Use in catalytic systems

Lead tetraalkyl derivatives are used in catalytic systems to polymerise olefines, as catalysts of re-etherification and polycondensation, to speed up the alkylation of lateral chains of alkylbenzenes with ethylene and its derivatives. An addition of lead tetraalkyl derivatives (0.05-2% of alkylben-zene quantity) to catalysts of the liquid-phase oxidation of alkylbenzenes speeds up the oxidation. Tetraethyllead proved to be a good initiator for Diels-Alder reactions to join polymers with alkenylsiloxy chains and can be used as an additive to reduce the attrition and wear of rubbing metal parts. Tetrabutyllead is an active cross-linking agent for polyethylene and modifying agent for plastics. 

Ceramic membranes can be used in catalytic systems in three modes ... 

Cerium is also used in catalytic systems. A catalyst is a substance used to speed up a chemical reaction. The catalyst does not undergo any change itself during the reaction. Compounds of cerium are used in the refining of petroleum. They help break down compounds found in petroleum into simpler forms that work better as fuels. 

It was claimed that la can be epoxidized with cumene hydroperoxide in the presence of catalytic amounts of [VO(acac)2] to give 98% l,2-epoxy-3-methyl-3-butanol, 3a (at 70% conversion). This epoxide could be separated in 99% purity from the reaction mixture by distillation [22]. Due to the chelating effect of the acetylacetonate ligand, it may be better to use oxovanadium alkoxide complexes. The V 0(0R) moiety has been known for many years [23-25], but only recently has its chemistry attracted significant attention. For example, it was found useful in catalytic systems for the oxidative cleavage of C-C bonds of ketones [26] and... 

Chiral pyridine addition to cobalt catalysts derived from dicobalt octacarbonyl has a positive influence on cherno- and regioselectivity, but not on stereoselectivity. Thus, no asymmetric induction is observed with Co ( + )-(.S )-3-.vtx-butylpyridine in the hydroformylation of styrene23. Again no induction is observed with chiral cobalt clustersl08. Cobalt is also used in catalytic systems of the type Co(An)n(alkene)m(CO)pLq [An = coordinating or noncoordinat-ing anions, e.g., BPhJ with various chiral ligands [L = ZR4R5R6 (Z = As, N, P, Sb)]166. Low catalytic activities and inductions are observed with supported catalysts of cobalt on silica gel or alumina [in situ preparation from salt, modified with phosphanes, e.g., (+)- or (—)-Diop]89. 

Fig. 1 Glycosylated metalloporphyrins which have been used in catalytic systems...

The discovery that it is possible to ortholithiate dibromoferrocene with LDA has allowed the synthesis of a range of new ligands/ In very elegant synthetic work, the asymmetric synthesis of (6, 6 -(+)-l>l -bis(methylphenylphos-phinoferrocene) shown as 35 has been achieved. The same synthetic methodology allowed the synthesis of the ligands of the type shown in 36 which have been structurally characterized and their use in catalytic systems examined. ... 

A wide variety of solid materials are used in catalytic processes. Generally, the (surface) structure of metal and supported metal catalysts is relatively simple. For that reason, we will first focus on metal catalysts. Supported metal catalysts are produced in many forms. Often, their preparation involves impregnation or ion exchange, followed by calcination and reduction. Depending on the conditions quite different catalyst systems are produced. When crystalline sizes are not very small, typically > 5 nm, the metal crystals behave like bulk crystals with similar crystal faces. However, in catalysis smaller particles are often used. They are referred to as crystallites , aggregates , or clusters . When the dimensions are not known we will refer to them as particles . In principle, the structure of oxidic catalysts is more complex than that of metal catalysts. The surface often contains different types of active sites a combination of acid and basic sites on one catalyst is quite common. 

