Selectivity relationship

Brown developed the selectivity relationship before the introduction of aromatic reactivities following the Hammett model. The former, less direct approach to linear free-energy relationships was necessary because of lack of data at the time. 

The selectivity relationship merely expresses the proportionality between intermolecular and intramolecular selectivities in electrophilic substitution, and it is not surprising that these quantities should be related. There are examples of related reactions in which connections between selectivity and reactivity have been demonstrated. For example, the ratio of the rates of reaction with the azide anion and water of the triphenylmethyl, diphenylmethyl and tert-butyl carbonium ions were 2-8x10 , 2-4x10 and 3-9 respectively the selectivities of the ions decrease as the reactivities increase. The existence, under very restricted and closely related conditions, of a relationship between reactivity and selectivity in the reactions mentioned above, does not permit the assumption that a similar relationship holds over the wide range of different electrophilic aromatic substitutions. In these substitution reactions a difficulty arises in defining the concept of reactivity it is not sufficient to assume that the reactivity of an electrophile is related... 

Volume 67 Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. 

A Quantitative Treatment of Reactivity of the Electrophile the Selectivity Relationship... 

A modification of the Hammett approach, suggested by Brown, called the selectivity relationship is based on the principle that reactivity of a species varies inversely with selectivity. Table 11.3 " shows how electrophiles can be arranged in order of selectivity as measured by two indexes (1) their selectivity in attacking toluene rather than benzene, and (2) their selectivity between the meta and para positions in toluene. As the table shows, an electrophile more selective in one respect is also more selective in the other. In many cases, electrophiles known to be more... 

Given this behavior (little selectivity in distinguishing between different substrate molecules), the selectivity relationship would predict that positional selectivity should also be very small. However, it is not. For example, under conditions where nitration of p-xylene and 1,2,4-trimethylbenzene takes place at about equal rates, there was no corresponding lack of selectivity at positions within the latter. Though... 

Polar effects of alkyl groups 243 Steric effects of alkyl groups 246 Kinetic effects of aryl substituents 252 Selectivity relationships and transition-state shifts in arylolefin bromination 256... 

Finally, as shown in Table 13, p for an aromatic ring is also strongly dependent on the other substituents at the double bond it varies from —1.6 to — 5.5 on going from a-methoxystyrenes to stilbenes. This variation, which is related to the well-known non-additivity of multiple substituent effects, and contrasts with what is observed for alkene bromination, is discussed in the next paragraph, devoted to substituent interaction and selectivity relationships in bromination. 

SELECTIVITY RELATIONSHIPS AND TRANSITION STATE SHIFTS IN ARYLOLEFIN BROMINATION... 

Equations (37)—(39), where the non-additivity of multiple substituent effects is described by a cross-term, express correctly the rate data for bromination and other reactions of polysubstituted substrates. The question arises, therefore has the interaction constant, q, any physicochemical meaning in terms of mechanism and transition state charge To reply to this question, selectivity relationships (42) that relate the p-variation to the reactivity change and not to any substituent constant, have been considered (Ruasse et al., 1984). 

Substituent effects on rates (Nishida, 1967) and equilibria (Mindl and Vecera, 1972) of the heterolytic formation of benzhydryl cations, analogous to those obtained in the bromination of 1,1-diphenylethylenes, can be analysed in terms of selectivity relationships (50) and (51). Here ak is... 

On the other hand, transition-state positions in bromination can be evaluated from solvent effects and their Winstein-Grunwald m-coefficients, since these latter are related mainly to the magnitude of the charge in the activated complexes (p. 274). The p- and m-values for most olefins included either in selectivity relationship A (44) or in B (45) are compared in Table 17. The m-value varies significantly with the reactivity as does p. Since m-variations arise from transition-state shifts, p-variations necessarily come, at least in part, from the same effect. 

Table 17 Selectivity relationships substituent" and solvent effects in bromination of arylolefins as indexes of transition state shifts with reactivity.

Callicott, J. H., Bertolino, A., Egan, M. F. et al. Selective relationship between prefrontal N-acetylaspartate measures and negative symptoms in schizophrenia. Am. J. Psychiatry 157 1646-1651,2000. 

In order to fully exploit the information contained in the X-ray structures, interdisciplinary research is required, involving such disciplines as computational chemistry, medicinal chemistry, and molecular pharmacology in partnership with X-ray crystallography. In this context, the X-ray structures provide an excellent springboard for studying structure-activity/selectivity relationships, as well as the structure-based design of new ligands. 

Automatic flexible docking of molecules into the binding pocket, e.g., in order to study structure-activity/selectivity relationships... 

Structure-Selectivity Relationship in Xylene Isomerization and Selective Toluene Disproportionation... 

Menziani, M.C., Cocchi, M., Fanelli, F. and De Benedetti, P.G. (1995) Quantitative structure-affinity/ selectivity relationship analysis on three-dimensional models of the complexes between the ETA and ETB receptors and C-terminal endothelin hexapeptide antagonists. Journal of Molecular Structure (Theochem), 333, 243—248. 

Menziani, M.C., De Benedetti, P.G. and Karelson, M. (1998) Theoretical descriptors in quantitative structure-affinity and selectivity relationship study of potent N4-substituted arylpiperazine... 

Furthermore, Venuto (77, 78) has applied a special type of LFER, the Brown selectivity relationship (79), to the deuteration (series 50) and ethylation (series 51) of toluene and benzene catalyzed by zeolites. The Brown equation... 

The transition states of free radical reactions generally show evidence of polar character wherein electron transfer to or from the radical has occurred (20). Thus, the electron affinity or ionization potential of the radical involved should affect the reaction. The much higher electron affinity (16) of ROo than CH3 radicals no doubt alters the transition state so that the reactivities toward it show less selectivity. The results of Szwarc and Binks (22) center around the fact that only carbon radicals were used for the correlation, and thus the electron affinity does not vary sufficiently to show in the correlation any deviation from the expected reactivity-selectivity relationship. 

Because of the overall first-order dependence of reaction rate on pressure (specifically, on hydrogen partial pressure) in combination with the rather complex selectivity relationships among primary products, it is regarded as quite probable that all of the primary products (and the separate inter-... 

For example, in the ring isomerization reaction, methylcyclopentane forms a methylcyclopentene intermediate in its reaction sequence to cyclohexane. The intermediate can also further dehydrogenate to form methylcyclo-pentadiene, a coke precursor. Bakulen et al. (4) states that methylcyclo-pentadiene can undergo a Diels-Alder reaction to form large polynuclear aromatic coke species. Once any olefinic intermediate is formed, it can either go to desired product or dehydrogenate further and polymerize to coke precursors. This results in a selectivity relationship between the desired products and coke formation as shown on the next page. 

I would like to emphasize the simplicity of Simon s molecules more than I did in my previous comment. These are by far the simplest class of ion-selective molecules that act in bilayers and they open up very promising prospects for the study and understanding of structure-selectivity relationships. 

In the chapter, we report a successful application of the Free-Wilson (26-30) methodology to model structure-activity/selectivity relationships. The Fujita-Ban (31-34) modification of Free-Wilson coupled with multiple linear regression... 

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