Selectivity enhancement

A few years after the first articles of Breslow had appeared, Grieco elegantly demonstrated that the astonishing rate and selectivity enhancements of Diels-Alder reactions in water can be exploited sirccessfully in organic synthesis. He extensively studied the reactivity of dienes containing... 

Nonchemical or traditional practices, such as weed seed removal, optimal crop seeding rates, crop selection, enhanced crop competitiveness, crop rotation, and mechanical weed control are all important components of an effective weed management program (458,459). In the context of modern intensive chemical herbicide appHcation, nonchemical practices may even represent an innovative approach to weed management and should receive careful consideration. 

Pindur U., Lutz G., Otto C. Acceleration and Selectivity Enhancement of Diels-Alder Reactions by Special and Catalytic Methods Chem. Rev. 1993 93 741 761... 

As discussed before, very high turnover numbers of the catalytic site and a large active electrode area are the most important features for effective catalysis. In the following sections three relatively successful approaches are illustrated in detail, all of which make use of one or both of these parameters. A further section will deal with non-redox modified electrodes for selectivity enhancement of follow-up reactions. 

Selectivity Enhancement by Addition of Water-Miscible Organic Cosolvents... 

Several other examples have been reported in the literature and most of them have been already revievred (see, for instance, [11]). Hovrever, it must be mentioned that, of course, the selectivity enhancement via addition of vater-misdble organic cosolvents may not be taken for granted, as sometimes this approach may be unsuccessfij] [11a]. 

Selectivity Enhancement in Organic Media with Low Water Activity... 

Van Sickle BJ, Tietz El Selective enhancement of AMPA receptor-mediated function in hippocampal CAl nemons from chronic benzodiazepine-treated rats. Nemo-pharmacology 43 11—27, 2002... 

Table II. Luminescence Parameters that can be Combined for Selectivity Enhancement...

Scheme 9 Selectivities enhanced by Lewis acid catalysts...

Saponins la 7,411,430 -, bioautographic determination la 109 Sarcosine Ia435 lbl24 Scandium cations, detection la 144 Scanner, optical trains la 30,39 S-Chamber (small chamber) la 126,127 SCHiFF s bases lb 52 Scintillators la 12 Scopolamine lb 231,252,255,323 Scopoletin lb 216-218,365 Screening process lb 45 Sebacic acid la 178,233,249,308 Sebuthylazine lb 418 Selectivity, enhancement by derivatiza-tion la 55... 

The characteristic nature of gold as a catalyst can be depicted by four properties low-temperature activity, unique selectivity, enhancement by water, and preference to basic conditions [7]. The main questions are why gold NPs are usually more... 

One promising extension of this approach Is surface modification by additives and their Influence on reaction kinetics. Catalyst activity and stability under process conditions can be dramatically affected by Impurities In the feed streams ( ). Impurities (promoters) are often added to the feed Intentionally In order to selectively enhance a particular reaction channel (.9) as well as to Increase the catalyst s resistance to poisons. The selectivity and/or poison tolerance of a catalyst can often times be Improved by alloying with other metals (8,10). Although the effects of Impurities or of alloying are well recognized In catalyst formulation and utilization, little Is known about the fundamental mechanisms by which these surface modifications alter catalytic chemistry. 

Principles and Characteristics A sample can contain a great number of compounds, but analysts are usually interested only in the qualitative presence (and the quantitative amount) of a small number of the total compounds. Selectivity is an important parameter in analytical separations. The total analytical process clearly benefits from selectivity enhancement arising from appropriate sample preparation strategies. Selective separation of groups or compound classes can simplify a mixture of analytes before analysis, which in turn enhances analytical precision and sensitivity. Selective fractionation, in some cases, allows easier resolution of the compounds of interest, so analysts can avoid the extreme conditions of high-resolution columns. 

Improved sensitivity and selectivity (enhanced separation efficiency)... 

Moss MC, Scholey AB and Wesnes K (1998). Oxygen administration selectively enhances cognitive performance in healthy young adults a placebo-controlled, double blind crossover study. Psychopharmacology, 138, 27-33. 

---