Availability, catalysts

The mechanism by which Lewis-acids can be expected to affect the rate of the Diels-Alder reaction in water is depicted in Scheme 2.6. The first step in the cycle comprises rapid and reversible coordination of the Lewis-acid to the dienophile, leading to a complex in which the dienophile is activated for reaction with the diene. After the irreversible Diels-Alder reaction, the product has to dissociate from the Lewis-acid in order to make the catalyst available for another cycle. The overall... 

Hydrogenation. Hydrogenation is one of the oldest and most widely used appHcations for supported catalysts, and much has been written in this field (55—57). Metals useflil in hydrogenation include cobalt, copper, nickel, palladium, platinum, rhenium, rhodium, mthenium, and silver, and there are numerous catalysts available for various specific appHcations. Most hydrogenation catalysts rely on extremely fine dispersions of the active metal on activated carbon, alumina, siHca-alumina, 2eoHtes, kieselguhr, or inert salts, such as barium sulfate. 

For a chiral molybdenum-based catalyst available in situ from commercial components, see (a) Aeilts SL, Cefalo DR, Bonitatebus PJ, Houser JH, Hoveyda AH, Schrock RR (2001) Angew Chem Int Ed 40 1452 (b) For the first enantiomerically pure solid-sup-ported Mo catalyst, see Hultzsch KC, Jernelius JA, Hoveyda AH, Schrock RR (2002) Angew Chem Int Ed 41 589 (c) For a chiral Mo catalyst, allowing RCM to small- and medium-ring cyclic amines, see Dolman SJ, Sattely ES, Hoveyda AH, Schrock RR (2002) J Am Chem Soc 124 6991 (d) For a novel adamantyl imido-molybdenum complex with advanced selectivity profiles, see Tsang WCP, Jernelius JA, Cortez GA, Weatherhead GS, Schrock RR, Hoveyda AH (2003) J Am Chem Soc 125 2591... 

Six catalysts available from Evonik Degnssa were tested initially (Catalysts A-F in Table 10.1). 

Initial studies of the polymerization of propylene with transition metal allyl compounds suggested that this monomer could not be polymerized by any of the soluble catalysts available. Subsequent work (16) has revealed, however, that the propylene polymerization is much more susceptible to impurities, in particular traces of ether which compete with the monomer for the coordination sites. When this and other impurities are removed, weak activity is detected. These results are summarized in Table XIII. 

It is clear that the high-throughput approach is only possible if there are sufficient catalysts available. Fortunately, in hydrogenation it is usually possible to form the pre-catalyst simply by mixing a metal precursor and the ligand in a... 

The main components of FCC catalysts are Zeolite Y, e.g., REY orUSY as the major active component (10 to 50%), and a binder that is typically an amorphous alumina, silica-alumina, or clay material. In addition to these main components, other zeolite components, e.g., ZSM-5, and other oxide or salt components are quite frequently used additives in the various FCC catalysts available on the market. The addition of 1 to 5% ZSM-5 increases the octane number of the gasoline. ZSM-5 eliminates feed compounds with low octane numbers because it preferentially center-cracks n-paraffins producing butene and propene [14], These short-chain olefins are then used as alkylation feedstocks... 

Whereas we might in principle have a catalyst available today for every synthetic conversion, it will probably take another 10-20 years before enjoying the full potential of cascades of catalysis on an industrial scale (Fig. 13.18). 

There are many different homogeneous catalysts available that can be used for hydrogenation and hydroformylation reactions. The most widely used metals that are present in these catalysts are the transition metals, especially Co, Ni, Ru, Rh and Ir [1],... 

Several conclusions can be reached from these data. The first is that on a per mole basis, the quaternary ammonium salt is the most favorable catalyst for this reaction. Among the other compounds, the catalytic activity of 1.5 mole-% of crown, PEG, or PEG-MME are similar. If equal weights of PEG-400 and PEG-3400 are used, quite different reaction rates are observed. This is because each polymer chain is capable of transporting one cation across the phase boundary at a time. The ratio of molecular weights is 8.5, so there are 8.5 more catalysts available in the PEG-400 catalyzed reaction than in the one involving the higher molecular weight compound. The actual ratio of rates for these two processes is 12.5, or nearly the expected value. 

