Other Metals as Catalysts for Oxidation with

In addition to ruthenium, other late and first-row transition elements are capable of dehydrogenating alcohols via an oxometal pathway. Some are used as catalysts, in 

Similarly, Co(acac)3 in combination with N-hydroxyphthalimide (NHPI) as cocatalyst mediates the aerobic oxidation of primary and secondary alcohols to the corresponding carboxylic acids and ketones, respectively (see, for example, Eq. (5.16) [122]). 

By analogy with other oxidations mediated by the Co/NHPI catalyst studied by Ishii and coworkers [123, 124], Eq. (5.16) probably involves a free-radical mechanism. We attribute the promoting effect of NHPI to its ability to efficiently scavenge alkylperoxy radicals, suppressing the rate of termination by combination of these radicals. The resulting PINO radical subsequently abstracts a hydrogen atom from the a-C-H bond of the alcohol to propagate the autoxidation chain (Eqs. (5.17)-(5.19)). 

A nickel-substituted hydrotalcite was also reported as a catalyst for the aerobic oxidation ofbenzylic and allylic alcohols [125]. Analogous to cobalt, nickel is expected to catalyze oxidation via a free-radical mechanism. 

After their leading publication on the osmium-catalyzed dihydroxylation of alkenes in the presence of dioxygen [126], Beller et al. [127] reported that alcohol oxidations could also be performed using the same conditions (see Eq. (5.20)). The reactions were carried out in a buffered two-phase system with a constant pH of 10.4. Under these conditions a remarkable catalyst productivity (TON up to 16 600 for acetophenone) was observed. The pH value is critical in order to ensure the reoxidation of Os(VI) to Os(VIII). The scope of this system seems to be limited to benzylic and secondary alcohols. 

As heterogeneous oxidation catalyst, 5% Pt, 1% Bi/C has been identified as an efficient catalyst for the conversion of 2-octanol to 2-octanone and 1-octanol to octanoic acid (see Fig. 4.72) [210]. Also manganese-substituted octahedral mo- 

Scant attention has been paid to vanadium-catalyzed oxidation of alcohols, despite its ability to act according to the oxometal mechanism. Punniyamurthy recently reported that indeed vanadium turns out to be a remarkably simple and selective catalyst with a wide substrate scope, which requires few additives [212], albeit, the activities are still rather low. 

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Many other metal ions have been reported as catalysts for oxidations of paraffins or intermediates. Some of the more frequently mentioned ones include cerium, vanadium, molybdenum, nickel, titanium, and ruthenium [21, 77, 105, 106]. These are employed singly or in various combinations, including combinations with cobalt and/or manganese. Activators such as aldehydes or ketones are frequently used. The oxo forms of vanadium and molybdenum may very well have the heterolytic oxidation capability to catalyze the conversion of alcohols or hydroperoxides to carbonyl compounds (see the discussion of chromium, above). There is reported evidence that Ce can oxidize carbonyl compounds via an enol mechanism [107] (see discussion of manganese, above). Although little is reported about the effectiveness of these other catalysts for oxidation of paraffins to acetic acid, tests conducted by Hoechst Celanese have indicated that cerium salts are usable catalysts in liquid-phase oxidation of butane [108]. 

Use of alcohol as a solvent for carbonylation with reduced Pd catalysts gives vinyl esters. A variety of acrylamides can be made through oxidative addition of carbon monoxide [630-08-0] CO, and various amines to vinyl chloride in the presence of phosphine complexes of Pd or other precious metals as catalyst (14). 

The Guerbet reaction is an important industrial process for increasing the carbon numbers of alcohols. Thus, a primary or secondary alcohol reacts with itself or another alcohol to produce a higher alcohol (Scheme 23). Alkaline earth metal oxides have been used as catalysts for the condensation of alcohols. Ueda et al. (158,159) reported the condensation of methanol with other primary or secondary alcohols having a methyl or methylene group at the )S-position they used MgO, CaO, and ZnO as catalysts. The reactions were performed with gas-phase reactants at 635 K only MgO was found to be both active and selective (>80%). 

An alternative method for the epoxidation of enones was developed by Jackson and coworkers in 1997 , who utilized metal peroxides that are modified by chiral ligands such as diethyl tartrate (DET), (5,5)-diphenylethanediol, (—)-ephedrine, ( )-N-methylephedrine and various simple chiral alcohols. The best results were achieved with DET as chiral inductor in toluene. In the stoichiometric version, DET and lithium tert-butyl peroxide, which was generated in situ from TBHP and n-butyllithium, were used as catalyst for the epoxidation of enones. Use of 1.1 equivalent of (-l-)-DET in toluene as solvent afforded (2/f,35 )-chalcone epoxide in 71-75% yield and 62% ee. In the substo-ichiometric method n-butyllithium was replaced by dibutylmagnesium. With this system (10 mol% Bu2Mg and 11 mol% DET), a variety of chalcone-type enones could be oxidized in moderate to good yields (36-61%) and high asymmetric induction (81-94%), giving exactly the other enantiomeric epoxide than obtained with the stoichiometric system (equation 37). 

