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Polyoxometalate Catalysis 

Polyoxometalate Catalysis
Some general properties make POMs as a class an attractive target for catalysis. POMs are anions, and their countercations may be exchanged by metathesis reactions. This allows for the preparation of both inorganic and organic salts. Selection of countercation can make the POM soluble in either water or organic phases [8]. Typically tetrabutylammonium (TBA) is chosen as the countercation for organic phase reactions, and Na+ or K+ is chosen for aqueous phase reactions. Existing as an anion is also useful because the POM can then be attached to a positively charged support medium, thus changing a homogeneous catalyst to a heterogeneous catalyst [9]. An example of this is TBA8[(FeIII(OH2)2)3(A-a-PW9O34)2]/(Si/AlO2), a sandwich POM bound to cationic silica used as a sulfoxidation catalyst [9].

Another property is the ease with which POMs are synthesized and the variety of compounds possible. POMs are usually self-assembled structures at the appropriate pH. This allows for the creation of complex molecules in only one synthetic step. Another characteristic is that the negatively charged oxygen atoms are polarized towards the positive tungsten atoms on the interior of the structure. Therefore, the oxygen atoms are relatively inert, making them resistant to acidic or basic decomposition.
In addition to their structural robustness, POMs possess important electronic characteristics. The incorporation of transition metals provides a source of weakly attached electrons which can be transferred to other compounds (reduction). Also, the POM is capable of absorbing and stabilizing electrons from substrates in oxidation reactions, thus lowering energy state of intermediates [10]. The last, and perhaps most important, feature of POMs is their ability to transfer and accept multiple electrons at a time [10]. This prevents the formation of radical intermediates, which result in non-stereospecific products.
Perhaps the largest benefit from POMs comes from their ability to catalyze green (i.e. environmentally friendly) reactions. Many POMs are able to activate molecular oxygen or hydrogen peroxide as reagents in oxidation reactions. The by-products from these reactions are water as opposed to organic compounds. Another green chemistry goal is for “atom economy,” in which as much as possible of the reagents are incorporated into the final product [11]. An example will illustrate this notion in the following section on epoxidation catalysts. The following sections will detail the use of POMs in the preparation of epoxides, alcohols, and organic molecules.

A final, crucial consideration for a catalyst is that it must be cost effective. The cost of production of POMs varies greatly. Simple tungstophosphate POMs can be readily synthesized on a large scale from cheap salts. However, POMs that contain rare metals are of course much more expensive. The goal and assumption of attempting to produce catalysts is that since the catalyst is degraded at a very slow rate, the overall process is cheaper than if stoichiometric amounts of reagents are used.

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Oxidation of Alkyl Arenes
A system that oxidizes alkyl arenes is PMo12O40 in phenylmethylsulfoxide (PMSO) [21]. The PMSO serves as an oxygen source for the oxidation of alkyl arenes, via hydride removal and the formation of a benzylic carbocation mechanism. In most substrates, a carbonyl moiety was given at the benzylic position (Scheme 1.1). However, other products were observed for some substrates, most interesting of which is the product of oxidation of triphenylmethane, which results in the formation of a carbon-carbon bond between two of the phenyl rings creating a pentacyclic ring (Scheme 1.2). This result may lend itself to use in organic synthesis pathways of natural products, where the formation of polycyclic systems is often required. The final type of oxidation occurs where the formation of a double bond can give an extended conjugated system. A representative reaction is shown in Scheme 1.3, and tetrahydrophenanthracene and 1,2-diphenylethane react in the same manner go give anthracene and trans-stilbene [21]. 

تصویر

Scheme 1: The scheme shows the various reactions catalyzed by PMo12O40 oxidation. 1. shows the formation of the pentacyclic compound upon oxidation. 2. shows the typical reaction product for most substrates, the addition of a carbonyl functionality to the benzylic position. 3. shows the oxidation of tetrahydroanthracene yields anthracene, adding double bonds rather than a carbonyl function.