Catalyst binding

Cadogan and coworkers160 developed a fructose-derived l,3-oxazin-2-one chiral auxiliary which they applied in the Diels-Alder reactions of its iV-enoyl derivatives 246 with cyclopentadiene using diethylaluminum chloride as the Lewis acid catalyst. The reactions afforded mixtures of endo 247 and exo 248 (equation 68). The catalyst binds to the chiral dienophile in a bidentate fashion (co-ordination to both carbonyl groups). As a consequence, the dienophile is constrained to a rigid conformation which accounts for the almost complete diastereofacial selectivities observed. 

Another example of the use of Lewis acids in organic reactions in water is the lan-thanide(III) triflate catalysed aza-Diels-Alder reaction, exemplified in Scheme 14. In this reaction the hetero-dienophile is formed in situ from a primary ammonium hydrochloride and a carbonyl compound followed by the actual Diels-Alder reaction288,289. This type of reaction proceeds readily in aqueous media290-296, and a dramatic increase in the yield upon addition of lanthanide triflates was observed288,289. The exact role of the catalyst, however, is not entirely clear. Although it was suggested that the catalyst binds to the dienophile, other mechanisms, such as simple proton catalysis, are also plausible. Moreover, these reactions are further complicated since they are often heterogeneous. 

At this point it is impossible to guess the architecture of the active catalyst. The folding of 10-mers of leucine and alanine in organic solvents is clearly of critical importance, and studies are in progress to understand the preferred shapes. Obviously, in its active form the catalyst binds and activates peroxide anion and/or the electron-poor alkene near its chiral surface, perhaps in a chiral cavity, but the precise orientation of catalyst and reactants in the initial bondforming Michael reaction remains unsolved. 

According to this work the catalytic decomposition of nitrous oxide molecules proceeds in the following way a N2O molecule adsorbed by the catalyst binds metal electrons, and thus the bond between the 0 atom and N2 in the molecule is loosened, and N2 is thermally dissociated from O at sufficiently high temperature. The 0 atom is held to the surface through the influence of the metal electrons. It can combine with a neighboring... 

Coordination of the olefin to osmium results in additional substrate-catalyst binding and leads to arrangement 16 with one axial and one equatorial oxygen in proximity to the olefinic double bond [37]. Consequently, a minimal motion pathway from this arrangement via a [3+2]-cycloaddition would directly produce the pentacoordinate osmium(Vl) ester in the energetically most-favorable geometry. 

Product inhibition occurs if the catalyst binds to the product with a similar tightness to the substrate. In a similar geometric approach of reactants, however, catalysis was successful (Mackay, 1994). Besides the above-mentioned Diels-Alder reaction, the substrate-free porphyrin trimer also catalyzes the acetylation of 4-hydroxymethylpyridine with N-acetylimidazole. Although both reactants are bound to the porphyrin trimer, their ground states are bound less tightly than the... 

For monosubstituted ethylenes chiral products arise only when the hydrogen of the catalyst binds to the unsubstituted unsaturated carbon atom of the substrate. Since this product is, in general, the minor one, from the enantiomeric excess no prediction can be made of the face of the substrate preferentially attacked but only the face of the substituted unsaturated carbon atom preferentially formylated to form the chiral product can be established. Indications about all 4 transition states can be obtained using either labelled 2-(2H]-olefins or carrying out a deuterio-formylation instead of a hydroformylation (see Sect. 2.1.5.). 

Like an enzyme, an asymmetric catalyst binds its substrate, performs a reaction, and releases the product three steps. Chiral auxiliaries have three analogous steps attach, react, and cleave. The advantage of an asymmetric catalyst over an auxiliary is that binding to the substrate is reversible and involves weak, intermolecular forces instead of covalent bonds. Binding and release of the substrate occur in the same reaction vessel as the stereocenter-forming reaction. 

Some chiral transition metal catalysts bind with face-selectivity toward the substrate, which enables the enantioselective conversion of prochiral starting materials ... 

Chaudhari, R.V., Bhanage, B.M., Deshpande, R.M. and Delmas, H. (1995) Enhancement of interfacial catalysis in a biphasic system using catalyst-binding ligands. Nature, 373, 501. 

M/Si surface metallic dispersion factor, where M-Zn, Cu, or Pt in the particular catalyst, binding energies (B.E.) are shown for Zn 2p3/2, Cu2p3/2, or Pt3d5/2, respectively c calcined r reduced d deactivated ... 

The ion exchanger catalyst binds the chlorine in the waste plastic to form neutral salts. The catalyst splits the long-chain hydrocarbons into shorter chains at a maximum temperarnre of 390°C. 

In the generally accepted mechanism for catalytic hydrogenation, the surface of the metal catalyst binds both H2 and the alkene, and H2 is transferred to the n bond in a rapid but stepwise process (Mechanism 12.1). 

Second, microbial chemical transformations are accomplished by means of enzymes, proteins that act as catalysts. Catalysts bind with reactants and hold them in such an orientation that they more readily react. The products of the reaction are then released, leaving the catalyst ready to facilitate another transformation. (It is possible for an enzyme to be destroyed if a chemical mimics the proper substrate sufficiently to bind, but fails to react and subsequently release from the enzyme.) Because each enzyme is produced in response to a section of the genetic code (DNA) in the organism and many enzymes are extremely specific, it is possible that some strains of a species of bacteria may accomplish a certain chemical transformation while other individuals cannot. By using modern techniques of molecular biology, scientists can insert specific biotransformation capabilities into bacteria by means of genetic transfer. This procedure is easiest if the genetic material is associated with plasmids, which are small circular molecules of DNA that can exist independently within a bacterial cell. 

An efficient catalyst binds the transition state very tightly, and therefore is very small. This number may be directly compared to K y the dissociation con-... 

Why does the (EBTHI)Zr system induce such high levels of enantioselectivity in the C-C bond formation process It is plausible that the observed levels of enantioselection arise from minimization of unfavorable steric and torsional interactions in the complex that is formed between 3 and the heterocycle substrates (Scheme 3). The alternative mode of addition, illustrated in Fig. 1, would lead to costly steric repulsions between the olefin substituents and the cyclohexyl group of the chiral ligand [6]. Thus, reactions of simple terminal olefins imder identical conditions results in little or no enantioselectivity. This is presumably because in the absence of the alkenyl substituent (of the carbon that bonds with Zr in i) the aforementioned steric interactions are ameliorated and the olefin substrate reacts indiscriminately through the two modes of substrate-catalyst binding represented in Fig. 1. 

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