Base-type Catalysis

Didier TiCHiTa, Sara Iborra, Avelino Corma6 and Daniel Brunel  

2 Probe molecules combined with spectroscopic methods. 174 

Catalysts for Fine Chemical Synthesis, Vol. 4, Microporous and Mesoporous Solid Catalysts Edited by E. Derouane 2006 John Wiley Sons, Ltd 

While much effort has been put into obtaining materials with basic sites of different strength, the exact nature of the basic sites acting on those catalysts is not always known. For instance, reactions like aldolization, hydrogen transfer or double bond isomerization require either Brpnsted or Lewis type sites or even the association of strong or medium basic sites with weak acid sites/13-161 Many of these requirements (acid-base associations, versatility of the basic strength) can be accomplished by hydrotalcites and this would explain their significant development as catalysts precursors/13-15,171 

It must also be pointed out that other basic materials have been synthesized which present no surface oxygens and hydroxyls, but other types of active sites whose exact nature remains controversial. These type of solids are, for example, impregnated imides and nitrides on zeolites and alumina, amorphous oxynitrides obtained by treatment with ammonia or aluminium orthophosphate, zirconium phosphate, aluminium vanadate or galloaluminophosphate, and KF supported on alumina/1,3,41 One of the main advantages of these solids with respect to basic oxides is their resistance to carbon dioxide or water. 

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Proline is a stable, nontoxic, cyclic, secondary pyrrolidine-based amino acid with an increased pK value. Thus, proline is a chiral bidentate compound that can form catalytically active metal complexes (Melchiorre et al. 2008). Bidentate means that proline has not only one tooth but also a second one to bite and react. The greatest difference to other amino acids is a Lewis-base type catalysis that facilitates iminium and enamine-based reactions. It is especially noteworthy that cross-aldol condensations of unprotected glycoladehyde and racemic glyceralde-hyde in the presence of catalytic amounts of the Zn-(proline)2 gave a mixture of pentoses and hexoses (Kofoed et al. 2004). Again, proline seems to play the decisive role. The conditions are prebiotic the reaction proceeded in water for seven days at room temperature. It is remarkable that the pentose products contained ribose (34%), lyxose (32%), arabinose (21%), and xylose (12%) and that all are stable under the conditions. Thus, the diastereomeric and enantiomeric selection observed support the idea that amino acids have been the source of chirality for prebiotic sugar synthesis. 

EfiBdent hydrogen supply iiom decalin was only accomplished by the si terheated liquid-film-type catalysis under reactive distillation conditions at modaate heating tempaatures of 210-240°C. Caibcm-supported nano-size platinum-based catalysts in the si ietheated liquid-film states accelerated product desorption fixjm file catalyst surface due to its temperature gradient under boiling conditions, so that both hi reaction rates and conversions were obtained simultaneously. 

H30+ and OH-. Reactions that are dependent on the concentrations of HXt and X- are categorized as involving general acid and general base catalysis. Table 7.1, adapted from Ashmore (30), indicates a number of catalytic reactions of the specific and general acid-base types in order to provide some orientation as to the types of reactions in the various categories. A thorough discussion of these reactions is obviously... 

In order to develop an artificial restriction enzyme that can cleave a desired sequence, an oligonucleotide tag needs to be attached to the catalysis site. The artificial enzyme shown in Fig. 6.15 has an oligonucleotide tag (the rectangle) connected to a metal-chelate-type catalysis site (the circle). The catalytic site was fixed to a particular site on the substrate upon base pairing between the artificial enzyme and the substrate. When the Lu-chelate site was connected to single-stranded DNA, and the DNA moiety was hybridized to RNA with the complementary sequence, the RNA was hydrolyzed at the desired site. If the DNA sequence in the artificial enzyme is designed appropriately, RNA can be cleaved at any site desired. 

Many solution reactions are catalyzed by hydrogen or hydroxyl ions and consequently may undergo accelerated decomposition upon the addition of acids or bases. The catalysis of a reaction by hydrogen or hydroxyl ions is known as acid-base specific catalysis. In many cases, in addition to the effect of pH on reaction rate, there may be catalysis by one or more species of the buffer system. This type of catalysis is known as the acid-base general catalysis. Solutions of vitamin were found to be... 

Amine oxidase crystallizes in two different forms, an active and an inactive form. In the inactive form, the type 2 copper center is coordinated in a distorted tetrahedral conformation by the three histidines and the TOPA cofactor. The conformation of the active type 2 copper center, however, is a distorted planar square. A water molecule and three histidines coordinate the copper ion equa-torially, while a second water molecule forms an apical ligand. TOPA is not a direct copper ligand in the active enzyme form, but occupies a position near aspartate 383. This residue is conserved in amine oxidases and possibly functions as a base during catalysis (Figs 19 and 20) [28,28 a]. 

Recently, the structure of a putative third type of thioesterase from M. tuberculosis was determined.This enzyme, shown to be a long-chain fatty acyl-CoA thioesterase (FcoT), has a hotdog fold similar to type II thioesterases, but does not have a carboxylate in its active site that can act as a nucleophile or general base during catalysis. Instead, it has been suggested that hydroxide ions are formed in the active site which subsequently attack the thioester this mechanism is similar to the one described for the hydrolytic antibody D2.3, which have a similar active site architecture to that of FcoT. This difference in mechanism suggests that FcoT in fact represents a third type of thioesterase. 

These proceedings from the 19 th Conference represent the work of over 150 scientists from fourteen countries. The proceedings are organized into the following topics immobilized and supported catalysts, practical aspects of catalysis, solid acid and base catalysis, heterogeneous catalysis and commercial processes, Raney-type and base metal catalysis, stereoselective catalysis, and hydrogenation and amination. 

Vasihev and Gahnsky [224] and Palomares et al. [225] studied X-type zeolites exchanged with K, Rb, and Cs and found that the Cs-exchanged X zeolite was the most active for side chain alkylation, producing nearly no xylene isomers. On the other hand, toluene alkylation over Na-X mainly led to the formation of xylenes. An IR-spectroscopic study in [225] revealed that on Na-X, methanol was by far the most abimdant species on the catalyst smface, while on the more basic Cs,Na-X it was toluene. Miyamoto et aL [226] visualized the acid-base cooperative catalysis in the side chain alkylation of toluene by computer graphics and also used it to explore geometrical factors. 

Surprisingly, very few publications reporting the use of bismuth catalysts have appeared in the field of polymer chemistry although bismuth(III) salts, due to their ability to form weak complexes with various heteroatoms and functional groups commonly encountered in polymer chemistry, clearly have potential as catalysts (Bronsted or Lewis cationic activation) for the synthesis of these special multi-Lewis base type of macromolecules. In one of the few reported examples, the polycondensation of dimethyl terephthalate with 1,4-butanediol has been carried out under BiCls catalysis and leads to telechelic poly(butylenes terephthalates) [125]. Bi(0Tf)3 xH20 has been patented as a catalyst for the cationic polymerization of THF using selected phosphorus-containing compounds [126]. 

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