Enforced catalysis

When the nucleophile is not as good (Figure 9.13 B), is comparable to or larger than C2[HA], and the acid does enter the kinetic equation. We now have acid catalysis, and it is general-acid catalysis because protonation is involved in the rate-determining step. This particular scenario is called enforced catalysis, because the short lifetime of the intermediate requires that the acid trap it in order for the product to be formed. In other cases, the... 

Figure 10.15 A shows the steps and relative rates involved in the addition with the strongly basic amines. The addition step has three possible pathways (step 1) direct addition, general-acid catalyzed addition, or specific-acid catalyzed addition. Because the amines are good nucleophiles, they add directly at all pHs, but below pHs around 4 this direct addition becomes rate-determining. This is because there is a low concentration of unproton-ated amine present at low pHs. In some circumstances, enforced general-acid catalysis of the first step is found (see Section 9.3.6 for the definition of enforced catalysis). At the high pHs...

Related results of promotion (catalysis) and inliibition of stereonuitation by vibrational excitation have also been obtained for the much larger molecule, aniline-NHD (CgH NHD), which shows short-time chirality and stereonuitation [104. 105]. This kind of study opens the way to a new look at kinetics, which shows coherent and mode-selective dynamics, even in the absence of coherent external fields. The possibility of enforcing coherent dynamics by fields ( coherent control ) is discussed in chapter A3.13. 

Dendrimers are not only unreactive support molecules for homogeneous catalysts, as discussed in the previous paragraph, but they can also have an important influence on the performance of a catalyst. The dendrons of a dendrimer can form a microenvironment in which catalysis shows different results compared to classical homogeneous catalysis while peripheral functionalized dendrimers can enforce cooperative interactions between catalytic sites because of their relative proximity. These effects are called dendritic effects . Dendritic effects can alter the stability, activity and (enantio)selectivity of the catalyst. In this paragraph, different dendritic effects will be discussed. 

The factors — or at least some of them — which control reactivity in intramolecular reactions are relevant to enzyme catalysis, which also involves reactions between functional groups brought together in close and precisely defined proximity (Kirby, 1980). This has been an area of lively discussion in the recent literature [for a brief summary and leading references see Paquette et al. (1990)]. The main difficulty in making generalizations about the dependence of reactivity on geometry based on results from systems in which proximity is covalently enforced lies in the constraints imposed by particular systems. These may well affect reactivity... 

The properties of a surface are influenced by the surface groups to a very great extent. Knowledge of their existence and of their chemistry is important for many technological processes. Apart from heterogenous catalysis, surface chemistry is important in lubrication, in re-enforcement of rubber and other elastomers, in flotation, in the behavior of pigments in laquers, printing inks, and textile additives, and in many other applications. 

At the turn of the century, studies were made by Johannes Brode (in Ostwald s Institute, Leipzig) on combined catalysts (8). Continuing the work of Price with catalysis in solutions, this author demonstrated the existence of re-enforcing and of weakening influences exerted on a given catalyst by added compounds. He showed that a catalyst can be... 

Jencks, W.P. (1976) Enforced general acid-base catalysis of complex reactions in water. Accounts of Chemical Research, 9, 425. 

The high-throughput production of data at each level of the combinatorial loop enforces an automated flow of data, which requires automated acquisition and retrieval. Since no database adapted to the specific case of heterogeneous catalysis is currently available (except those included in soft- and hardware packages provided by specialised companies, but at prices that academic groups cannot afford), the only solution was to develop an in-house database that fitted exactly the laboratory requirements. 

Perhaps the most important point in these studies was the discovery that two discrete amine catalysts could be employed to enforce cycle-specific selectivities (Scheme 3.18) [20]. Conceptually, this achievement demonstrates that these cascade-catalysis pathways can be readily modulated to afford a required... 

The X-ray crystal structure for AZ-28 has a variety of structural features that are consistent with the proposed mechanism operative for the oxy-Cope rearrangement. The antibody binds the transition stage analog in a chair-like conformation, consistent with the preferred chair transition state for this pericyclic reaction (Doering and Roth, 1962). The positions of the C-2 and C-5 atoms are fixed in the antibody-bound hapten molecule in a similar fashion, the C-2 and C-5 positions in the hexadiene substrate should be held in a fixed position by conserved van der Waals interactions locking in the two phenyl substituents in the antibody combining site. This bound conformation of the acyclic (47T + 2er) system of the hexadiene substrate should enforce a molecular conformation close to the transition state for the rearrangement reaction, consistent with the catalysis observed for AZ-28. 

H.F. Gilbert and W.P. Jencks, Mechanisms for Enforced General Acid Catalysis of the Addition of Thiol Anions to Acetaldehyde, J. Am. Chem. Soc., 99, 7931. 

Aryl but not alkyl tetrahydropyranyl acetals show general acid catalysis, for the same reason [13] but aryl methyl acetals do not, because the methoxymethyl car-benium ion is not sufficiently stable. (This situation can lead to enforced general acid catalysis, when the specific acid catalyzed reaction requires nucleophilic assistance if the nucleophile is the conjugate base of the general acid this will be observed as general acid catalysis.) At the other extreme, sufficient stabilization of the carbenium ion can have the same effect, as shown by the observation of general acid catalysis of tropolone diethyl acetal 2.1 (Scheme 2.10) [14]. And even... 

However, general acid catalysis is observed for the (1375 times slower) cyclization of the C-methyl compound 4.3 (R = CH3, Scheme 2.32) evidently methyla-tion destabilizes the vinyl carbanion to the point where it no longer has a significant lifetime in water, and the preassociation-concerted mechanism is enforced once more [67]. 

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