Vlhen the chiral methylation is carried out with 30% aqueous NaOH the indanone is deprotonated at the interface but does not precipitate as the sodium enolate (Figure 11). In this system there are 3 to 4 molecules of H2O per molecule of catalyst available while in the 50% NaOH reactions the toluene is very dry with only 1 molecule of H2O available per catalyst molecule thus forcing the formation of tight ion pairs. Solvation of the ion pairs in the toluene/30% NaOH system should decrease the ee which we indeed observe with an optimum 78% versus 94% in the 50% NaOH reaction. In the 30% NaOH reactions the ee decreases from 78% to 55% as the catalyst concentration increases from 1 mM to 16 mM (80 mM 5, 560 mM CH3CI, 20 C). Based on these ee s rates of formation of (-h)-enantiomer and racemic product can be calculated. When the log of these rates are plotted versus the log of catalyst concentrations (Figure 13) we find an order of about 0.5 in the catalyst for the chiral process similar to that found using 50% NaOH consistent with a dimer-monomer pre-equilibrium. The order in catalyst for the... 

Poly (acetylenes) [16], There are several catalysts available for polymerization of substituted acetylenes. Whereas Ziegler-Natta catalysts are quite effective for polymerization of acetylene itself and simple alkylacetylenes, they are not active towards other substituted acetylenes, e.g. phenylacetylenes. Olefin-metathesis catalysts (Masuda, 1985 Masuda and Higashimura, 1984, 1986) and Rh(i) catalysts (Furlani et al., 1986 Tabata, 1987) are often employed. In our experience, however, many persistent radicals and typical nitrogen-containing functional groups serve as good poisons for these catalysts. Therefore, radical centres have to be introduced after construction of the polymer skeletons. Fortunately, the polymers obtained with these catalysts are often soluble in one or other organic solvent. For example, methyl p-ethynylbenzoate can be polymerized to a brick-coloured amorph- See the Appendix on p. 245 of suffixes to structural formula numbers. 

In many instances, the formation of inactive dimers from active, monomeric catalytic species is observed during catalysis. When weak or unstable ligands are used, even larger rhodium carbonyl clusters like Rh4(CO)i2 and Rh5(CO)i5 can be observed [42-44]. The formation of dimers is often a reversible equilibrium (Scheme 6.2). This only leads to a reduction in the amount of catalyst available and does not kill the catalyst. One of the first examples was the formation of the so-called orange dimer from HRh(PPh3)3CO, already reported by Wilkinson [45]... 

We began with examination and evaluation of all catalysts available on the market. Detailed analysis of catalysts showing promising performance in our test units gave us insight as to which properties and ingredients should receive greater attention and which were contributory or noncontributory. 

The 3-aza-Claisen rearrangement concerning the rearrangement of aUyl vinyl amines, contrary to its Cope analogue, has not been used extensively to form carbon-carbon bonds, probably due to the high temperatures (>180°C) required for the uncatalysed variant and to the limited number of catalysts available for promoting the reaction at moderate temperatures (equation 4). 

Though there are many problems related to solid catalysts, we consider only those which are related to the development of kinetic rate equations needed in design. We simply assume that we have a catalyst available to promote a specific reaction. We wish to evaluate the kinetic behavior of reactants in the presence of this material and then use this information for design. 

A catalyst available from Engelhard Industries was used. The catalyst was dried in a vacuum oven at 115° for 48 hours. Caution Palladium-on-carbon is pyrophoric, and vacuum drying increases this hazard. Catalysts kept in the oven for longer periods of time were extremely pyrophoric. 

