benzene with cyclic alkenes using a Pd catalyst co-ordinating to a chiral sulfonylamide-oxazoline ligand (Scheme 7.24). With PhCOs Bu as the oxidant, the coupling reaction occurred with moderate enantioselectivity (up to 49% ee), although the chemical yield was low. The reaction is considered to involve the formation of a phenyl-Pd species via electrophilic C-H substitution by Pd(ii), and Heck-type cyclization followed by re-oxidation of the resultant Pd(0) species. 

Isolated Pd-atoms in sodalite cages obtained by reducing Pd + ions on SI sites were re-oxidized into Pd + cations located on ST sites merely by heating the zeolite in oxygen above 450 K [21]. In the same way, Pd-clusters filling adjacent supercages were re-oxidized into Pd + cations by calcination in oxygen above 470 K [21]. The mechanism of oxidative redispersion of metal clusters into cations involved the participation of zeolite protons ... 

Hydrogenation. Gas-phase catalytic hydrogenation of succinic anhydride yields y-butyrolactone [96-48-0] (GBL), tetrahydrofiiran [109-99-9] (THF), 1,4-butanediol (BDO), or a mixture of these products, depending on the experimental conditions. Catalysts mentioned in the Hterature include copper chromites with various additives (72), copper—zinc oxides with promoters (73—75), and mthenium (76). The same products are obtained by hquid-phase hydrogenation catalysts used include Pd with various modifiers on various carriers (77—80), Ru on C (81) or Ru complexes (82,83), Rh on C (79), Cu—Co—Mn oxides (84), Co—Ni—Re oxides (85), Cu—Ti oxides (86), Ca—Mo—Ni on diatomaceous earth (87), and Mo—Ba—Re oxides (88). Chemical reduction of succinic anhydride to GBL or THF can be performed with 2-propanol in the presence of Zr02 catalyst (89,90). 

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