Oxygen source, cerium oxide

Cerium(III) tris(/3-diketonate) complexes are readily oxidized to the cerium(IV) compounds. They are volatile with vapor pressures high enough for MOCVD use. A number of diketonates are now better characterised, prompted by the possibility of using them as CVD materials and petrol additives, as well as a source of cerium oxide as an oxygen store for catalytic converters. [Ce Me3CCOCHCOCMe2(OMe) 4], Ce(acac)4 (Figure 32), Ce(dbm)4, [Ce(pmhd)4], and Ce(tmhd)4 all have square-antiprismatic coordination of cerium (in the [Ce(catecholate)4]" ion the coordination is dodecahedral). 

Polynuclear aromatic systems are oxidized to the corresponding quinones by ammonium hexanitratocerate(IV) in aqueous THF under mild conditions (25-60 °C) (scheme 1) (Ho et al., 1973). Water is the source of oxygen in the oxidation process. The yields are reasonable, but not very high, and the reaction is only useful for symmetric molecules like naphthalene and anthracene. Substituted and asymmetric substrates give often an intractable mixture of oxidation products (Balanikas et al., 1988). A problem associated with the use of ammonium hexani-tratocerate(IV) is that nitration of the aromatic ring can also occur. Cerium(IV) ammonium... 

Krohn and Vitz (2000) used tert-butyl hydroperoxide (TBHP) with catalytic amounts of CAN for the oxidation of hydroquinones and hydroquinone monomethyl ethers to quinones. TBHP is a cheap and convenient oxygen source (Sherrington, 1988). It will not oxidize organic substrates in the absence of a catalyst. High regioselectivity can be obtained by a careful choice of a catalyst. Cerium(IV) impregnated on Naflon is an example of a supported catalyst for the oxidation of alcohols by tert-butyl hydroperoxide (Kanemoto et al., 1984, 1989). 

Phenols (e.g., phenol itself [CeHs-OH or Ar-OH], Table 6.10, item 2) and their esters (e.g., the trifluoroacetate ester of phenol [C6H5-O2CCF3 or Ar02CCH3], Table 6.10, item 3) have been oxidized with air and oxygen (O2), in neutral and alkaUne solutions, with and without ionic and/or radical catalysts and/or irradiation and in a variety of solvents. Enzymes (this chapter and Chapter 12) from a wide variety of sources have also been used. Frequently, oxidation of aromatic systems to phenols cannot be stopped before quinones and products of ring fragmentation occur and numerous, sometimes ill-defined, products result. Thus, as shown in Equation 6.80, oxidation of the polynuclear hydrocarbon chrysene with anunonium cerium(IV) sulfate [ceric ammonium sulfate, Ce(NH,)4(S04)4] is reported to produce 6H-benzo[d]naphtho[l,2-/>]pyran-6-one (8% yield) and a quinone (23% yield). The remainder of the product(s) (69%) was unidentified. 

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