An area, in which catalytic olefin metathesis could have a significant impact on future natural product directed work, would be the desymmetrization of achiral molecules through asymmetric RCM (ARCM) with chiral molybde-num-(for a chiral molybdenum-based catalyst available in situ from commercial components, see Refs 156 and 156a-156c) or ruthenium-based catalysts. 

Effects of polymer structure on reaction of phenylacetonitrile with excess 1-bromo-butane and 50% NaOH have been studied under conditions of constant particle size and 500 rpm stirring to prevent mass transfer limitations I03). All experiments used benzyltrimethylammonium ion catalysts 2 and addition of phenylacetonitrile before addition of 1-bromobutane as described earlier. With 16-17% RS the rate constant with a 10 % CL polymer was 0.033 times that with a 2 % CL polymer. Comparisons of 2 % CL catalysts with different % RS and Amberlyst macroporous ion exchange resins are in Table 6. The catalysts with at least 40% RS were more active that with 16 % RS, opposite to the relative activities in most nucleophilic displacement reactions. If the macroporous ion exchange resins were available in small particle sizes, they might be the most active catalysts available for alkylation of phenylacetonitrile. 

This kinetics expression indicates the rate s dependency on catalyst availability through the [Enz]tot term imbedded in Vmax (see Illustrative Example 17.8 on page 765). 

There is a wide range of possible catalysts available for initiating the reaction. In addition to organic salts, oxidants such as hydrated or anhydrous FeClg063 and ceric ammonium nitrate1063,124 have proven effective. 

A direct comparison of catalysis of olefin epoxidation with a homogeneous chemical catalyst (Mn salen), an enzyme (CPO), and an antibody resulted in sufficiently high enantioselectivity for all three catalysts, a higher turnover number for the enzyme, and a slightly higher substrate/catalyst ratio for the homogenous catalyst. Criteria for comparison should be quantitative and include catalyst lifetime as well as volumetric productivities, but have been found to depend on the different needs of laboratory synthetic chemists, who need a broadly specific catalyst quickly, versus those of process chemists, who often control catalyst availability and can allow narrow specificity (provided their substrate is accepted) but need high productivity. 

Raw material suppliers have a large range of catalysts available, each with special properties. They should be consulted when required. Some of the catalysts are heat-activated and start very slowly and then react more quickly as the temperature increases. 

Here the reaction rate depends not only on the concentration of the S02 but also on the surface area of any catalyst available, such as airborne dust particles. The efficiency of a catalyst depends upon its specific surface area, Asp, defined as the ratio of surface area to mass [30], Accordingly, this property is frequently used as a basis for comparing different kinds of catalysts, or catalyst supports, and for diagnosing practical problems in catalysts being used in a process (since both agglomeration and poisoning reduce Asp). The specific surface area of the airborne dust particles, considering n spheres of density p and radius R, would be ... 

The specific surface area depends on both the size and shape and is distinctively high for colloidal-sized species. This is important in the catalytic processes used in many industries for which the rates of reactions occurring at the catalyst surface depend not only on the concentrations of the feed stream reactants, but also on the surface area of catalyst available. Since practical catalysts are frequently supported catalysts, some of the surface area is more important than the rest. Also, given that the supporting phase is usually porous, the size and shapes of the pores may influence the reaction rates as well. The final rate expressions for a catalytic process may contain all of these factors surface area, porosity, and permeability. 

Ru-MeO-Biphep (Ru-96a) was used by Roche to reduce 98 to the corresponding P-hydroxy ester with >98% ee at 240-kg scale. Turnover numbers of 50,000 were achieved in this reduction. A process was developed by PPG-Sipsy to reduce 99 with Ru-MeO-Biphep for Pfizer in an approach to candoxatril. The olefin was reduced in >99% ee at 230-kg scale (S/C = 1000-2000).118 Although catalysts that contain DuPhos had been determined to be more effective based on overall yields and isomerization to an enol by-product, the Ru-MeO-Biphep catalyst was preferred as a result of catalyst availability at scale and more favorable licensing agreements.119... 

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