Table 2 reports the catalytic activities of the catalysts prepared for 2.6-DTBP oxidation. All the titanium grafted materials were active as catalysts for liquid phase oxidation of 2.6-DTBP, and catalytic activity decreased in the order of MCM-48 (24.5% conversion) > HMS (22.8%) > KIT-1 (16.0%) > MCM-41 (14.3%) > SBA-1 (5%). Apparently. 3 dimensional channel system of MCM —48, and HMS with small particle size and textual mesoporosity proved to be useful in liquid phase reaction [1,2,3], Chemical analysis of the titanium-grafted SBA-1 by EDX showed far less titanium at the surface than the others it seems surface nature of SBA-1 synthesized in acidic medium is different from the rest. All Ti-grafted samples suffered from titanium leaching during the liquid phase oxidation HMS host resulted in over 4 % loss in metal content while the rest showed 2%. 

Ruthenium is commonly used with other platinum metals as a catalyst for oxidations, hydrogenations, isomerizations, and reforming reactions. The synergetic effect of mixing ruthenium with catalysts of platinum, palladium, and rhodium lias been found for the hydrogenations of aromatic and aliphatic nitro compounds, ketones, pyndine, and nitriles. 

The natural clay minerals are hydrous aluminum silicates with iron or magnesium replacing aluminum wholly or in part, and with alkali or alkaline earth metals present as essential constituents in some others. Their acidic properties and natural abundance have favored their use as catalysts for cracking of heavy petroleum fractions. With the exception of zeolites and some specially treated mixed oxides for which superacid properties have been claimed, the acidity as measured by the color changes of absorbed Hammett bases is generally far below the superacidity range. They are inactive for alkane isomerization and cracking below 100 °C and need co-acids to reach superacidity. 

In natural processes, metal ions are often in high oxidation states (2 or 3), whereas in chemical systems the metals are in low oxidation states (0 or 1). This fact inverts the role of the metal center, such that it acts as a one-electron sink in a natural system, but as a nucleophile in an artificial ones (see other chapters of this book and the review by Aresta et al. [109]). Nevertheless, important biochemical processes such as the reversible enzymatic hydration of C02, or the formation of metal carbamates, may serve as natural models for many synthetic purposes. Starting from the properties of carbonic anhydrase (a zinc metalloenzyme that performs the activation of C02), Schenk et al. proposed a review [110] of perspectives to build biomimetic chemical catalysts by means of high-level DFT or ah initio calculations for both the gas phase and in the condensed state. The fixation of C02 by Zn(II) complexes to undergo the hydration of C02 (Figure 4.17) the use of Cr, Co, or Zn complexes as catalysts for the coordination-insertion reaction of C02 with epoxides and the theoretical aspects of carbamate synthesis, especially for the formation of Mg2+ and Li+ carbamates, are discussed in the review of Schenk... 

Compared with other kinds of catalysts, for example homogeneous catalysts, in which ligands are responsible for specificity, and heterogeneous catalysts, in which catalyti-cally active centers are attached to solid carriers such as zeolites or metal oxides, enzymes feature the advantages and disadvantages listed in Table 1.1. 

Electron transfer from the substrates to 02 proceeds by a redox cycle that consists of copper(II) and copper(I). The high catalytic activity of the copper complex can be explained as follows (1) The redox potential of Cu(I)/Cu(II) fits the redox reaction. (2) The high affinity of Cu(I) to 02 results in rapid reoxidation of the catalyst. (3) Monomers can coordinate to, and dissociate from, the copper complex, and inner-sphere electron transfer proceeds in the intermediate complex. (4) The complex remains stable in the reaction system. It may be possible to investigate other catalysts whose redox potentials can be controlled by the selection of ligands and metal species to conform with these requisites several other suitable catalysts for oxidative polymerization of phenols, such as manganese and iron complexes, are candidates on the basis of their redox potentials. 

Both the discovery of new synthesis processes for nanostructured materials and the demonstration of the highly reactive properties of these materials have increased rapidly within recent years. The new synthesis processes have made available nanostructured materials in a wide variety of compositions of metal oxides and metals supported on metal oxides, which have led to recognition of their exceptional chemical, physical, and electronic properties. The objective of this review is to provide recent results on synthesis of nanostructured materials using the novel processes that were developed in these laboratories recently and to contrast them to other important, new methods. Because some of the most important applications of nanostructured materials are as catalysts for chemical processing, several key reports on enhanced catalytic reactivity of nanostructured grains will be discussed along with the pertinent theory responsible for controlling both activity and selectivity of these new catalysts. 

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