Electrophilic solvation

In these solvents at sufficiently low Br2 concentration (< 10-3 m) the kinetics are first order both in the olefin and in Br2 and the main solvent effect consists of an electrophilic solvation of the departing Br ion. A nucleophilic assistance by hydroxylic solvents has also been recognized recently (ref. 26) (Scheme 10). So far, return during the olefin bromination in methanol had been admitted only for alkylideneadamantanes, and was ascribed to steric inhibition to nucleophilic attack at carbons of the bromonium ion (ref. 26). 

In contrast, in protic solvents and at low bromine concentration, the addition process is characterized by a second order rate law (first order in bromine), Scheme 2, path b. In this case, due to the ability of the solvent to provide a specific electrophilic solvation to the leaving bromide ion, the reaction occurs via an SN1 -like unimolecular ionization of the 1 1 it complex to form a bromonium or P-bromocarbenium bromide ion pair. It is worth noting that protic solvents can also give nucleophilic assistance, depending on their specific solvent properties. 

However, the factors of electrophilic solvation and unexpected molar volume have the little influence too. The dependence of IgQ on the solvent property can be satisfactory described by the two-parametric equation too and... 

Significant factor as same as in a case of swelling degree is the solvents basicity. With the solvents basicity increasing, the process rate is also increased. The less essential is a role the solvents ability to electrophilic solvation although this factor increases the process rate but it exclusion from the consideration decreases R till 0,928. The value IgQ calculated in accordance with the equation (15) is represented in Table 3. 

Changes in intramolecular selectivity in the bromination and nitration of alkyl-benzenes in acidic media have been attributed to changes in medium polarity or changes in electrophile solvation. Mass spectrometric studies of the first stage in the gas-phase reactions of halobenzenes, furan, thiophene and pyrrole with alkyl cations have been rationalized in terms of co-existing a- and tt-complexes. The extent of... 

Heterolysis rates of r-butyl bromide, 1-bromo-l-methylcyclohexane, and 2-bromo-2-methyladamantane increase in the order of solvents MeCN < y-butyrolactone < sulfolane, but heterolysis rates of 2-bromo-2-phenyladamantane decrease in the same order of solvents. The observed effects are considered to be caused by superposition of dipolar and electrophilic solvations. 

In most of the examples of superelectrophilic reactions involving Lewis acids, they are conducted using an excess of the Lewis acid. This is in accord with electrophilic solvation by the Lewis acid, i.e. activation of the electrophile requires interaction with two or more equivalents of Lewis acid. As an example, superelectrophilic nitration can be accomplished with NO2CI and at least three equivalents of AICI3 (eq 23).46 This powerful nitrating reagent involves a superelectrophilic complexed nitronium ion (33). 

Scheme 10. Electrophilic solvation of the acetyl cation and reactions with alkane.

When nitronium tetrafluoroborate was attempted to react with the trityl cation in CH2CI2 or sulfolane, no nitration occurred due to the deactivating effects of the carbenium ion center in 215. Nitration of deactivated substrates is also readily accomplished by reaction with NO2CI with three mole excess AICI3 suggesting Lewis acidic electrophilic solvation of the nitronium cation (217, eq 62).105... 

Our book is about the emerging field of Superelectrophiles and Their Reactions. It deals first with the differentiation of usual electrophiles from superelectrophiles, which show substantially increased reactivity. Ways to increase electrophilic strength, the classification into gitionic, vicinal, and distonic superelectrophiles, as well as the differentiation of superelec-trophilic solvation from involvement of de facto dicationic doubly electron deficient intermediates are discussed. Methods of study including substituent and solvent effects as well as the role of electrophilic solvation in chemical reactions as studied by kinetic investigations, spectroscopic and gas-phase studies, and theoretical calculations are subsequently reviewed. Subsequently, studied superelectrophilic systems and their reactions are discussed with specific emphasis on involved gitionic, vicinal, and distonic superelectrophiles. A brief consideration of the significance of superelectrophilic chemistry and its future outlook concludes this book. 

Consequently there appears to be a sound empirical basis for the use of the OTs scale of solvent ionizing power. Its use should be restricted to sulphonates, however, because of the differential effects of electrophilic solvation in acidic solvents (see Section 4). The importance of these effects can be seen by comparing the Y and Iqxs values for carboxylic acids (Table 5) it appears that, relative to 80% ethanol/water, a carboxylic acid ionizes a tosylate about ten times more rapidly than a chloride. 

Because of the different effects of electrophilic solvation of the various negative charges (i.e. Cl- for Y, "OTs for Tots I" for Z, 0 for Ej), direct comparisons between the various scales should be done cautiously. A wide variety of correlations giving clear indications of trends, has been reported by Reichardt and Dimroth (1968), but the significance of a recent general survey of scales of solvent polarity is doubtful, because of the many parameters used in the correlations (Fowler et al., 1971). The multi-parameter approach has also been adopted and reviewed by Koppel and Palm (1972). 

Inevitably this specific electrophilic solvation must be included in the Y, Z and T scales of solvent ionizing power. Ideally some correction for the different extents of solvation of the anions Cl-, OTs-, I-) should be made, and then the various scales of solvent polarity may be more comparable. 

Probably both reactant stabilization and the already evaluated relative instability of the cationic transition state contribute to the slowness of the solvolysis of vinyl components, but other factors are certainly involved. The most obvious experimental problem is whether the compounds compared react by a unimolecular mechanism or nucleophilic attack by the solvent is involved to a certain extent. In the case of vinylic systems, for instance, nucleophilic solvation from the rear is in general much more hindered than in the case of saturated compounds and the transition state is likely to be stabilized only by electrophilic solvation of the leaving group (Rappoport and Atidia, 1970). The low m values observed in the case of vinyl halides or sulphon-ates may be taken as a strong indication of poor solvation of the transition state in solvolytic reactions of vinyl derivatives. These and other complications, such as differences in hyperconjugation, differences in electronegativity of the -—C= and —- bonds (Jones and Maness,... 

It has been shown that in the case of bromine addition to 1-pentene in solvents of different polarity, the overall rate constant varies by a factor of 10 ( ) [81]. This dramatic solvent effect has been taken - together with other findings - as strong evidence for the so-called AdnCl-mechanism, which involves considerable charge separation in the activation step. It has also been demonstrated that protic solvents enhance this addition by a specific electrophilic solvation of the anionic part of the activated complex... 

For the analysis of SN1 solvolyses, Abraham et al. (9) have proposed an equation (equation 3) based on sensitivities toward solvatochromatic properties. In equation 3, tr is a measure of solvent dipolarity-polarization, a is a measure of solvent hydrogen bond donor acidity, and P is a measure of solvent hydrogen bond acceptor basicity. More recently, a term governing cavity effects has been added, and this term is considered to represent an important contribution (10, 11). The cavity term can be directly related to the square of the Hildebrand solubility parameter (10-12). A similar analysis by Koppel and Palm (13, 14) involves terms governed by solvent polarity, solvent polarizability, electrophilic solvation ability, and nucleophilic solvation ability. Recently, a cavity term has also been added to this analysis (12). 

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