Strain catalysis

A reaction interface is the zone immediately adjoining the surface of contact between reactant and product and within which bond redistributions occur. Prevailing conditions are different from those characteristic of the reactant bulk as demonstrated by the enhanced reactivity, usually attributed to local strain, catalysis by products, etc. Considerable difficulties attend investigation of the mechanisms of interface reactions because this thin zone is interposed between two relatively much larger particles. Accordingly, many proposed reaction models are necessarily based on indirect evidence. Without wishing to appear unnecessarily pessimistic, we consider it appropriate to mention here some of the problems inherent in the provision of detailed mechanisms for solid phase rate processes. These difficulties are not always apparent in interpretations and proposals appearing in the literature. 

The entropic hypothesis seems at first sight to gain strong support from experiments with model compounds of the type listed in Table 9.1. These compounds show a huge rate acceleration when the number of degrees of freedom (i.e., rotation around different bonds) is restricted. Such model compounds have been used repeatedly in attempts to estimate entropic effects in enzyme catalysis. Unfortunately, the information from the available model compounds is not directly transferable to the relevant enzymatic reaction since the observed changes in rate constant reflect interrelated factors (e.g., strain and entropy), which cannot be separated in a unique way by simple experiments. Apparently, model compounds do provide very useful means for verification and calibration of reaction-potential surfaces... 

Catalysis of strained bond re- 8 arrangements by silver(I) ion (43) The chemistry of organogold com- 13 pounds (94)... 

Figure 7-5. Two-dimensional representation of Koshland s induced fit model of the active site of a lyase. Binding of the substrate A—B induces conformational changes In the enzyme that aligns catalytic residues which participate in catalysis and strains the bond between A and B, facilitating its cleavage.

Catalytic mechanisms employed by enzymes include the introduction of strain, approximation of reactants, acid-base catalysis, and covalent catalysis. 

Selective aromatic functionalization has been a permanent object of research since the ninenteenth century. Catalysis has offered a powerful tool to achieve this goal. Over the years we have worked out a complex catalytic system consisting of an inorganic compound such as a palladium salt and an organic molecule containing a strained double bond such as norbomene (1,2). We have seen that these two catalysts cooperatively react with an aromatic iodide, an alkyl iodide and a terminal olefin. The following equation reports an example (L = solvent and/or olefin) (3). 

Finally, in the sense that the imposition of conformational restrictions or specific solvent effects on an organic molecule are forms of strain, non-covalent catalysis by the cycloamyloses may provide a simple model for the investigation of strain and distortion effects in enzymatic reactions. 

The catalytic specificity of the cycloamyloses has led to their utilization as a model for understanding enzymatic catalysis. It is the authors expectation that the cycloamyloses will continue to serve as an enzyme model as well as a model for designing more efficient catalytic systems. Toward this end, it would seem profitable to pursue the idea that the cycloamyloses may lower the activation energy of a chemical reaction by inducing strain into the substrate. 

The study of both carbonyl and carbon acid participation in ester hydrolysis has been used by Bowden and Last (1971) to evaluate certain of the factors suggested for important roles in enzymic catalysis. A first model concerns a comparison of the three formyl esters and shows that the proximity of the formyl to the ester group and internal strain increase in passing along the series, 1,2-benzoate, 1,8-naphthoate and 4,5-phenanthroate. The very large rate enhancements result from the proximity of the internal nucleophile once formed and from internal strain. Strain is increased or induced by the primary... 

Strain and Distortion Transition-State Stabilization Transition-State Analogs Chemical Catalysis... 

The STRAIN AND DISTORTION model for catalysis involves pushing, pulling, or twisting a bond that is to be made or broken during the reaction. Parts of the substrate not involved directly in the chemical reaction are required to hold the substrate on the enzyme in the distorted form. The distortion and strain make it easier to reach the transition state. 

X-ray line broadening provides a quick but not always reliable estimate of the particle size. As Cohen [9] points out, the size thus determined is merely a ratio of two moments in the particle size distribution, equal to /. Both averages are weighted by the volume of the particles, and not by number or by surface area, as would be more meaningful for a surface phenomenon such as catalysis. Also, internal strain and instrumental factors contribute to broadening. 

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