Cross-selection

The enyne cross metathesis was first developed in 1997 [170,171]. Compared to CM it benefits from its inherent cross-selectivity and in theory it is atom economical, though in reality the aUcene cross-partner is usually added in excess. The inabihty to control product stereochemistry of ECM reactions is the main weakness of the method. ECM reactions are often directly combined with other transformations like cyclopropanation [172], Diels-Alder reactions [173], cychsations [174] or ring closing metathesis [175]. 

Whereas detection of electroinactive ions was principally worked out at the end of last century, the use of transition metal hexacyanoferrates as sensors for various electroactive compounds still attracts particular interest of scientists. Although the cross-selectivity of such compounds must be low, a number of them have been successfully used for analysis of real objects. 

As mentioned earlier, a random and statistical cyclization with two different and regio-defined Ti-compounds would produce a synthetically unattractive mixture of ten different zirconacycles. In reality, however, there are a few factors that can be exploited to produce a single desired zirconacycle. A systematic investigation has revealed that there are several discrete types offive-membered zirconacycle formation, as shown in Scheme 1.53 [88,89] (Generalization 20). In the Type I reaction, the cross-selective cyclization is kinetically favored. Presumably, little ethylene is displaced during the reaction. Type I reactions cannot be readily observed with ZrCp2 complexes with 1-butene. In contrast, Type II cyclization must be thermodynamically controlled, as 1-butene is readily displaced by a number of better Ti-ligands. It is predicted, however, that the cross-combination of the two Ti-com-... 

Type I Kinetically favored cross-selective cyclization... 

Type IV Cross-selective cyclization (the original alkene is released)... 

Arrays were introduced in the mid-eighties as a method to counteract the cross-selectivity of gas sensors. Their use has since become a common practice in sensor applications [1], The great advantage of this technique is that once arrays are matched with proper multivariate data analysis, the use of non-selective sensors for practical applications becomes possible. Again in the eighties, Persaud and Dodds argued that such arrays has a very close connection with mammalian olfaction systems. This conjecture opened the way to the advent of electronic noses [2], a popular name for chemical sensor arrays used for qualitative analysis of complex samples. 

Figures 2 and 3 show the situation in the case of an array of TSMR sensors exposed to different compounds at various concentration levels [13]. The cross-selectivity of the sensors makes their individual responses ambiguous. Namely different samples, due to a combination of qualita-...

The application of this concept to liquid samples is what we already refer to electronic tongue . It entails the use of multidimensional information coming from an array of chemical sensors, mimicking the animal sense of taste. As several possibilities exist on the side of which sensors form the array, the general response shown by the different sensors used is of paramount importance that is, cross-selectivity features are needed in order to profit from the multidimensional aspects of the information [7]. The performance of electronic tongues can be suited not only to qualitative purposes like identification of species and classification of sample varieties, but also to quantitative uses, normally the multidetermination of a set of chemical species, an interesting objective for process control. A more bioinspired trend is the artificial taste [8] in order to perform automated taste perception, especially in the industrial field. 

The values in the parentheses are the cross selectivity factors for the primary guests of each group. 

Although molecular imprinting is a fascinating tool for tailoring the enantioselectivity of a CSP, from a practical standpoint MlP-type CSPs are problematic for analytical applications. This is mainly due to (i) their poor efficiency, in particular for the high-affinity enantiomer and print molecule, and (ii) the limited range of applicability, i.e. only for the racemate of the print molecule and structurally closely related SAs for which cross-selectivity exists. These major limitations are the main reasons why there are no MlP-type CSPs currently available on the market. 

Higher-order chemical sensing can alleviate some inherent problems of chemical sensors. For example, a sensor array can mathematically correct for systematic drift. It also provides cross selectivity for elimination of interference. 

Note that, inside the cell area of the spreadsheet, the mouse pointer usually shows as a cross. Select a cell, then move the pointer away from it and back again. You will see that, near the border of the active cell, the pointer changes its shape and becomes an arrow. When the pointer shows... 

Voltammetric sensors based on chemically modified electrodes (conducting polymers, phthalocyanine complexes) with improved cross-selectivity were developed for the discrimination of bitter solutions [50], The performance and capability were tested by using model solutions of bitterness such as magnesium chloride, quinine, and four phenolic compounds responsible for bitterness in olive oils. The sensors gave electrochemical responses when exposed to the solutions. A multichannel taste sensor was constructed using the sensors with the best stabilities and cross-selectivities and PCA of the signals allowed distinct discrimination of the solutions. 

Di Natale C., Paolesse R., D Arcangelo G., Comandini P., Pennazza G., Martinelli E., Rullo S., et al.. Identification of schizophrenic patients by examination of body odor using gas chromatography-mass spectrometry and a cross selective gas sensor array, Med. Sci. Monit., 11, 366-375, 2005. 

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