Polarizing influences

The polarizing influence of an electronegative atom decreases with the number of inteiwening rr-bonds. This is called the inductive effect and is indicated in Figure 3-6b by a progression of 6 symbols, (t is generally accepted that the inductive effect is attenuated by a factor of 2-3 by each intervening bond. The inductive ctlcct is not... 

TRISPHAT anion 8 seems to be more particularly efficient with cationic metallo-organic and organometallic substrates. BINPHAT 15 has often-supe-rior chiral shift properties than 8 when associated with organic cations such as ammonium cation 68 (Fig. 24). In all these examples, solvent polarity influences the quality of the separation since ion association is crucial. Solvent or solvent mixtures of low polarity are preferred for these experiments. 

There is also evidence that single, carbon-bound fluorine substituents, particularly when on an aromatic ring, can exhibit specific polarity influences, including H-bonding, that can strongly influence binding to... 

The trifluoromethyl group has become an important structural component of many bioactive compounds, mainly because of its polar influence and its effect on the lipophilicity of compounds. 

It has been shown that the SF5 group has a similar but often enhanced effect on the lipophilicity of compounds, and on essentially any physical or chemical property that derives from its strong polar influence. It also has the advantage of greater hydrolytic stability than a trifluoromethyl group. 

At this early stage of comprehension of the interrelation between metathesis and cyclopropanation, many questions remain. Why is the formation of cyclopropanes such a rare occurrence with typical metathesis catalysts, yet favored with some zero-valent carbene complexes What is the role of prior complexation of the olefin with the metal in determining the reaction course for metal-carbene species How are typical metathesis carbenes polarized, and how does this polarization influence selectivity of metathesis reactions (e.g., regenerative metathesis of a-... 

We see that the ester peroxyl radical is nearly five times more active than the cumyl peroxyl radical. Two reasons are possible for such a difference difference in the BDE of the formed O—H bond and polar influence of the ester group on the polar transition state. One can suppose that the BDE of the formed O—H bond in ester hydroperoxide is more than that of... 

Qualitatively, aN may be considered to vary with solvent because solvent polarity influences the balance between the contributions from structures [5a] and [5b]. The effect of substituents on this balance contributes to the substituent-dependence of aN (Janzen, 1969). 

F. Perrin Theory of fluorescence polarization (influence of viscosity)... 

Figure 19. Multifunctional nature of quinine as a catalyst. The various parts of the molecule fill the following roles (a) hydrogen bond forming ligand forming with metals (b) basic amine (c) aliphatic hydrocarbon—bulk (d) handle to modify (e,f) epimers available few conformers (g) aromatic hydrocarbon—bulk, polarizable (h) handle to modify steric and polar influence.

This high sensitivity to polar effects of the homolytic alkylation of protonated heteroaromatic bases has been interpreted in terms of the transition state.This would be similar to a w-complex in which an enhanced contribution of polar forms (5) would explain the high sensitivity to polar influence. 

In summary, none of the observations made to date would be inconsistent with the concept of a concerted but nonsynchronous cycloaddition in which polar influences are of great importance in determining regiospecificity and in some cases stereospecificity. 

Correspondin y the linear plots for E, and E2 in Fig. 1 converge since pf = 0.256 is nearly twice as large as pf = 0.151. Thus the redox step RED/SEM is much more sensitive to substituent effects than SEM /OX . This result is plausible since the polar influence of a substituent will be dominated by the larger amount of positive charge in the higher oxidation levels of 1. 

H bond and polar influence of the ester group on the polar transition state. One can suppose that the BDE of the formed —H bond in ester hydroperoxide is more than that of... 

The methylene group of a-amino acid metal complexes such as the copper(II) glycinato complex is activated to some extent by the polarizing influence of the metal.46,47 Consequently, carban-ion-type reactions can be carried out at this position, while the amino and carboxyl groups are protected by coordination. These reactions cannot be performed on either the free ligand or the conjugate base. 

Another reason for the apparent inconsistencies in experimental data is that, depending on the nature and position of the substituent, steric and conjugative effects often outweigh polar influences. In order to study pure polar effects, Holtz and Stock have carried out rate studies of displacements by thiophenoxide ion on 4-Z-bicyclo-[2.2.2]-octylmethyl toluenesulfonate (Equation 4.17).37... 

Solvent polarity influences the rate of copolymerization. Thus with increasing dielectric constant of the solvent, the copolymerization rate rises as a result of the increase in the dissociation constants of the active species. The apparent rate constant for the copolymerization of 2-hydroxy-4-(2,3-epoxypropoxy)benzophenone with phthalic anhydride, initiated by hexadecyltrimethylammonium bromide56), increases from 4.65 x 10 4 s 1 in o-xylene to 6.84x 10 4 s-1 in nitrobenzene. Hilt et al.S4) proposed a suitable model illustrating the effect of solvent polarity in the copolymerization of phthalic anhydride with ethylene glycol carbonate in a mixture of nitrobenzene and trichlorobenzene (Table 4). With increasing fraction of the more polar nitrobenzene, the rate of copolymerization increases. 

Within the framework of the bond polarization the shift components of the unpolarized bond and the parameter Aai giving the polarization influence on the chemical shift are determined empirically from solving a set of linear equations 1 for a number of substances where both the chemical shift tensor and the molecular structure are known. The bond polarization energies Vai are calculated as effect of surrounding net atomic charges qx on atomic hybrids %. With the bond polarity parameter the polarization energy can be calculated. 

Solvent polarity influences the dissociation of the intimate ion pair, while the fate of the ion-radicals depends mainly on the solvent basicity. In non-basic solvents (benzene or dichloromethane), cationic polymerization of VCZ takes place. In moderately basic solvents (acetone or acetonitrile), VCZ cyclodimerizes. Radical polymerization occurs along with cycloaddition in strongly basic solvents, such as DMF and DMSO, while only radical polymerization takes place in the extremely basic solvent hexamethylphosphoric triamide (HMPA). 

Figure 3.4 Order of increasing nudeophilicity of halide ions is influenced by polarizing influences such as solvent effects.

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