Lewis acids, electrophilic solvation

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... 

The electron withdrawing inductive effects of the fluorine substituents render the carboxonium ion 3 more electrophilic than carboxonium ion 2, and consequently it reacts with benzene. Thus, the electrophilic reactivity of the carbonyl group can be greatly enhanced by Brpnsted or Lewis acid solvation and by substitution with electron withdrawing groups. 

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). 

Aubke has rationalised this approach by postulating that under superacidic conditions highly reactive, extremely electrophilic naked or very weakly solvated cations of low charge are formed (as shown in Sec. 11.3.4.2) and that they function as strong Lewis acids towards CO to form metal carbonyl cations in solution. Aubke and Wang [45] reviewed synthesis and spectroscopic characterisation of several of the compounds described briefly below. 

The position of this equilibrium depends on the electrophilicity or nucleophilicity of A and B , respectively, as well as the solvation capability of the surrounding medium. The solvent can influence the association as well as the electron-transfer step (or in the reverse reaction the ionization and dissociation step). The position of the Lewis acid/base equilibrium given in Eq. (4-30) will depend mainly on the differential solvation of the ionic and covalent species (a) and (b). 

Superelectrophilic Activation or Superelectrophilic Solvation. Trifluoromethanesulfonic acid (triflic acid, TfOH) has been extensively employed as a superacid Ho= —14.1) in superelectrophilic activation (or superelectrophilic solvation), both concepts advanced by Olah. Superelectrophilic activations may occur when a cationic electrophile reacts with a Bronsted or Lewis acid to give a dicationic (doubly electron-deficient) superelectrophile. However, it should be recognized that the activation may proceed through superelectrophilic solvation without necessarily forming limiting dicationic intermediates. The frequently used depiction of protosolvated species as their limiting dications is just for simplicity. ... 

The positive charge of the cationic solvation centre is partly delocalized over the pyridinium ring and shielded by the phenyl substituents. Therefore, the E,p values depend strongly on the electrophilic solvation power of the solvents, i.e. on their hydrogen-bond donor (HBD) ability or Lewis acidity, rather than on their nucleophilic solvation capability, for which the Donor Numbers of Gutmann (19) are the better empirical solvent parameters. 

As discussed above, measuring acidities in the gas phase allows a determination of intrinsic acidity without solvation effects. In many synthetic reactions, Lewis acids are used to increase the enolization of an aldehyde or ketone as well as to enhance electrophilicity of the carbonyl carbon. Although the acidity of the a carbon to a carbonyl will undoubtedly increase with carbonyl oxygen coordination to an electrophile, quantitative information has been lacking. 

Taking into account the fact that the solvation of ambident anions in the activated complex may differ considerably from that of the free anion, another explanation for the solvent effect on orientation, based on the concept of hard and soft acids and bases (HSAB) [275] (see also Section 3.3.2), seems preferable [366]. In ambident anions, the less electronegative and more polarizable donor atom is usually the softer base, whereas the more electronegative atom is a hard Lewis base. Thus, in enolate ions, the oxygen atom is hard and the carbon atom is soft, in the thiocyanate ion the nitrogen atom is hard and the sulfur atom is soft, etc. The mode of reaction can be predicted from the hardness or softness of the electrophile. In protic solvents, the two nucleophilic sites in the ambident anion must interact with two electrophiles, the protic solvent and the substrate RX, of which the protic solvent is a hard and RX a soft acid. Therefore, in protic solvents it is to be expected that the softer of the two nucleophilic atoms (C versus O, N versus O, S versus N) should react with the softer acid RX. 

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