Dicationic species, protosolvated

The reaction of the parent compound did not take place in trifluoroacetic acid (TFA), but did proceed on the addition of triflic acid to give the alkylated product in low yield (Table 5.24). In neat triflic acid the reaction is faster and the product is formed in much higher yield. Similar changes were observed with the other 1,3-diphenylpropanones. Substituents on the benzoyl group also induce significant differences in reactivities (compare the results for R = Me and CF3 in Table 5.24). These observations suggest that the protosolvated dicationic species 79 is involved in the rate-determining step of cyclization (Scheme 5.35). 

In mixed acid and related acidic solutions the formation of the nitronium ion can, indeed, be considered to proceed through a dicationic protosolvated species, which in the absence of a suitable nucleophile deprotonates to the more stable nitronium ion. 

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. ... 

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