Several catalytic systems have been reported for the enantioselective Simmons Smith cyclopropanation reaction and, among these, only a few could be used in catalytic amounts. Chiral bis(sulfonamides) derived from cyclo-hexanediamine have been successfully employed as promoters of the enantioselective Simmons-Smith cyclopropanation of a series of allylic alcohols. Excellent results in terms of both yield and stereoselectivity were obtained even with disubstituted allylic alcohols, as shown in Scheme 6.20. Moreover, this methodology could be applied to the cyclopropanation of stannyl and silyl-substituted allylic alcohols, providing an entry to the enantioselective route to stannyl- and silyl-substituted cyclopropanes of potential synthetic intermediates. On the other hand, it must be noted that the presence of a methyl substituent at the 2-position of the allylic alcohol was not well tolerated and led to slow reactions and poor enantioselectivities (ee<50% ee). ... 

Trimethyl-l,3-dioxane (146) is the major product formed when the palladium complex of 3-methy 1-1 -butene is treated with formaldehyde. The dioxane products (146, 147) can also be obtained directly in improved yield from the olefin using the catalytic system PdCl2-CuCI2 (Scheme 169).240... 

Molecular catalysis. The term molecular catalysis is used for catalytic systems where identical molecular species are the catalytic entity, like the molybdenum complex in Figure 8.1, and also large molecules such as enzymes. Many molecular catalysts are used as homogeneous catalysts (see (5) below), but can also be used in multiphase (heterogeneous) systems, such as those involving attachment of molecular entities to polymers. 

The idea of enantioselective activation was first reported by Mikami and Matsukawa111 for carbonyl-ene reactions. Using an additional catalytic amount of (R)-BINOL or (/ )-5.5 -dichloro-4,4, 6,fi -tctramcthyl biphenyl as the chiral activator, (R)-ene products were obtained in high ee when a catalyst system consisting of rac-BINOL and Ti(OPri)4 was employed for the enantioselective carbonyl ene reaction of glyoxylate (Scheme 8-54). Amazingly, racemic BINOL can also be used in this system as an activator for the (R)-BINOL-Ti catalyst, affording an enhanced level of enantioselectivity (96% ee). 

Using the catalytic system described above, the enantioselective opening of meso epoxides could also be pursued. Although many excellent examples of ring-opening of meso epoxides by Sn2 reactions have recently been reported, the reaction planned here is conceptually different [40]. In the SN2 reaction, the path of the incoming nucleophile has to be controlled. In the titanocene-catalyzed reaction, the intermediate radical has to be formed selectively. If an intermediate similar to that invoked in the Bartmann ring-open-... 

Selective oxidation of allylic alcohols.1 This zircononcene complex when used in catalytic amount can effect an Oppenauer-type oxidation of alcohols, including allylic ones, in the presence of a hydrogen acceptor, usually benzaldehyde or cyclohexanone. This system oxidizes primary alcohols selectively in the presence of secondary ones. Thus primary allylic alcohols are oxidized to the enals with retention of the configuration of the double bond in 75-95% yield. The method is not useful for oxidation of propargylic alcohols. 

Recent advances have led to the development of microcalorimeters sensitive enough for low-surface-area ( 1 cm2) solids [71]. This instrumentation has already been used in model systems to determine the energetics of bonding of catalytic particles to the support, and also in adsorption and reaction processes [72,73],... 

Engler and colleagues256 demonstrated that the way in which catalyst 406 is prepared has a strong effect on the regioselectivity and enantioselectivity of quinone Diels-Alder reactions. The most effective catalyst was prepared from a 1 1 1 mixture of titanium tetrachloride, titanium tetraisopropoxide and chiral diol 416. The cycloadditions of 2-methoxy-l,4-benzoquinones such as 414 with simple dienes to give adducts like 415 proceeded with high yields and enantioselectivities of up to 80% ee using this catalytic system (equation 123). 

Having used their catalytic systems with dienolates derived from unsaturated esters, Denmark performed aldol reactions with the dioxanone-derived dienol ether described above in the context of Carreira s and Campagne s vinylogous aldol reactions (Scheme 21). Here, exclusively, the y product was formed with the nucleophile attacking from the Re face. For all three aldehydes, very good yields (83-92%) and selectivities (74-89% ee) were observed with only 0.01-0.05 mol% of the catalyst. 

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