Dual activation catalysts

The optimal catalyst for this new family of dual activation reactions was an open problem. For these reactions, the ideal ratio of gold acetylide to LAu is 1 1. On the typical small scale of methodology development or mechanistic studies, this was difficult to adjust by mixing different precatalysts or a precatalyst and a 

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Scheme 9.14 Synthesis of dual-activation catalysts (DACs) based on IPr according to Hashmi et al. X = BF4, PFe, SbF. OTs, N(Tf)2 [32],...

Scheme 19.10 Asymmetric cyanosilylation of ketones catalysed by a dual-activation catalyst of Al(salen) and JV-oxide.

In many cases there is an interaction between the carrier and the active component of the catalyst so that the character of the active surface will change. For example, the electronic character of the supported catalyst may be influenced by the transfer of electrons across the catalyst-carrier interface. In some cases the carrier itself has a catalytic activity for the primary reaction, an intermediate reaction, or a subsequent reaction, and a dual-function catalyst is thereby obtained. Materials of this type are widely employed in reforming processes. There are other cases where the interaction of the catalyst and support are much more subtle and difficult to label. For example, the crystal size and structure of supported metal catalysts as well as the manner in which the metal is dispersed can be influenced by the nature of the support material. 

Based on Chien s research results, Collins et al. modified the basic structure of the catalysts and also achieved elastic material [8,18,19]. In both cases the elastic properties of the polymers are justified in a block structure with isotactic and atactic sequences. In 1999 Rieger et al. presented a couple of asymmetric, highly active metallocene catalysts, e.g., the dual-side catalyst rac-[l-(9-r 5-fluorenyl)-2-(5,6-cyclo-penta-2-methyl-l-q5-indenyl)ethane]zirconium dichloride (Fig. 3). These catalysts allowed building of isolated stereoerrors in the polymer chain to control the tacticity and therefore the material properties of the polymers [9],... 

Higher heat fluxes require a modified ring shape to sustain the reforming reaction conversion. A dual charge of catalyst may also be used. The tube s top half has a high-activity catalyst to prevent carbon formation in the maximum flux zone. The bottom half may be a more conven-... 

Dual function catalysts contain metallic and acidic active sites. No distinction will be made here between mono- and multimetallic catalysts—all the more since our knowledge on the latter type of catalysts is far from perfect. 

Sinfelt et al. (120) observed a twofold increase in the -heptane aromatiza-tion rate when the platinum content of their alumina-supported catalyst increased from 0.10 to 0.60%. At the same time, the rate of methylcyclo-pentane ring expansion remained constant. This result also serves as evidence for metal-catalyzed aromatization over dual-function catalysts without the participation of any Cg cyclic intermediate. The cyclization activity of platinum itself was independent of the nature of the support (109). Pure acidic cyclization prevailed with olefin feed (30, 109). 

Newer types of the dinuclear vanadium(IV) complex catalysts 84 have been developed. The abovementioned dinuclear vanadium complexes possess a VO V linkage whereas the ESR study on the catalyst 84 revealed no V—O—V linkage. The sense of enantioselection by the catalyst 84 of the (R,5,5)-structure is opposite to that of the binuclear complex 78a of the same (R,5,5)-structure. These results suggested two active sites attached to the binaphthyl skeleton in the catalyst 84 performed the dual activation of 2-naphthols in the oxidative couphng to achieve high enantioselectivity ... 

We reported a catalytic enantioselective cyanosUylation of ketones that produces chiral tetrasubstituted carbons from a wide range of substrate ketones [Eq. (13.31)]. The catalyst is a titanium complex of a D-glucose-derived ligand 47. It was proposed that the reaction proceeds through a dual activation of substrate ketone by the titanium and TMSCN by the phosphine oxide (51), thus producing (l )-ketone cyanohydrins ... 

Corey reported a catalytic enantioselective cyanosilylation of methyl ketones using combination of a chiral oxazaborolidinium and an achiral phosphine oxide, [Eq. (13.23)]. An intermolecular dual activation of a substrate by boron and TMSCN by the achiral phosphine oxide (MePh2PO) is proposed as a transition-state model (54). The same catalyst was also used for cyanosilylation of aldehydes ... 

Gas Oil Cracking by the Dual Zeolite Catalyst. After the steam pre-treatment, the gas oil cracking activity of REY was about 100 times greater than the activity of the pure matrix catalyst. 

Dual-function catalysts possessing both metallic and acidic sites bring about more complex transformations. Carbocationic cyclization and isomerization as well as reactions characteristic of metals occurring in parallel or in subsequent steps offer new reaction pathways. Alternative reactions may result in the formation of the same products in various multistep pathways. Mechanical mixtures of acidic supports (silica-alumina) and platinum gave results similar to those of platinum supported on acidic alumina.214,215 This indicates that proximity of the active sites is not a requirement for bifunctional catalysis, that is, that the two different functions seem to operate independently. 

The application of zeolitic materials cls catalysts in paraffin isomerization is discussed. Particular attention is given to catalyst preparation variables such as sodium removal for zeolite Y and mordenite. Dual function catalysts based on these zeolites are compared with respect to activity. A reaction mechanism for paraffin isomerization over zeolitic dual function catalysts, on the basis of literature and own data, is presented. 

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