Origins of Selectivity

There are two general classes of selectivity equilibrium-based and kinetically based. Both types depend on specific interactions of the analyte molecule with the selective layer. In addition, there is also physical selectivity based on the highly specific interaction of the molecule with the electrostatic and electromagnetic fields. The topic of selectivity is broad enough to deserve its own chapter. We examine issues pertaining to selectivity in Chapter 2. 

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The carbo-Diels-Alder reaction of acrolein with butadiene (Scheme 8.1) has been the standard reaction studied by theoretical calculations in order to investigate the influence of Lewis acids on the reaction course and several papers deal with this reaction. As an extension of an ab-initio study of the carbo-Diels-Alder reaction of butadiene with acrolein [5], Houk et al. investigated the transition-state structures and the origins of selectivity of Lewis acid-catalyzed carbo-Diels-Alder reactions [6]. Four different transition-state structures were considered (Fig. 8.4). Acrolein can add either endo (N) or exo (X), in either s-cis (C) or s-trans (T), and the Lewis acid coordinates to the carbonyl in the molecular plane, either syn or anti to the alkene. 

Estiu, G., Greenberg, E Harrison, C.B., Kwiatkowski, N.P., Mazitschek, R., Bradner, J.E. et al. (2008) Structural origin of selectivity in class 11-selective histone deacetylase inhihitors. Journal of Medicinal Chemistry, 51, 2898—2906. 

The potential of using free radical reactions as synthetic methods will be increased when the origin of selectivity, especially in acyclic systems, is better imderstood [100,101]. 

In this paper we present a molecular view of the origin of selectivity differences based on bonding of intermediates. The suggestion is that hydrocarbon intermediates bonded to metal centers through oxygen atoms... 

Here asx is the activity of the bound species and ax and as are the activities of the species in the sample and of the binding site in the sensor, respectively. For the purpose of this discussion, the binding site can be thought of as a defined but separate component of the selective layer, such as in heterogeneous selective layers, or it may be a specific part of the uniform matrix, as in homogeneous selective layers. (More on the origins of selectivity are discussed later.) The free energy of interaction for reaction depicted in (1.1) is... 

With this group of electrochemical sensors, information is obtained from the current-concentration relationship. The two most important issues to discuss are (1) the origin of the signal for various types of amperometric sensors and (2) the origins of selectivity. To begin our examination of these issues, we briefly reiterate some of the information presented in the Introduction to Electrochemical Sensors (Chapter 5). 

Fig. 7.8 Origins of selectivity of amperometric sensors based on charge-transfer resistance and mass transport resistance...

In principle, there is no requirement on how fast the electrochemical reaction should be because, as we have seen, it is usually possible to apply the working potential E at which the kinetics of the charge transfer are fast. The origin of selectivity is in the enzymatic reaction. 

One further complication concerning the origin of selectivity changes during structural isomerization reactions has been indicated by the work of Kramer and Zuegg.352 They observed that the percentage of n-hexane produced from methyl cyclopentane increases with the amount of interface between Pt and the support. They propose that isomerization occurs by two parallel routes, one on the metal the other at the metal/oxide interface. Such effects, if confirmed, should be more important for small metal particles, and may have influenced the selectivity observed in the other work quoted above. 

Worz N, Brandner A, Claus P. Platinum-bismuth catalyzed oxidation of glycerol kinetics and the origin of selective deactivation. J Phys Chem C. 2010 114 1164-72. 

Cheong PH, Houk KN (2004) Origins of selectivities in proline-catalyzed alpha-aminoxylations. J Am Chem Soc 126 13912-13913... 

Bimey, D. M. Houk, K. N. Transition structures of the Lewis acid-catalyzed Diels-Alder reaction of butadiene with acrolein. The origins of selectivity, 7. Am. Chem. Soc. 1990,112, 4127-4133. 

Saphier S, Hu Y, Sinha SC, Houk KN, Keinan E. Origin of selectivity in the antibody 20F10-catalyzed Yang cyclization. J. Am. Chem. Soc. 2005 127(1) 132-145. 

Process Feed/Strip Pore size Origin of Selectivity Pressure Gradient Operation C/) CO... 

The origin of selectivity with the tartrate-based allylboronates is believed to result from the minimization of unfavorable electronic interactions. The two transition structures shown in Fig. 10-5 illustrate the interactions that contribute to the major and minor pathways. The electronic interactions are reasonable as they would arise from a favored conformation of the a-heteroatom-substituted carbonyl system, i.e. one in which the heteroatom and the carbonyl group are. yn-coplanar. It is interesting to note that the selectivity which would be predicted on steric grounds is opposite to that observed with the tartrate-based reagents. 

Investigation of the Origins of Selectivity in Ethylene Epoxidation on Promoted and Unpromoted Ag/a-Al2O3 Catalysts A Detailed Kinetic, Mechanistic and Adsorptive Study... 

Origins of Selectivity in Ethylene Epoxidation on Ag/a-Al2O3 Catalysts... 

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