Chromium support-bound

Other reagents that have been used to reduce support-bound aromatic nitro compounds include phenylhydrazine at high temperatures (Entry 5, Table 10.12), sodium borohydride in the presence of copper(II) acetylacetonate [100], chromium(II) chloride [196], Mn(0)/TMSCl/CrCl2 [197], lithium aluminum hydride (Entry 3, Table... 

The earliest Ziegler-Natta catalysts were insoluble bimetallic complexes of titanium and aluminum. Other combinations of transition and Group I-III metals have been used. Most of the current processes for production of high-density polyethene in the United States employ chromium complexes bound to silica supports. Soluble Ziegler-Natta catalysts have been prepared, but have so far not found their way into industrial processes. With respect to stereo-specificity they cannot match their solid counterparts. 

In addition, several groups have bound ir-arene complexes to solid supports and constructed libraries of products derived from ir-arenes by the chemistry described in this section (Equation 11.57). - Oxidation of the final product releases the arene from the metal. The arene-chromium was bound to the support by a pendant phosphine or isocyanide. The isocyanide provides the benefit of maintaining the ir-accepting property of the three ancillary ligands. 

Finally, it should be noted that this chemistry may have biological relevance. Several metalloenzymes are believed to contain more than one metal ion bound at the active site. One relevant example is the glucose tolerance factor (GTF) which is important for the metabolic degradation of glucose (398-401). GTF is a low-molecular-weight protein which contains chromium(III). Its structure is not known, but it has been suggested that the active site contains a dinuclear chromium(III) complex (401). The fact that hydroxo-bridged dinuclear chromium(III) complexes exhibit reactions which are often very fast compared with those observed for the parent mononuclear species seems to support such a proposal. 

The transport or release of iron has been much discussed. One view is that the iron is transferred from the siderophore complex at the outer membrane to another membrane-bound protein, for example in the uptake of iron by rhodotorulic acid in Rhodotorula. The other view is that the intact Fein-siderophore complex is taken up into the cell. This is supported by studies with inert chromium(III) complexes in E. coli and also by labelling studies. 

Many oxidants, especially the older and stronger chromium oxidizing agents, may have their reactivity and selectivity modified by adsorption on to inert supports.Reactions utilizing supported oxidants have the advantage that the residual chromium salts remain bound to the support and thus work-up often becomes reduced to a mere filtration. Many of these systems are discussed in detail later (Volume 7, Chapter 7.3). 

The high initial activity of the fresh alumocopperchromium catalyst in the oxidation of carbon monoxide is due to Cufll) cations within the copper chromite and in CuO surface clusters (Fig 1) Centres with chromium cations in the highest oxidation degrees are bound to the support surface and solid solutions of the aluminate type. These contain cations of bivalent copper and are less active in the CO oxidation reaction. It was shown by IRS and XPS that during the use of this catalyst in CHG, as well as during the unsteady-state oxidation reaction/ a partial reduction of Cu(II) to Cu I occurs (Fig, 2), which leads to a decrease in catalyst activity in the oxidation of the carbon monoxide. In the fresh alumomagnesiumchromium catalyst, the activity for carbon monoxide is determined by the centres containing chromium cations in... 

The polymer boxmd peroxocomplexes prepared in the present work have excellent catalytic potentiality and selectivity in the production of quinones. The chromium peroxocomplexes loses its activity on reaction and could be regenerated. The pwlymer bound vanadium catalyses sharpless epoxidation reactions. The high pore volume and a marginally good surface area shows that GMA-EGDM copolymers are good choice as supports. 

Chromium(III) is oxophilic and can occupy sites in nucleotide complexes similar to those bound by Mg. Since Cr -nucleotide complexes are relatively inert, however, they do not support the phosphoryl transfer chemistry catalyzed by Mg in polymerases or other nucleotide-dependent enzymes. Thus, Cr -nucleotide complexes have been used to study structure and mechanism, particularly in polymerases. As one example, a co-crystal of DNA polymerase /3 with Cr -dTTP substrate shows Cr occupying one site in the two metal binding active site as a replacement of Mgii 44 structure, Cr binds to the /3,7-phosphate oxygens of the 5 -dTTP substrate. 

The use of chiral esters as 2tx-substrates permit the production of diastereomer-ically enriched cycloadducts. For example, the menthyl ester 48 was obtained as a 4 ldiastereomeric mixture. The most intriguing feature of polystyrene-bound catalyst 46 is the fact that it can be removed from the reaction mixture by simple filtration and repeatedly reused. This means that the two basic problems of homogeneous catalysis, separation and recycling of the catalyst, can be solved by using the solid-supported complex 46. Additionally, the environmental problems associated with chromium can also be effectively eliminated. 

As carbene complexes have become proficient catalysts, their incorporation into recyclable catalyst systems has grown into a field of its own. The most common strategy for recovering a catalyst for subsequent use is to append it to a heterogeneous support (e.g., a PS bead), and both Fischer carbenes and NHCs have been incorporated into polymers for this purpose. The first polymer-bound Fischer carbene complexes were reported by Mariorana and Seneci in 1999 [51], for use in soHd-phase organic synthesis for combinatorial chemistry. These materials were prepared via the attachment of chromium carbene complexes to a polymer resin... 

Because this chapter focuses on molecular transition metal complexes that catalyze the formation of polyolefins, an extensive description has not been included of the heterogeneous titanium systems of Ziegler and the supported chromium oxide catalysts that form HDPE. However, a brief description of these catalysts is warranted because of their commercial importance. The "Ziegler" catalysts are typically prepared by combining titanium chlorides with an aluminum-alkyl co-catalyst. The structural features of these catalysts have been studied extensively, but it remains challenging to understand the details of how polymer architecture is controlled by the surface-bound titanium. This chapter does, however, include an extensive discussion of how group(IV) complexes that are soluble, molecular species polymerize alkenes to form many different types of polyolefins. 

This acylation strategy was used to prepare pol5mier-supported chromium carbenes. Microwave irradiation on Wang resin shows the same regioselectivity as solution chemistry but fewer side products. The resin-bound phenol 21 is simultaneously released and oxidized to the benzoquinone 22 with ceric ammonium nitrate. Microwave irradiation has been shown to accelerate the DBR and provide high yields of benzannulated products in short reaction times (ca. 5 min). ... 

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