Oxidation using inorganic supports

Table 2.2 Examples of heterogeneous asymmetric oxidation using inorganic supports. 

Various heterogeneous asymmetric oxidation reactions using inorganic supports are listed in Table 2.2. A few selected examples will be described herein on the basis of reachon type. 

The desire to convert benzene directly to phenol with 30% hydrogen peroxide was mentioned in Chap. 4. A polymer-supported salicylimine vanadyl complex (1 mol%) was used to catalyze this reaction. Phenol was obtained in 100% yield at 30% conversion.217 There was no leaching of the metal. The catalyst was recycled ten times after which it started to break up. Oxidation of ligands is often a problem with oxidation catalysts. Inorganic supports not subject to such oxidation need to be tried to extend the life of such catalytic agents. 

Silica is the most commonly used solid support. Treatment with complexes bearing alkoxy- or chlorosilane functional groups is a common way to generate supported complexes on silica or any inorganic oxide containing surface silanol (Si-OH) groups (Scheme 7.2).23,24... 

Most industrial catalysts are heterogeneous catalysts consisting of solid active components dispersed on the internal surface of an inorganic porous support. The active phases may consist of metals or oxides, and the support (also denoted the carrier) is typically composed of small oxidic structures with a surface area ranging from a few to several hundred m2/g. Catalysts for fixed bed reactors are typically produced as shaped pellets of mm to cm size or as monoliths with mm large gas channels. A catalyst may be useful for its activity referring to the rate at which it causes the reaction to approach chemical equilibrium, and for its selectivity which is a measure of the extent to which it accelerates the reaction to form the desired product when multiple products are possible [1],... 

The pattern for outer-sphere oxidation by Co(NH3)5 compared with Co(en)j+ (usually it is —10 times slower) towards inorganic reductants can be used to support an estimate of the proportion of electron transfer (Marcus-dependent) and charge transfer which Ru(bpy) + displays towards these oxidants (45 and 11%, respectively). Sec. 2.2.1(b). Finally, Eqn. 5.35 can be used to determine K 2 for a reaction in which the other kinetic parameters are known. The value of A, 2 can be used, in turn, to estimate the oxidation potential of one couple, which is normally inaccessible. Thus the potentials of the o-, m- and /7-benzene diol radicals 1T2A4 were determined from kinetic data for the oxidation of the diols (H2A) by Fe(phen) + (5.45) ... 

The interaction of Co2(CO)g and Co4(CO)i2 with many common inorganic supports as oxides and zeolites has been reported, and catalysts prepared from cobalt carbonyls have been used in the main reactions in which cobalt is used as active... 

A variety of industrial catalytic processes employ small metal-particle catalysts on porous inorganic supports. The particle sizes are increasingly in the nanometre size range which gives rise to nanocatalysts. As described in chapter 1, commonly used supports are ceramic oxides, like alumina and silica, or carbon. Metal (or metallic) catalysts in catalytic technologies contain a high dispersion of nanoscopic metal particles on ceramic oxide or carbon supports. This is to maximize the surface area with a minimum amount of metal for catalytic reactions. It is desirable to have all of the metal exposed to reactants. 

The Anelli oxidation of alcohols to aldehydes and ketones has been accomplished using polymer-supported nitroxyl radical catalysts. The practicality of removing polymer-supported reagents by filtration to simplify product purification is highlighted by these examples. Bolm and coworkers11 demonstrated that a silica-supported nitroxyl catalyst is easily filtrated after use from the reaction solution, recovered and recycled, and the residual inorganic salts present in the reaction mixture are separated from the organic product by aqueous extraction (Table II, entry 7). 

Inorganic supports have been used less, but they have undeniable advantages, such as the relatively high oxidative stability of the support. A first attempt was made with salen complexes that are vinyl functionalized at both aromatic rings. By coupling in a thioether, catalyst 7f is obtained. For... 

Benzene hydroxylation to give phenol has been performed with Mo-substituted mesoporous silicas and H2O2 in the absence of solvent (267). However, as explained earlier, reports of anchoring of Mo in an inorganic support must be treated with great caution, particularly if there is no clear concept for immobilizing both Mo and peroxo Mo. The same holds true for the Mo silicalite MoS-1, which has been used for sulfide oxidation with H2O2 (268). 

Sn(PC)] and [Sn(PC)Cl2] were prepared by the method of Kroenke and Kenney. (20) Hexadecane (certified) and quinoline were distilled prior to use. Inorganic oxide supports were from commercial sources and have the following properties AI2O3,... 

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