Catalysts, general types

Acid Halides (Lewis Acids). AH metal haUde-type Lewis catalysts, generally known as Friedel-Crafts catalysts, have an electron-deficient central metal atom capable of electron acceptance from the basic reagents. The most frequendy used are aluminum chloride and bromide, followed by... 

The catalysts generally used in catalytic reforming are dual functional to provide two types of catalytic sites, hydrogenation-dehydrogenation sites and acid sites. The former sites are provided by platinum, which is the best known hydrogenation-dehydrogenation catalyst and the latter (acid sites) promote carbonium ion formation and are provided by an alumina carrier. The two types of sites are necessary for aromatization and isomerization reactions. 

Recently Teles found that cationic gold(l) complexes of the general type [L-Au ] (where L is a phosphine, a phosphite or an arsine) were excellent catalysts for the ad-... 

The above example outlines a general problem in immobilized molecular catalysts - multiple types of sites are often produced. To this end, we are developing techniques to prepare well-defined immobilized organometallic catalysts on silica supports with isolated catalytic sites (7). Our new strategy is demonstrated by creation of isolated titanium complexes on a mesoporous silica support. These new materials are characterized in detail and their catalytic properties in test reactions (polymerization of ethylene) indicate improved catalytic performance over supported catalysts prepared via conventional means (8). The generality of this catalyst design approach is discussed and additional immobilized metal complex catalysts are considered. 

Complexation of an amino acid derivative with a transition metal to provide a cyanation catalyst has been the subject of investigation for some years. It has been shown that the complex formed on reaction of titanium(IV) ethoxide with the imine (40) produces a catalyst which adds the elements of HCN to a variety of aldehydes to furnish the ( R)-cyanohydrins with high enantioselectivity[117]. Other imines of this general type provide the enantiomeric cyanohydrins from the same range of substrates11171. 

Gabriele Centi and Siglinda Perathoner (University of Messina, Italy) look at how quasi-one-dimensional catalyst morphologies can be synthesized. Though they focus on titania as an example, they also show how this technique can be used for more general types of supports and catalysts. 

In 2006, Yamamoto and Nakashima picked np on this and designed a chiral A -triflyl phosphoramide as a stronger Brpnsted acid catalyst than the phosphoric acids based on this concept. In their seminal report, they disclosed the preparation of new chiral BINOL-derived A -triflyl phosphoramides and their application to the asymmetric Diels-Alder (DA) reaction of a,p-unsaturated ketones with sily-loxydienes [83], As depicted in Scheme 59, chiral A-triflyl phosphoramides of the general type 4 are readily synthesized from the corresponding optically active 3,3 -substituted BINOL derivatives 142 through a phosphorylation/amidation route. 

C. Several important physiologic catalysts are made of RNA, and these RNA-based enzymes or ribozymes are of two general types. 

Rh complexes are examples of the most effective catalysts for the polymerization of monosubstituted acetylenes, whose mechanism is proposed as insertion type. Since Rh catalysts and their active species for polymerization have tolerance toward polar functional groups, they can widely be applied to the polymerization of both non-polar and polar monomers such as phenylacetylenes, propiolic acid esters, A-propargyl amides, and other acetylenic compounds involving amino, hydroxy, azo, radical groups (see Table 3). It should be noted that, in the case of phenylacetylene as monomer, Rh catalysts generally achieve quantitative yield of the polymer and almost perfect stereoregularity of the polymer main chain (m-transoidal). Some of Rh catalysts can achieve living polymerization of certain acetylenic monomers. The only one defect of Rh catalysts is that they are usually inapplicable to the polymerization of disubstituted acetylenes. Only one exception has been reported which is described below. 

CdO is used in connection with the stabilization of poly(vinyl chloride). This is discussed below in more detail. It also finds application in modifying the thermal properties of teflon and some rubbers. CdS is used in some smoke detectors, in lasers and in phosphors. The cadmium(II) halides are important as catalysts and are also used in pyrotechnics. Cadmium borates of the general type (Cd0)x(B203), are also used as phosphors. CdS04 is employed in the Weston cell, which is important as a voltage standard.137... 

Molybdena catalysts generally need to be activated by reduction or sulfidation in order to obtain an active catalyst for most reactions in which they are employed (except for oxidation-type reactions). Therefore, it is important to determine what changes occur in the state of the oxidized catalyst when it is subjected to these activation pretreatments. 

Diels-Alder reactions.6 The Diels-Alder reaction of cyclopentadiene with dienophiles of the general type CH2=CHR results in endo/exo adducts in the ratio of co. 3 1, and this rcgioselecti vity is not affected by catalysts. The ratio is affected when the reaction is conducted heterogeneously with the dienophile adsorbed on nonactivated surfaces such as A1203, Si02, and cellulose. The most striking effect is shown by methyl acrylate adsorbed on alumina (equation I). 

Complex 49 catalyzes, among other reactions, the addition of morpholine to methylacrylonitrile giving the amination product with modest selectivity (37% ee) (Scheme 35). In order to obtain catalytically active species with palladium, complex 48 was converted into dicationic derivatives of the general type [Pd(NCCH3)(PCP)](PF6)2. Using this catalyst, the addition of morpholine to methylacrylonitrile could be achieved with 47% ee. Further improvement in selectivity was obtained by the introduction of methyl substituents at the 3- and 5-positions of the phenyl groups in diphenylphosphanyl derivative leading to ee s of over 70% [94]. 

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