Palladium complexes hydrocarbon oxidation

Palladium complexes are generally superior catalysts for oxidation reactions, whereas other noble metals are more active for other reactions, e.g., rhodium for hydroformylation. All of these reactions seemingly involve activation of the olefin substrate by rr-complex formation with the noble metal catalyst.513 The oxidation reactions discussed in the following generally depend on nucleophilic attack on the coordinated olefins (or other hydrocarbons) to effect oxidation of the substrate. 

As a matter of fact, olefin-consuming reactions (by H2) may be a serious problem in some technical reactions. Palladium complexes and Co2(CO)g (commercial products) are typical catalysts. Problems may also arise in the Fischer-Tropsch reaction [19, 20] where iron oxides of a certain basicity (alkaline-metal doping) are being used to catalyze the formation of hydrocarbons according to (the simplified) eq. (15). More details are provided in Section 3.1.8. Since water is inevitably formed, carbon dioxide can also occur. On the other hand, it is doubtful whether the CO/H2O system will be used for directed reductions of organic compounds, since hydrogen is an extremely abundant industrial chemical. The water-gas shift reaction is thus to be avoided in the vast majority of cases. 

Palladium(II) complexes with heteropolyanions PWnOs and PW9034 , and originally synthesized bimetallic Pd(II)-Fe(III) complexes with PW9034 were used for the preparation of Si02 supported catalysts of hydrocarbons oxidation. The composition of the complexes in aqueous solution was characterized by P NMR and IR spectroscopy. 

Addition of anionic Fischer carbene complexes, of their CS2 adducts and of an anionic thiocarbyne complex to cationic organometallic Lewis acids and to coordinated, unsaturated hydrocarbons gives novel hydrocarbon bridged heterodimetallic compounds. Oxidative addition of halocarbyne complexes to zerovalent platinum and palladium complexes provides a synthesis of carbido bridged complexes. 

The discussion of the activation of bonds containing a group 15 element is continued in chapter five. D.K. Wicht and D.S. Glueck discuss the addition of phosphines, R2P-H, phosphites, (R0)2P(=0)H, and phosphine oxides R2P(=0)H to unsaturated substrates. Although the addition of P-H bonds can be sometimes achieved directly, the transition metal-catalyzed reaction is usually faster and may proceed with a different stereochemistry. As in hydrosilylations, palladium and platinum complexes are frequently employed as catalyst precursors for P-H additions to unsaturated hydrocarbons, but (chiral) lanthanide complexes were used with great success for the (enantioselective) addition to heteropolar double bond systems, such as aldehydes and imines whereby pharmaceutically valuable a-hydroxy or a-amino phosphonates were obtained efficiently. 

In most palladium-catalyzed oxidations of unsaturated hydrocarbons the reaction begins with a coordination of the double bond to palladium(II). In such palladium(II) olefin complexes (1), which are square planar d8 complexes, the double bond is activated towards further reactions, in particular towards nucleophilic attack. A fairly strong interaction between a vacant orbital on palladium and the filled --orbital on the alkene, together with only a weak interaction between a filled metal d-orbital and the olefin ji -orbital (back donation), leads to an electrophilic activation of the alkene9. 

Uses in Synthesis and Catalysis Table 6 Palladium-catalyzed Oxidation of Hydrocarbons Catalytic reaction Complex Nucleophile Refs. 

This section is concerned with the activation of hydrocarbon molecules by coordination to noble metals, particularly palladium.504-513 An important landmark in the development of homogeneous oxidative catalysis by noble metal complexes was the discovery in 1959 of the Wacker process for the conversion of ethylene to acetaldehyde (see below). The success of the Wacker process provided a great stimulus for further studies of the reactions of noble metal complexes, which were found to be extremely versatile in their ability to catalyze homogeneous liquid phase reaction. The following reactions of olefins, for example, are catalyzed by noble metals hydrogenation, hydroformylation, oligomerization and polymerization, hydration, and oxidation. 

Phosphine ligands are oxidized under the reaction conditions, however, palladium(II) complexes bearing enantiomerically pure hydrocarbons as ligands (e.g., complexes 4 and 5) can be employed as the catalysts63. 

Henry, P. M. Catalysis hy Metal Complexes, Vol. 2 Palladium Catalyzed Oxidation of Hydrocarbons ed. Reidel, D.) (Dordrecht, Holland, 1980)41. 

As trialkyl(aryl)stannanes have never been reported to undergo oxidative addition with a palladium(O) or nickel(O) complex, the corresponding arylstannylation is not known. As shown in Scheme 5.7.24, Kosugi and coworkers found that the use of aryl(trichloro)stannanes instead of the trialkyl derivatives enables the palladium-catalyzed arylstannylation of norbomene. The reaction using other unsaturated hydrocarbons including ordinary alkenes has not been reported, but the observation that aryl-tin bonds can be activated by palladium(O) complexes has certain significance, and will lead to future development of other arylstannylations. 

Other type of complexes have also been used for the oxidation of hydrocarbons. For instance, Fujiwara and coworkers employ a coordinated complex of palladium with o-phenanthroline as an efficient catalyst for the direct conversion of benzene into phenol. Moro-oka and coworkers use an oxo-binuclear iron complex, whereas Machida and Kimura work with macrocyclic polyamines. Sasaki and coworkers employ Pd-Cu composite catalysts, which are prepared by impregnating the respective metal salts on silica gel. 

Oxidative biomimetic aryl coupling reactions of electron-rich (phenolic) aromatic hydrocarbons have occasionally been reported for selected examples see the reviews by Bringmann et al. [93 a] and the review on the ellagitannin chemistry by Quideau and Feldman [93b]. Oxidative homocoupling has also been achieved with palladium(II) acetate via the 5-arylpalladium complexes [78 d], but stoichiometric amounts of the palladium reagents are often required and acetylation can occur as a side reaction [93a]. 

The interaction of SO and an oxidizable hydrocarbon with the catalyst surface also illustrates how complex such behavior can be. In the oxidation of CH and SO, mixtures on vanadium, palladium/vanadium, and palladium cataly.sts, SO increased the CH oxidation rate on all catalysts.- How ever, SO, inhibited CO oxidation on the palladium/vanadium catalyst while it increased the CO oxidation rate on the vanadium catalyst. Also, SO, inhibits alkene oxidation on these types of catalysts as well. These results are explained by competition between SOj and the hydrocarbons for adsorbed forms of oxy gen on the catalyst surface. First, SO, can reduce the... 

Nickel complexes act as catalysts for the oxidation of ditertiary diphosphines. Palladium(o) complexes catalyse the autoxidation of hydrocarbons. The reactivity is sensitive to the nature of the arylphosphine ligands. The mechanism involves intermediate formation of a palladium-molecular oxygen complex. ... 

In these and related reactions, the ether and alcohol ligands dissociate readily. The coordinated THF dissociates from the zirconium methyl complex to allow olefin polymerization, albeit more slowly than in the absence of THF. The ether dissociates from palladium and nickel to allow olefin to bind to the cationic palladium and nickel species, and alcohol and water are easily displaced from related Pt(II) complexesby hydrocarbons prior to C-H activation processes. Likewise, the THF and phosphine oxide ligands reversibly dissociate from the zirconium knido complex (Equation 2.18) prior to [2+2] additions with alkynes. - ... 

---