Field-enhanced reactions

Lewis acids have served as catalysts in numerous fields of organic synthesis. By temporary coordination to basic sites substrates can be activated resulting in enhanced reaction rates. Immobilization of Lewis acids to a solid support has been carried out in order to overcome work-up problems, for complexing ligands or for generating distinct reaction environments. 

There are some difficulties of studying non-field-enhanced surface reactions with field ion emission techniques. Aside from the uncertain effects of the very high applied field, there is also the disturbing effect of field dissocation, which will complicate a correct identification of the true desorbed species. However, some of these difficulties are not insurmountable. A few precautionary procedures which can be followed are listed below ... 

Another illustration of the power of molecular dynamics simulation can be drawn from the sphere of enzyme catalysis. Many enzyme-catalyzed reactions proceed at a rate that depends on the diffusion-limited association of the substrate with the active site. Sharp et al. [28] have carried out Brownian dynamics simulations of the association of superoxide anions with superoxide dismutase (SOD). The active center in SOD is a positively charged copper atom. The distribution of charge over the enzyme is not uniform, and so an electric field is produced. Using their model, Sharp et al. [28] have shown that the electric field enhances the association of the substrate with the enzyme by a factor of 30 or more. Their calculations also predict correctly the response of the association rate to changes in ionic strength and amino... 

Fig. 6-3. Magnetic field effects observed in the radiation reaction of a squalane (S) solution of fluorene (M) for pulse radiolysis with a 4-MeV electron accelerator. The reaction temperature is not described in the present papers, but may be room temperature, (a) Time profile of fluorine fluorescence during and after pulse radiolysis of a squalane solution (1) at the minimum field less than 0.05 mT, where the residual field of an electromagnet is cancelled by passing a small reverse current through the magnet s coils (2) at 0.3 T. (b) The time dependence of the magnetic field enhancement of the fluorescence intensity (A) 15-ns pulse ( ) 50-ns pulse, (c) The MFE on the increase in fluorescence intensity at 200 ns after the pulse. (Reproduced from Ref. [18b] by permission from The American Chemical Society)...

Chen, C. J. and Osgood, R. M. (1983). Direct observation of the local-field-enhanced surface photochemical reactions. Phys. Rev. Lett. 50 1705-1708. 

The use of ultrasound to enhance performances of titania (or preferably Ti02-loaded zeolites and mesoporous materials) have been instead reviewed recently by Smirniotis et al. It was found that the presence of an ultrasonic field enhances the rate of photodegradation. A threshold of specific energy that should be provided in the reaction medium exists. This threshold is about 0.1 kW/1 and operation under this volume will ensure structural stability of these catalysts used. The use of ultrasound also avoids the formation of toxic intermediates observed when photocatalytic degradation of phenol alone is used. ... 

The field of reaction enhancement is more complex and developments more difficult to predict. The ability of ceramic membranes to run at higher temperatures greatly increases the number of reactions, which can be the considered as enhancement candidates. These reactors can be further enhanced, for example, by using the ceramic tube to support a catalyst as well as a membrane. Equipment of this type is already under development. 

A concerted effort is now underway to establish the benefits that microreactors can bring to the field of reaction chemistry. Many reactions (reviewed in detail in Refs. [1, 2, 11-13]) have been demonstrated to show enhanced reactivity, product yield and selectivity when performed in microreactors as compared with convenhonal bench-top glassware. But in the pharmaceutical industry the speed at which candidates can be prepared (and screened) is clearly most critical and this is where microreactors offer a real advantage. For instance, in the Wittig reaction the yield was of the order of 70% for both batch and micro-reactions however, in the microreactor the product was generated in approximately 6 s compared with several hours for the batch reaction [14]. Comparable results were observed in the Suzuki reaction (Table 14.2) [15] clearly this would enable more compounds to be prepared in a given period of time, which is one of the aims if the pharmaceutical industry wishes to screen more compounds. 

Sometimes magnetic fields enhance ECL intensities, and studies along this line have been used for mechanistic diagnosis (2). The effects seem to arise from field-dependent rate constants for certain reactions involving triplets hence they are associated with the T route. 

This chapter reviews the field of heterogeneous catalytic reactions in SCFs [5-8]. By exploiting the unique solvent properties of SCFs, it may be possible to enhance reaction rates while maintaining or improving selectivity. The following benefits can be expected. 

Electrically-enhanced reactions Use of electric fields for induction heating, or ultrasonic agitation Bioreactions (e.g.) 5... 

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