Catalysis multifunctional

Metallocenes Mg(II)-catalyzed Microwave irradiation MO calculation Montmorillonite Moraceous plants Morphinadienes Mulberry tree Multifunctional catalysis Multi-step... 

The source of the enormous rate enhancements in enzymatic catalysis has been discussed from physical organic points of view (Jencks, 1969 Bruice, 1970). The kinetic behavior is attributed to factors such as an orientation effect, a microenvironmental effect and multifunctional catalysis. The active sites of enzymes are generally located in a hydrophobic hole or cleft. Therefore, the microenvironmental effect is mainly concerned with the behavior of enzyme catalytic groups in this hydrophobic microenvironment and the specific... 

Dehydration into ethylene (over acid catalysts) [71-73] followed by oligomerization (via coordination or multifunctional catalysis) [74-77]. The main problem is the need to remove water from the feed and the vaporization of ethanol, making the process costly. 

One aspect of asymmetric catalysis has become clear. Every part of the molecule seems to fulfill a role in the process, just as in enzymic catalysis. Whereas many of us have been used to simple acid or base catalysis, in which protonation or proton abstraction is the key step, bifunctional or even multifunctional catalysis is the rule in the processes discussed in this chapter.Thus it is not only the increase in nucleophilicity of the nucleophile by the quinine base (see Figures 6 and 19), nor only the increase in the electrophilicity of the electrophile caused by hydrogen bonding to the secondary alcohol function of the quinine, but also the many steric (i.e., van der Waals) interactions between the quinoline and quinuclidine portions of the molecule that exert the overall powerful guidance needed to effect high stereoselection. Important charge-transfer interactions between the quinoline portion of the molecule and aromatic substrates cannot be excluded. 

Many of the basic elements of enzyme catalysis have been illustrated here, including binding of substrate, multifunctional catalysis, microenvironmental effects, covalent catalysis, and strain effects. The most remarkable rate enhancements reported to date are those brought about by apolar derivatives of PEI, a polycation. These rate enhancements are very en-... 

Intramolecular Catalysis Multifunctional Catalysis Conformation and Strain Effects... 

Koshland (1962) has calculated, however, that such a propinquity effect will not explain the large rate enhancements observed with enzymes unless there are more than two functional groups involved with utilization of five functional groups (2 substrates and 3 catalytic groups) a rate increase of 10 would be possible. Such multifunctional catalysis would, of course, be impossible to demonstrate... 

Another fundamental idea that has been invoked to explain enzymatic catalysis is that such reactions utilize bifunctional or multifunctional catalysis that is, several functional groups in the active site are properly aligned with the substrate so that concerted catalysis may occur. Mutarotation of tetramethyl glucose is frequently cited as an example of bifunctional catalysis. Lowry and... 

It is clear that bifunctional catalysis does not necessarily represent a favourable process in aqueous solution even when a second functional group is held sterically in proper position to participate in the reaction. Caution should then be used in assuming that most enzymes are utilizing bifunctional or multifunctional catalysis. Nevertheless, this idea has played a leading role in concepts of enzymatic catalysis. 

Calixcrown 5, featuring two diethylaminomethyl side-arms at the polyether bridge, testifies an attempt at a higher order multifunctional catalysis of ester cleavage, namely, from nucleophilic-electrophilic to nucleophilic-electrophilic-general acid catalysis [20]. 

The lesson to be learned by the above experiments is that designed multifunctional catalysts may lead to unsuccessful results, and awareness of imperfections and drawbacks in the design comes only a posteriori. Progress towards a higher order multifunctional catalysis requires a most careful design of intra- and intermolecular interactions for optimal positioning of catalytic units in a molecular framework. 

If the epoxide rearrangement (see chapter 15.2.1) of styrene oxide is carried out in the presence of hydrogen and by use of a bifunctional boron-pentasil zeolite catalyst having a hydrogenation component such as Cu, then 2-phenylethanol is obtained in one step. This hydro-isomerization renders high yields (> 85%) at 250 °C under the gas phase conditions. It is an example for multifunctional catalysis in a one pot-reaction, that means simultaneous rearrangement and hydrogenation. 

C-C Bond Formation Through Multifunctional Catalysis By Mixed Metal Oxides... 

