Other Catalytic Applications

Considering the industrial importance of cyclopropanes in the pesticide field, it is not surprising that chiral ferrocenylphosphines have been applied as control ligands for the palladium-catalyzed enantioselective formation of cyclopropanes from the dicarbonate of 2-butene-1,4-diol and malonates, leading to 70% ee (Fig. 4-32e) [179]. Ferrocenylphosphines also induce chirality in the reaction of sulfonyl-substituted propenyl carbonates and acrylic esters to methylenecyclopentanes (up to 78% ee (Fig. 4-321)) [180], with potential applications in natural product synthesis. These examples show that the synthetic potential of chiral ferrocene derivatives is not yet fully exploited, and one may look forward to new applications. 

In several respects, the cylindrical molecule ferrocene (di( 7 -cyclopentadienyl)iron, FeCpj) is analogous to the planar molecule benzene (CgHg). Both ferrocene and benzene are electron-rich aromatic systems that undergo electrophilic substitution. Ferrocene reacts about 10 times faster than benzene in Friedel-Crafts acetylation and about 10 times faster in mercuration with Hg(OAc)2- 

However, in contrast to benzene, ferrocene is sensitive to oxidation, and the ferrocenium cation, FeCpj, a paramagnetic 17-electron species, is readily formed in the presence of various oxidants. The ferrocenium cation is reluctant to undergo electrophilic substitution, and therefore reactions such as halogenation and nitration, which are important routes to substituted benzene derivatives, cannot be used for the synthesis of substituted ferrocenes. Only electrophilic substitution under nonoxidizing conditions (e.g., Friedel-Crafts acylation, Mannich reaction, borylation, lithiation or mercuration), and radical substitution are available as an entry into the chemistry of substituted ferrocenes. 

The most frequently studied ferrocene derivatives are the monosubstituted and the l,l -disubstituted ferrocenes, for which the abbreviations Fc-X and fcXj (X = substituent) will be used throughout the following discussion (Fig. 5-1). 

In general, the ferrocenyl compounds, Fc-X, are easily converted to the l,l -ferro-cenediyl compounds, fcX2, and isolation of the monosubstituted product Fc-X may become difficult even if an excess of ferrocene (FcH) is used. 

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Metal oxides possess multiple functional properties, such as acid-base, redox, electron transfer and transport, chemisorption by a and 71-bonding of hydrocarbons, O-insertion and H-abstract, etc. which make them very suitable in heterogeneous catalysis, particularly in allowing multistep transformations of hydrocarbons1-8 and other catalytic applications (NO, conversion, for example9,10). They are also widely used as supports for other active components (metal particles or other metal oxides), but it is known that they do not act often as a simple supports. Rather, they participate as co-catalysts in the reaction mechanism (in bifunctional catalysts, for example).11,12... 

Hydrogen fluoride also is used as a catalyst in alkylation of aromatic compounds and for dimerization of isobutene. Other catalytic applications are in isomerization, polymerization, and dehydration reactions. Other uses are in... 

As deduced from Tables 4.1 and 4.2, the studies specifically dealing with the other pure higher rare earth oxide supports (praseodymia and terbia) are much scarce. However, a number of recent investigations have shown that the incorporation of praseodymium (69,100,274-278) and terbium (115,187,279-281) ions into the ccria lattice may improve its redox behaviour very significantly, thus becoming materials with potential interest in TWC technology and several other catalytic applications. 

The number of zeolites used has been continuously expanding. More than eight types are used today in oil refining processes, and the number will be larger if other catalytic applications in the field of chemicals and fine chemicals arc considered. 

The importance of bridged, binuclear complexes for decarbonylation reactions (see Chapter 11), the water-gas shift reaction (see Chapter 5), and other catalytic applications is only now beginning to be investigated. 

Other Catalytic Applications ofMetallosilicates. Recent publications describe the synthesis (25-27,29-30,32,33,35) catatytic appHcation of a variety of metaUosHicates. 

The extraordinary photocatalytic performance of AEROXIDE TiOj P 25 in comparison to other nanoscaled titania particles has been published in several papers It is, for example, useful in the degradation of humic acid [71], of phenol and salicylic acid [72], of l,4dichlorobenzene [73], and in the photocatalytic reduction of Hg(II) [74]. It is also used in the oxidation of primary alcohols to aldehydes [75] or in the photopolymerization of methyl methacrylate [76]. Its use in cement can help reduce environmental pollution [77, 78]. A detailed study is reported by Bolte [79]. The results show that crystal size and filling ratio in mass are more important than the modification of the titania. Pyrogenic titania is not only useful in photocatalysis but also in other catalytic applications. 

The DFT mechanism shows that the quartet and doublet transition state species are close in energy, which is understandable since both S = 3/2 and 1/2 ground states are known for RU2 paddlewheels. Considering the Ru-Ru-N core, the spin density at the N atom is minimal, despite its participatory role in C-H bond cleavage and N-H/N-C bond formations. Instead, both Ru centers harbor most of the spin density during the catalytic cycle. The spin flexibility afforded by the two Ru centers joined by a metal-metal bond may potentially be exploited in other catalytic applications. 

Achievements and scientific impact acquired from three-way catalysis research have become methodologies, prodders, and benchmark for other catalytic applications. 

Catalysis For technical reasons it can be necessary to have a catalytic active sohd material supported to improve, e.g., its mechanical stability and reduce its flow resistance when used in a flow reactor. The sol-gel fluorination synthesis provides a convenient way for depositing high surface area metal fluorides on supports. For example, HS-AIF3, which as fine powder makes problems when used as catalyst in flow systems, could be supported by Y-AI2O3 whereby its Lewis acidity and consequently its catalytic activity remains almost unchanged [64]. For other catalytic applications, like micro-reactor techniques, deposition of catalytically active thin layers of metal fluorides is also of interest. 

Other catalytic applications of HBF4 include alkene isomerizations, alkylation of alcohols with diazoalkanes, preparations of substituted pyridines, hydrolysis of a-hydroxyketene or a-(methylthio)ketene thioacetals to a,p-unsaturated thioesters, and terpene formation from isoprenic precursors. ... 

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