Palladium catalysis oxygenation

Scheme 9. Oxygen heterocycle by tandem palladium catalysis.

Oxygen heterocycles can be prepared in several different ways via palladium catalysis, even though oxygen is a poorer nucleophile than nitrogen. For example, p, y-alkynic ketones are cyclized to fhrans in the presence of Pd(dba)2 and PPhs (equation 96). 

Oestxeich found that the direct arylation of indolines could be accomplished without over oxidation to the corresponding indole under palladium-catalysis with air (open flask), oxygen (balloon), or copper(II) acetate as the oxidant. Indolines can be unsubstituted or substituted as C2/C3 and the reaction performs well on gram scale (250, 18 examples, 18—90% isolated yield) (140L6020).A directed C2-functionalization/C7-alkenylation was discovered by Xu,Yi, and colleagues. With a rhodium catalyst, indole derivatives were functionalized with acetates at C2 (22 examples, 62—92% yield) the newly obtained products could be alkenylated at C7 with a rhodium/copper system (251,3 examples, 68—78% yield) (14CC6483). 

The impurities usually found in raw hydrogen are CO2, CO, N2, H2O, CH, and higher hydrocarbons. Removal of these impurities by shift catalysis, H2S and CO2 removal, and the pressure-swing adsorption (PSA) process have been described (vide supra). Traces of oxygen in electrolytic hydrogen are usually removed on a palladium or platinum catalyst at room temperature. 

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. 

We showed that the application of PEG/CO2 biphasic catalysis is also possible in aerobic oxidations of alcohols [15]. With regard to environmental aspects it is important to develop sustainable catalytic technologies for oxidations with molecular oxygen in fine chemicals synthesis, as conventional reactions often generate large amoimts of heavy metal and solvent waste. In the biphasic system, palladium nanoparticles can be used as catalysts for oxidation reactions because the PEG phase both stabilises the catalyst particles and enables product extraction with SCCO2. 

Catalysis. The most important industrial use of a palladium catalyst is the Wacker process. The overall reaction, shown in equations 7—9, involves oxidation of ethylene to acetaldehyde by Pd(II) followed by Cu(II)-catalyzed reoxidation of the Pd(0) by oxygen (204). Regeneration of the catalyst can be carried out in situ or in a separate reactor after removing acetaldehyde. The acetaldehyde must be distilled to remove chlorinated by-products. 

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