Catalyst-substrate interactions

Alternatively, substrate control of diastereoselectivity can rely on attractive catalyst substrate interactions. This requires in general special functional groups which allow for a directed hydroformylation, which is summarized in Sect. 6 (vide infra). 

For a poly- or oligonucleotide catalyst-substrate interaction the problem of binding-specificity is in principle solved simply by using the complementary sequence, and chemistry based on classical... 

Note how the key interaction boosting the catalytic effect is the protonation of the carbonyl group on the TG. Such catalyst-substrate interaction increases the electrophilicity of the adjacent carbonyl carbon atom, making it more susceptible to nucleophilic attack. Compare this to the base-catalyzed mechanism where the base catalyst takes on a more direct route to activate the reaction, creating first an alkoxide ion that directly acts as a strong nucleophile (Figure 4). Ultimately, it is this crucial difference, i.e., the formation of a more electrophilic species (acid catalysis) v.s. that of a stronger nucleophile (base catalysis), that is responsible for the differences in catalytic activity. 

Decarboxylation of indole-3-acetic add enhanced by thermal polylysine in darkness (without irradiation of white light) follows Michaelis-Menten kinetics, thus indicating a reversible catalyst-substrate interaction 22). 

For example, Samuel French and co-workers used a combined QM/MM approach for modeling the catalyst/substrate interactions in the methanol synthesis process [8]. The annual worldwide production of methanol exceeds 32 M tons, most of which is... 

A knowledge of the kinetics of the reaction at the active sites is of primary importance in determining the nature of catalytic action in heterogeneous catalysis. Information about the nature of the catalyst-substrate interaction can be obtained from the way in which the rate constants in the kinetics change on variation of such parameters as temperature, catalyst treatment, and catalyst composition. In addition, these constants are the quantities which should correlate with structural information such as that obtained by the methods of solid state physics. However, the true kinetics at the active sites is not always obtained unless certain precautions are taken, as has been pointed out in a recent volume of Advances in Catalysis (1). 

The most common method to transfer asymmetry from a catalyst to the substrate relies on steric biasing. Other catalyst-substrate interactions, such as ir-interactions between aromatic groups on the catalyst and substrate or hydrogen bonding between the catalyst and substrate, can also play important roles and may be used in combination with steric biasing. 

This work represents a landmark in the area of stereoselective metal-free (i.e., aminocatalysis) alkylation of benzenes based on Michael-type condensation via covalent catalyst-substrate interaction [22]. Subsequently, asymmetric acid catalysis based on hydrogen bond catalyst-substrate recognitions has found elegant applications in 1,4-conjugated additions and direct condensation of arenes with carbonyl compounds. The following sections will be organized based on the reactivity exploited in the arene functionalization. 

Among the synthetic catalysts used for cyclohexanones, the organocatalyst developed by Peris and Miller is noteworthy [54]. Recently, an efficient system for the oxidation of 4-substituted cyclohexanones (3) to lactones (4) was reported by Stmkul and coworkers [5 5], in which they use a chiral enantiopure Pt(II) catalyst in water with added surfactant and hydrogen peroxide as the terminal oxidant (up to 92% ee), (Eq. (10.6)). The surfactant enables solubilization of the otherwise insoluble chiral catalyst and enhances the enantioselectivity of the reaction because of tighter catalyst-substrate interactions favored by the micellar supramolecular aggregate. 

Covalent activations of substrates can be achieved through different methods. One such method includes the formation of intermediates between an otherwise relatively inert substrate and the catalyst. This is comparable with the formation of an enzyme-substrate complex in an enzymatic reaction. In organocatalytic reactions, the substrates can be classified as nucleophilic and electrophiUc partners. Each of these substrates can be activated by interactions with the catalyst The catalyst-substrate interaction can either be covalent or non-covalent While the non-covalent activations are described elsewhere in other chapters of these volumes, the focus herein is on covalent activations. In covalent activation, the substrate forms a bond with the catalyst and thereby becomes activated. For instance, in the following example of the formation of the prohne enamine of acetone the substrate (acetone) reacts with proline to generate the activated nucleophile (enamine) (Scheme 17.1). 

To date, several different catalysts, both organocatalysts and metal-based catalysts, are available for the asymmetric Michael-type addition reactions. Indeed, a high level of achievement has been reached in terms of enanatioselectiv-ity and product yield. However, specihc windows for particular substrates, especially in natural product motif synthesis, are stdl in great demand. Thus, the exploration of more gen-eraL as well as more operationally simple (e.g., moisture stable and air stable), catalysts is attainable. Through the further in-depth structural investigation of catalyst-substrate interaction in Michael addition, a more sophisticated, yet more efficient, catalyst can be developed, and thus, the Turn Over Number (TON) can be expected to be increased. These future developments certainly will be fmitfiil to pharmaceutically and industrially related processes. 

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