Selectivity, factors controlling

Mukaiyama aldol reactions, whereby trimethylsilyl enol ethers react with aldehydes in aqueous solution to form -ketoalcohols, have been promoted by new chiral lanthanide-containing complexes and a chiral Fe(II)-bipyridine complex with 0 outstanding diastereo- and enantio-selectivities. Factors controlling the diastereoselec-tivity of Lewis-acid-catalysed Mukaiyama reactions have been studied using DFT to reveal the transition-state influences of substituents on the enol carbon, the a-carbon of the silyl ether, and the aldehyde. The relative steric effects of the Lewis acid and 0 trimethyl silyl groups and the influence of E/Z isomerism on the aldol transition state were explored. Catalytic asymmetric Mukaiyama aldol reaction of difluoroenoxysilanes with /-unsaturated a-ketoesters has been reported for the first time and studied extensively. ... 

Often it is possible to resolve vibrational structure of electronic transitions. In this section we will briefly review the symmetry selection rules and other factors controlling the intensity of individual vibronic bands. 

Options. Traditional control options for overexposure are material substitution, process change, containment, enclosure, isolation, source reduction, ventilation, provide personal protection, change work practices, and improve housekeeping. A simple way of looking at selection of control options is to find the cheapest option that results in the desired amount of exposure reduction. It is not actually that simple, however, because the various options differ in ways other than cost and degree of control. Some of the other factors to consider in selection of control options are operabiUty, rehabiUty, and acceptabihty. 

List the important economic factors to consider when selecting emissions control equipment. 

In the selection of control equipment, the most important waste-gas characteristics are volumetric flow rate, concentration and composition of organic compounds in the waste-gas, waste-gas temperature and humidity, and rbe content of particulate matter, chlorinated hydrocarbons, and toxic pollutants. Other factors influencing the equipment selection are the required removal efficiency, recovery requirements, investment and operating costs, ease of installation, and considerations of operation and maintenance. The selection of a suitable control method is based on the fundamental selection criteria presented as well as the special characteristics of the project. 

According to Equation 3, the resolution of two peaks in column separation is controlled by three major variables retention defined in terms of the retention factor k column efficiency expressed as the number of theoretical plates N and selectivity characterized by the selectivity factor a [48] ... 

One of the key factors controlling the reaction rate in multiphasic processes (for reactions talcing place in the bulk catalyst phase) is the reactant solubility in the catalyst phase. Thanks to their tunable solubility characteristics, the use of ionic liquids as catalyst solvents can be a solution to the extension of aqueous two-phase catalysis to organic substrates presenting a lack of solubility in water, and also to moisture-sensitive reactants and catalysts. With the different examples presented below, we show how ionic liquids can have advantageous effects on reaction rate and on the selectivity of homogeneous catalyzed reactions. 

The lEP provides the monitoring fi amework to characterize the status and trends of Delta aquatic ecosystems. The lEP strives to provide information on the many factors that affect ecological resorrrces in the Delta. Key near-term goals for the lEP include (1) collecting and analyzing data needed to understand factors controlling the distribution and abundance of selected fish and wildlife resources, (2)... 

In the case of the reverse-electron-demand Diels-Alder reactions, the secondary orbital interaction between the Jt-HOMO of dienophile and the LUMO of 114 or the effect of the orbital phase enviromnents (Chapter Orbital Phase Enviromnents and Stereoselectivities by Ohwada in this volume) cannot be ruled out as the factor controlling the selectivity (Scheme 55). 

Bard AJ, Wrighton MS (1977) Thermodynamic potential forthe anodic dissolution of n-type semiconductors - A crucial factor controlling durability and efficiency in photoelectrochem-ical cells and an important criterion in the selection of new electrode/electrolyte systems. J Electrochem Soc 124 1706-1710... 

The above syntheses of metal nanowires are based on the thermal hydrogen reduction. However, we found that the reproducible synthesis is difficult by this method, because the hydrogen reduction needs careful control of the reaction conditions. For the reproducible and selective synthesis of wires and particles, we need to clarify the factors controlling the sintering of metals. The key factors are the concentration of residual solvent and the relative rate of reduction and migration of metal ions. The details are shown in the next section. 

The variables that control the extent of a chromatographic separation are conveniently divided into kinetic and thermodynamic factors. The thermodynamic variables control relative retention and are embodied in the selectivity factor in the resolution equation. For any optimization strategy the selectivity factor should be maximized (see section 1.6). Since this depends on an understandino of the appropriate retention mechanism further discussion. .Jll be deferred to the appropriate sections of Chapters 2 and 4. 

Temperature variation may also be a relevant factor in flowrate stability. Since the viscosity of the solvent is temperature dependent, wide swings in the ambient temperature can directly affect pump performance. The direct effects of temperature on pump performance usually are far smaller, however, than the effects on retention and selectivity therefore, control of column temperature is generally sufficient to obtain high reproducibility. 

The number of subjects needed so that a study is likely to have an acceptable statistical power depends on a number of factors, including analytical parameters (precision, etc.), subject selection and control, and protocol design (cross-over, parallel). 

UDP and UTP are selective agonists of certain of the P2Y receptors. It is not yet clear what factors control the release of uridine nucleotides into the extracellular space. UTP can be formed from UDP in the extracellular space by the action of the enzyme nucleoside diphosphokinase, which catalyzes the transfer of the -phosphate of nucleoside triphosphates to nucleoside diphosphates, e.g., ATP + UDP —> ADP + UTP. 

Environmental factors control lineage decisions of neural crest cells. Several experimental approaches have shown that environmental factors are critical in determining neurotransmitter phenotype by altering existing cell properties and not by selecting different hypothetical subpopulations of NC cells. For example, during normal... 

The Waller et al. taxonic results do not appear to reflect a sample selection factor. If the findings resulted from combining predefined groups (individuals with multiple personality disorder and nonclinical controls), base rate estimates should have been about. 50. The. 37 taxon base rate suggests... 

Hydrophobic interactions are very sensitive to temperature. Retention of most proteins increases with temperature, but for some the opposite is true, and the magnitude of response is highly individual in any case.34-36 If you elevate temperature sufficiently (56°C and above) you may begin to denature proteins in the sample. This may expose more hydrophobic sites and alter selectivity to a greater degree. Whether or not you exploit temperature as a selectivity factor, good temperature control is essential for assay reproducibility. 

Although the intermolecular selectivity of the nitration of alkylbenzenes by nitric acid in trifluoroacetic acid is controlled by both electronic and steric factors, it is argued that intramolecular selectivity is controlled by steric effects on transition state solvation. 

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