Metal-free oxidation catalysts

Solid acids or bases often play a catalytic role in the activation of an oxidant such as H2O2. Although this topic is not discussed in detail here, we nevertheless illustrate it with two examples. First, LDHs can function as solid bases to catalyze the conversion of aromatic nitriles to peroxy-imidic acids the latter are excellent stoichiometric oxidants for olefin epoxidation (393)  

Alternatively, acids can activate peroxides, for example, in the Baeyer-Villiger reaction (394)  

The remainder of this section is devoted to cases in which an immobilized oxidizing reagent can be regenerated by the action of a simple oxidant, such as a peroxide or O2. For instance, the flavin 10-ethyl-isoalloxazine was immobilized on organic polymers and used in the air oxidation of 1 -benzyl-1, 4-dihydronicotinamide (395). However, peracids, dioxiranes, and nitroxyl radicals are of much more synthetic importance. 

Alternatively, a liquid biphasic approach may be adopted for water-insoluble reactants such as estrone 3-methyl ether. Although the immobilization of the As undoubtedly improves its handling safety, the disposal of this toxic element remains a problem. 

Highly fluorinated ketones such as hexafluoroacetone are oxidation catalysts even when H2O2 is used as an oxygen source. In this case, the reaction probably involves an a-hydroxyhydroperoxide, rather than a dioxirane, and the reactions require more drastic conditions (402, 403). Nevertheless, the observations demonstrate that fluorinated ketones are useful catalysts for activation of oxidants. 

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For a general review of metal-free oxidation, including achiral and chiral catalysts, see W. Adam, C. R. Saha-Moller, P. A. Ganeshpure, Chem. Rev. 2001, 101, 3499-3548. 

This is an important industrial reaction, alone or in combination with others. The CH3OH production is often coupled to oxidation to formaldehyde, methanol to gasoline (Mobil) process, methanol to olefins process, carbonylation, etc. Due to this, a large volume of information already exists on catalyst preparation, kinetics, reactors and all other aspects of the related chemical technology [53]. However, let us concentrate our attention here on just one selected problem the role of the promoter and the nature of the active site on the metal on oxides catalysts. Let us mention in passing that pure metals (promoter free) most likely do not catalyze the synthesis. 

Lin and coworkers prepared 2-substituted quinazolines via a metal-free oxidative sp C—H/sp C—H cyclization of amidines (Scheme 23) (140L2822). Two methods were developed. Method A uses iodobenzene diacetate as a source of hypervalent iodine(lll) and a nonpolar solvent, toluene. In Method B potassium persulfate is used as the oxidant in the presence of a catalyst, TEMPO, and a polar solvent, acetonitrile. Both methods... 

Saalfrank JW, Maier WF. 2004. Directed evolution of noble-metal-free catalysts for the oxidation of CO at room temperature. Angew Chem Int Ed 43 2028-2031. 

The activity of elemental carbon as a metal-free catalyst is well established for a couple of reactions, however, most literature still deals with the support properties of this material. The discovery of nanostructured carbons in most cases led to an increased performance for the abovementioned reasons, thus these systems attracted remarkable research interest within the last years. The most prominent reaction is the oxidative dehydrogenation (ODH) of ethylbenzene and other hydrocarbons in the gas phase, which will be introduced in a separate chapter. The conversion of alcohols as well as the catalytic properties of graphene oxide for liquid phase selective oxidations will also be discussed in more detail. The third section reviews individually reported catalytic effects of nanocarbons in organic reactions, as well as selected inorganic reactions. 

Two different mechanisms have been proposed for the ROP of (di)lactones depending on the nature of the organometalhc derivatives. Metal halides, oxides, and carboxylates would act as Lewis acid catalysts in an ROP actually initiated with a hydroxyl-containing compound, such as water, alcohol, or co-hydroxy acid the later would result more hkely from the in-situ hydrolysis of the (di)lac-tone [11]. Polymerization is assumed to proceed through an insertion mechanism, the details of which depends on the metal compound (Scheme la). The most frequently encountered Lewis acid catalyst is undoubtedly the stannous 2-ethylhexanoate, currently referred to as stannous octoate (Sn(Oct)2). On the other hand, when metal alkoxides containing free p-, d-, or f- orbitals of a favo-... 

Reduction. Benzene can be reduced to cyclohexane [110-82-7], C5H12, or cycloolefins. At room temperature and ordinary pressure, benzene, either alone or in hydrocarbon solvents, is quantitatively reduced to cyclohexane with hydrogen and nickel or cobalt (14) catalysts. Catalytic vapor-phase hydrogenation of benzene is readily accomplished at about 200°C with nickel catalysts. Nickel or platinum catalysts are deactivated by the presence of sulfur-containing impurities in the benzene and these metals should only be used with thiophene-free benzene. Catalysts less active and less sensitive to sulfur, such as molybdenum oxide or sulfide, can be used when benzene is contaminated with sulfur-containing impurities. Benzene is reduced to 1,4-cydohexadiene [628-41-1], C6HS, with alkali metals in liquid ammonia solution in the presence of alcohols (15). 

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