Steric selectivity parameter

H hydrophobic interaction parameter 5 steric selectivity parameter A H-bond donor (acidity) parameter B H-bond acceptor (basicity) parameter C cation-exchange parameter (at dissociated silanols)... 

Finally, we have constructed a QM/MM model of complex 3 whereby the phenyl phosphine groups are contained in the QM region, while the ferrocenyl and phenyl substituents on the pyrazole is accounted for on a steric basis only (model E). The agreement between the X-ray structure of 3 and model E is remarkable. For example, both the Pd-P distance and the twist of the coordination plane of the Pd center, 0, are virtually identical to those of the X-ray structure. In fact, the selected parameters displayed in Table 2 are generally better than those of the full QM calculation. The good agreement between the calculated and experimental structures is important for the detailed mechanistic study of the hydrosilylation that is presented in later sections of this chapter. 

Retention of Rohrschneider-McReynolds standards of selected chiral alcohols and ketones was measured to determine the thermodynamic selectivity parameters of stationary phases containing (- -)-61 (M = Pr, Eu, Dy, Er, Yb, n = 3, R = Mef) dissolved in poly(dimethylsiloxane) . Separation of selected racemic alcohols and ketones was achieved and the determined values of thermodynamic enantioselectivity were correlated with the molecular structure of the solutes studied. The decrease of the ionic radius of lanthanides induces greater increase of complexation efficiency for the alcohols than for the ketone coordination complexes. The selectivity of the studied stationary phases follows a common trend which is rationalized in terms of opposing electronic and steric effects of the Lewis acid-base interactions between the selected alcohols, ketones and lanthanide chelates. The retention of over fifty solutes on five stationary phases containing 61 (M = Pr, Eu, Dy, Er, Yb, n = 3, R = Mef) dissolved in polydimethylsiloxane were later measured ". The initial motivation for this work was to explore the utility of a solvation parameter model proposed and developed by Abraham and coworkers for complexing stationary phases containing metal coordination centers. Linear solvation... 

Steric substituent constants (or steric substituent parameters) are descriptors of substituent groups that measure the substituent steric effects on the reactivity centres of a molecule, based on differences in the rate and equilibrium constants of selected chemical reactions. 

The steric parameter S reflects the resistance of the stationary phase to undergo insertion of solute molectdes with a higher thickness . It exhibits very similar correlations with stationary phase properties such as the hydrophobicity parameter H. However, the S parameter cannot be related to the soshape-selectivity parameter defined by Sander and Wise [22-24], which describes the ability of phases to differentiate between planar and twisted polyaromatic hydrocarbons (PAH). These two parameters do not correlate at all and obviously characterize very different properties. 

Scheme 21.1 Heck arylation of acrolein and acrolein diethylacetal. was the most important parameter among all those evaluated (i.e. KCl, solvent...) that affect the selectivity (Scheme 21.1, Route 3). However, moderate activity and selectivity were achieved when using the 9-bromoanthracene whatever the olefin (8). This was attributed to the large steric hindrance of this substrate.

A classical Hansch approach and an artificial neural networks approach were applied to a training set of 32 substituted phenylpiperazines characterized by their affinity for the 5-HTiA-R and the generic arAR [91]. The study was aimed at evaluating the structural requirements for the 5-HTiA/ai selectivity. Each chemical structure was described by six physicochemical parameters and three indicator variables. As electronic descriptors, the field and resonance constants of Swain and Lupton were used. Furthermore, the vdW volumes were employed as steric parameters. The hydrophobic effects exerted by the ortho- and meta-substituents were measured by using the Hansch 7t-ortho and n-meta constants [91]. The resulting models provided a significant correlation of electronic, steric and hydro-phobic parameters with the biological affinities. Moreover, it was inferred that the... 

The results, presented in Table 8, show that in most cases the conformer with the lowest steric energy indeed corresponds to the experimentally most favored one. In addition, several molecules containing the N—C—N moiety were retrieved from the Cambridge Structural Database and calculated with the new parameter set. A comparison between MM2 and X-ray geometries (selected structural parameters only) for two conformers of 1,4,5,8-tetraazadecalin (25, 26) is provided in Table 9 and shows good fit between the experimental and calculated data. 

Dienophiles may contain more than one double or triple bond. This might result in multiple product formation, but in most instances the diene will attack one bond with high site specificity. This site selectivity is often controlled by substitution patterns and electronic or steric parameters. 

The most important parameter for the control of liquid chromatography is the composition of the eluent. Liquid chromatography is a powerful separation method with unlimited possibilities of eluent selection. However, it is not easy to choose a suitable eluent within a short time without a number of trial experiments. The crucial factor is to control the solubility of the analytes in the eluent. Increasing the solubility of analytes in the eluent decreases their retention times. The selection of the components of an eluent is described below, based on the properties of the analytes to be separated. The important properties are hydrophobicity, dipole moment, hydrogen bonding, ionization, and steric effects. 

Diastereoface selection has been investigated in the addition of enolates to a-alkoxy aldehydes (93). In the absence of chelation phenomena, transition states A and B (Scheme 19), with the OR substituent aligned perpendicular to the carbonyl a plane (Rl = OR), are considered (Oc-or c-r transition state R2 Nu steric parameters dictate that predoniinant diastereoface selection from A will occur. In the presence of strongly chelating metals, the cyclic transition states C and D can be invoked (85), and the same R2 Nu control element predicts the opposite diastereoface selection via transition state D (98). The aldol diastereoface selection that has been observed for aldehydes 111 and 112 with lithium enolates 99, 100, and 101 (eqs. [81-84]) (93) can generally be rationalized by a consideration of the Felkin transition states A and B (88) illustrated in Scheme 19, where A is preferred on steric grounds. 

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