Putative active catalyst

Most of the first-generation Mn-salen complexes such as 16 show poor enantioselectivity (<25% ee) [83]. The Mn-salen complex37, which has a small substituent at theethylenediamine moiety shows moderate enantioselectivity, but chemical yield is poor (Scheme 6B.37). Differing from the oxo-Mn species, the imino-Mn species that is the putative active catalyst species in aziridination has a substituent on its nitrogen atom. It is reasonable to assume that the size of the substituents at the ethylenediamine moiety directs the orientation of the substituent at the nitrogen atom, which influences enantioselectivity,... 

To put these problems into perspective, as already discussed, the PASCs remaining at the 0.20% S level are lower in reactivity, by a factor of 10-50, than the sulfur compounds that are now removed in lowering the sulfur level from 1.2 to 0.20%, Even with catalysts 2 times as active, the reactor volume may have to be doubled to convert the required 75% of the least-reactive PASCs to achieve the 0.05% S target. Based on the composition of a typical gas oil and using the first-order rate constants for the different classes of sulfur compounds (14), a theoretical HDS severity plot is presented in Fig. 9. It can be seen that reactor volumes will have to be increased by about a factor of 4 to meet the new specifications unless much more active catalysts can be developed than are presently available. 

Transition metal carbonyls and their derivatives are remarkably effective and varied in their ability to catalyze reactions between unsaturated molecules (e.g., CO and olefinic compounds) or between certain saturated and unsaturated molecules (e.g., olefins and H2 or H20). The carbonyl derivatives of cobalt are particularly active catalysts for such reactions and have been put to use in the industrial synthesis of higher aliphatic alcohols. In fact, much of the growth in knowledge concerning catalysis by metal carbonyls has been stimulated by the industrial importance of the Fischer-Tropsch synthesis, and by the economically less important, but chemically more tractable, hydroformylation reaction. 

While the molecular tantalum catalyst Ta(OCH2CH3)5 exhibited very poor activity for epoxidation under Sharpless conditions, the surface-supported analogue [a mixture of 70% =SiOTa(OCH2CH3)4 and 30% (=SiO)2Ta(OCH2CH3)3] was shown to have activity comparable with that of the molecular Ti catalyst. Furthermore, excellent enantiomeric ee values (up to 94%, compared with 96 % for Ti[OCH(CH3)2]4 under the same conditions) were obtained. An inversion of the the major enantiomer obtained was observed for both the molecular and supported tantalum catalysts, i. e., the association of tetraisopropyltitanium and (+)-diisopropyltartrate produces (/f)-epoxide whereas the Ti catalyst with (+)-diisopropyltartrate produces the (S)-epoxide. The putative active species, =SiOTa(OCH2CH3)2[(+)-(DET)] (Structure 18) has also been synthesized and tested (eq. (3) [23 a]) Further improvements of catalyst activity have been obtained by modification of the support and refinement of the synthesis of the supported tantalum alkoxide precursor. 

Cu(I). Carreira and co-workers have documented a class of Cu-mediated dieno-late aldol addition reactions that are postulated to proceed through an intermediate metalloenolate (Eq. (8.26)) [40]. The active catalyst is generated upon dissolution of p-tolbinap and Cu(OTf)2 in THE followed by addition of Bu4NPh3Sip2 as an anhydrous fluoride source. The putative Cu-fluoride complex initiates the formation of a Cu-dienolate that subsequently participates in a catalytic, enantioselec-tive addition reaction. Using as little as 0.5 mol% catalyst, the protected acetoace-tate adducts are isolated in up to 94% ee [41]. The use of the corresponding p-tol-binap-Cu(OrBu) complex prepared in situ from Cu(OfBu) and binap functions as a competent catalyst. This feature is consistent with an intermediate metal alkox-ide in the catalytic cycle, namely, the first-formed metal aldolate adduct. The... 

At this time it had become possible to determine experimentally total surface area and the distribution of sizes and total volume of pores. Wheeler set forth to provide the theoretical development of calculating the role of this pore structure in determining catalyst performance. In a very slow reaction, reactants can diffuse to the center of the catalyst pellet before they react. On the other hand, in the case of a very active catalyst containing small pores, a reactant molecule will react (due to collision with pore walls) before it can diffuse very deeply into the pore structure. Such a fast reaction for which diffusion is slower than reaction will use only the outer pore mouths of a catalyst pellet. An important result of the theory is that when diffusion is slower than reaction, all the important kinetic quantities such as activity, selectivity, temperature coefficient and kinetic reaction order become dependent on the pore size and pellet size with which a pellet is prepared. This is because pore size and pellet size determine the degree to which diffusion affects reaction rates. Wheeler saw that unlike many aspects of heterogeneous catalysis, the effects of pore structure on catalyst behavior can be put on quite a rigorous basis, making predictions from theory relatively accurate and reliable. 

In all of the structural models, the amino acid residues apparently constituting the catalytic triad or involved in covalent catalysis were identified as being adjacent to the core structure with the putative active site nucleophile cysteine located at the elbow of the strand-elbow helix motif. In the class II polyester synthase, the highly conserved histidine residue which functions as a general base catalyst in a/p-hydrolases was functionally replaced by an adjacent histidine residue, which too was close to the core structure. 

SCHEME 6.3 Activation of FI Catalysts (a) with MAO and (b) with /-Bu3Al/[Ph3C]+[B(C6F5)4] and formation of putative active species for olefin polymerizations. to are, for example, as defined in Table 6.1. R is presumably the isobutyl group. 

The aim of putting forward the above activity pattern is to design and develop highly active catalysts. To obtain high activity is the criterion to prove this model. The validity of above pattern has been proved positively by the successful development and wide application in industry of A301 and ZA-5 type Fei xO based... 

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