Catalysts, general transition states

These two examples, of aldol and a-amination, evidently illustrate that a generalized transition state model demands additional refinements, taking the reaction conditions as well into account While the hydrogen bonding transition state models remained successful for many prohnespecialized reaction conditions tends to surest the need for more investigations. Similarly, with a different series of pyrroUdine catalysts devoid of a-carboxylic acid, the need for alternative transition state models to the general hypothesis of carboxylic acid directed approach of the electrophile, is more readily evident. For example, one of the most popular pyrroUdine catalysts in use today carries bulkier a-substituents. 

Kinetic studies of the reaction of alcohols with acyl chlorides in polar solvents in the absence of basic catalysts generally reveal terms both first-order and second-order in alcohol. Transition states in which the second alcohol molecule acts as a proton acceptor have been proposed ... 

Various works has pointed out the role of the nanostructure of the catalysts in their design.18-26 There is a general agreement that the nanostructure of the oxide particles is a key to control the reactivity and selectivity. Several papers have discussed the features and properties of nanostructured catalysts and oxides,27-41 but often the concept of nanostructure is not clearly defined. A heterogeneous catalyst should be optimized on a multiscale level, e.g. from the molecular level to the nano, micro- and meso-scale level.42 Therefore, not only the active site itself (molecular level) is relevant, but also the environment around the active site which orients or assist the coordination of the reactants, may induce sterical constrains on the transition state, and affect the short-range transport effects (nano-scale level).42 The catalytic surface process is in series with the transport of the reactants and the back-diffusion of the products which should be concerted with the catalytic transformation. Heat... 

The reason for the high selectivity of zeohte catalysts is the fact that the catalytic reaction typically takes place inside the pore systems of the zeohtes. The selectivity in zeohte catalysis is therefore closely associated to the unique pore properties of zeohtes. Their micropores have a defined pore diameter, which is different from all other porous materials showing generally a more or less broad pore size distribution. Therefore, minute differences in the sizes of molecules are sufficient to exclude one molecule and allow access of another one that is just a little smaller to the pore system. The high selectivity of zeolite catalysts can be explained by three major effects [14] reactant selectivity, product selectivity, and selectivity owing to restricted size of a transition state (see Figure 4.11). 

General acid catalysis is schematized in Fig. 7J,b. Here, an acid A-H increases the polarity of the carbonyl group and, hence, the electrophilicity of the carbonyl C-atom. For entropy reasons, the reaction is greatly facilitated when it is an intramolecular one (Fig. 7J,b2), in other words, when the general acid catalyst is favorably positioned within the molecule itself. Such a mechanism is the one exploited and refined by nature during the evolution of the hydrolases, with the general acid catalyst and the H20 molecule replaced by adequate amino acid side chains, and the enzymatic transition state being de facto a supermolecule (see Chapt. 3). 

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