Polar effect of remote substituents

The polar effect of remote substituents on intrinsic rate constants is qualitatively similar to that in proton transfer, irrespective of whether the aryl group is attached to the a-carbon (e.g., 66) or the p-carbon (67) in both cases the partial negative charge in the transition state is closer to the... 

The Kochi reaction has also been used to demonstrate polar effects of remote substituents upon free-radical stereoselectivities in chlorine atom transfer. ... 

Polar Effects of Remote Substituents. Even though the identity of X and Y is the major factor in determining k0, intrinsic rate constants show a small dependence on the remote substituent Z in IV or VI. In analogy to the situation in proton transfers (8), the effect of Z on log k0 can be expressed by... 

The effect of remote substitutents on the complexing ability of crown ethers [256] towards t-butylammonium salts has been studied by Moore et al. (1977). The results (Table 52) show that electron-attracting substituents decrease the stability of the complex. The t-BuNHJ cation is even more sensitive to remote substituent effects than alkali-metal cations. This fact was attributed to the non-polar character of the alkyl group and to the different structure of the... 

One of the consequences of the imbalanced nature of the transition state is that the polar effect of a remote substituent may either increase or decrease the intrinsic barrier whether there is an increase or decrease depends on the location of the substituent with respect to the site of charge development. Let us consider a reaction of the type shown in Equation (4). In this situation an electron-withdrawing substituent Z will decrease AG or increase ka. This is because there is a disproportionately strong stabilization of the transition state compared to that of the product anion due to the closer proximity of Z to the charge at the transition state than in the anion. As discussed earlier, this also leads to an exalted BrlAnsted aCH value and is the reason why aCH > Pb for the deprotonation of carbon acids such as 11-13 and others (Table 2). 

There are other factors that can affect AG and kQ they do so not because they inherently develop nonsynchronously relative to bond changes but because of delayed charge delocalization. They include the effects of remote as well as adjacent polar and polarizable substituents, of remote and adjacent charge, and of substituents that impede charge delocalization by steric crowding. They are summarized in Table 27. 

The effect of remote polar substituents was also probed with substituted 7-methylenenorbornanes. The substituents are located in the endo position and cannot interact directly with the approaching reagent. Ester groups are strongly syn... 

A computational study has probed the effect of remote polar substituents on the... 

Closely related to the inductive effect and operating in the same direction is the field effect In the field effect the electronegativity of a substituent is communicated not by successive polarization of bonds but via the medium usually the solvent A substituent m a molecule polarizes surrounding solvent molecules and this polarization is transmit ted through other solvent molecules to the remote site... 

For more remote substitution p is expected to be smaller. Thus Hay and Porter-28 found p = 0.6 for the alkaline hydrolysis of the methyl esters of a-amino acids, RCH(NH2)COOCH.3 in water at 25°C. However, higher sensitivities are possible if the geometry of the system is such that the direct transmission of polar effects becomes more important. For example, Roberts and Moreland245 found that the effects of substituents in the 4 position of ethyl bicyclo-[2.2.2]octane-l-carboxylates (28) on the hydrolysis in 88% ethanol at 30°C are comparable to those observed for meta- anti para-substituted benzoates (p = 2.24), and a similar, though smaller, effect, is observed for methyl rrarts-4-substituted cyclohexanecarboxylates (29). 

In contrast, the need to evaluate the relative rates of competing radical reactions pervades synthetic planning of radical additions and cyclizations. Further, absolute rate constants are now accurately known for many prototypical radical reactions over wide temperature ranges.19,33 3S These absolute rate constants serve to calibrate a much larger body of known relative rates of radical reactions.33 Because rates of radical reactions show small solvent dependence, rate constants that are measured in one solvent can often be applied to reactions in another, especially if the two solvents are similar in polarity. Finally, because the effects of substituents near a radical center are often predictable, and because the effects of substituents at remote centers are often negligible, rate constants measured on simple compounds can often provide useful models for the reactions of complex substrates with similar substitution patterns. 

The effect of a remote substituent on the facial selectivities in a nucleophilic conjugate addition has been investigated for the reaction of EtSH with a series of dibenzobicyclo[2.2.2]octatrienes (84). Syn-addition proved to be favoured for nitro substituent and polar solvents increased the selectivity (Table 5).19... 

Stereoelectronic effects that cannot be viewed as dipole-dipole interactions most probably operate in many concerted epoxidations, however, they are mostly difficult to identify unambiguously because their influence is modest and easily masked by steric or polar effects they can be visualized in Hammett-type correlations by a series of compounds differing only in remote substitution125-127 or substitution with substituents in the plane of the double bond208. 

Direct fluorination at saturated carbon is possible with either elemental fluorine or CF30F and a radical inhibitor.57-8 The transformation is an electrophilic fluorination, regiospecific by virtue of the highly polar transition state sensitive to the inductive effects of nearby or remote polar substituents. Substitution occurs almost exclusively at tertiary... 

The greater positive character hence the increased acidity of the O—H proton of 2 2 2 tnfluoroethanol can be seen m the electrostatic potential maps displayed m Figure 1 8 Structural effects such as this that are transmitted through bonds are called indue tive effects A substituent induces a polarization m the bonds between it and some remote site A similar inductive effect is evident when comparing acetic acid and its trifluoro derivative Trifluoroacetic acid is more than 4 units stronger than acetic acid... 

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