Easy chemical and thermal manipulation of the active sites in a catalyst is desirable to carry out nearly uniform changes throughout the whole catalyst during catalyst synthesis and activation steps. This attribute is particularly important in the formulation of bifiinctional and multifunctional catalysts because ineffective interaction between relevant active sites is critical in multifunctional catalysis. The growth of multifunctional catalysis adds further emphasis to the need for uniform catalyst sites both in chemistry and in difihisivity. 

Bimetallic catalysts and multifunctional catalysis, whereby activation of substrate and reactant occurs simultaneously at Lewis acid and Bronstedt base sites, is an approach for giving more organization to the transition state and a higher stereocontrol [125]. 

Scheme 24. Heterobimetallic multifunctional catalysis of a Michael reaction by Na3[La(S)-BIN0L]3(thf)6(H20)...

So far, most approaches to enzyme-like activity have used just one of the functional groups which are present in enzymes. However, many enzymes only operate by a cooperation of functional groups (see for instance the catalytic triade in peptidases). There, the enzyme s functional groups perform a multifunctional catalysis. Therefore in (organo) catalysis, bifunctional catalysis has been developed, too. In the field of concave reagents, first bifunctional catalysts have been constructed (Figure 7.28), and future will tell how capable they are to catalyse reactions with their acidic and basic functionalities. 

The activity and selectivity of the HTC-supported Pd and Pt catalysts for the condensation of acetone, expressed in terms of reactant (acetone) conversion and selectivity to DAA, MO, MIBK, IPA, as well as other by-products is shown in Table III. The homogeneous reaction (i.e., no catalyst present) was also examined for comparison purposes. The results indicate that there was minimal acetone conversion in the absence of catalyst, with the direct hydrogenation of acetone to IPA being the only reaction. This result was not unexpected since, as indicated above, the MIBK formation requires multifunctional catalysis acidity/basicity for condensation, acidity for dehydration, and metal sites for selective hydrogenation (7,8). 

Combination of several individual reactions into one synthesis step, ie. shortening the synthesis route by using multifunctional catalysis with zeolites. 

Multifunctional catalysis, in which reactions consisting of several reaction steps are carried out by a shorter synthesis route, is becoming increasingly important in organic synthesis. Molecular sieve catalysts, too, help to combine several catalytic steps and tailor them optimally to one another [15, 18, 24], In this respect, molecular sieves like zeolites can be used as carriers for catalytically active components such as transition metals, noble metals. In addition the catalytic behaviour of these components the intrinsic acidic or basic or redox properties of the zeolites combined with shape selective feature are still present. 

Another type of bond that is ubiquitous in nature is phosphodiester bond making up the backbone of DNA or RNA. Enzymes can use more than one amino acid side chain in their active side for simultaneous bifimctional or multifunctional catalysis. Often an add-base catalyst is formed as in the enzyme ribonuclease A. The natural enzyme consists of 124 amino acids and catalyzes the hydrolysis of RNA phosphodiester bonds between the phosphorous atom and the 5 -oxygen atom. The mechanism of ester cleavage proceeds via a 2, 3 cyclo-phosphate intermediate. The histidine 119 and 12 function as acid and base to catalyze the formation of the cyclic intermediate while the lysine stabilizes the pentacoordinated transition state. The hydrolysis of the cyclic intermediate is then again catalyzed by both histidine residues (Figure 26). ... 

Pan Y, Yuan B, Li Y, He D. Multifunctional catalysis by Pd MIL-101 one-step synthesis of methyl isobutyl ketone over palladium nanoparticles deposited on a metal-organic framework. Chem Commun 2010 46 2280-2. 

Another classical example (72) of multifunctional catalysis is the hydrolysis of the monosuccinate ester of hexachlorophene ... 

In summary, for an enzyme model to be operative, a certain number of criteria, characteristic of enzyme catalysis, must be fulfilled, among which is substrate specificity—that is, selective differential binding. The enzymelike catalyst must also obey Michaelis-Menten kinetics (saturation behavior), lead to a rate enhancement, and show bi- and/or multifunctional catalysis (348). 

Altogether, the several features of calixarenes, notably their ability to form host-guest complexes and the ease of introducing a large variety of active units by means of selective derivatization, make this class of compounds most useful for the purpose of multifunctional catalysis, and excellent candidates as platforms for the design of artificial catalysts. 

---