Triflic acid superelectrophiles

Superelectrophilic dications 50 have been observed by Coustard,197 who used 1H and 13C NMR spectroscopy in triflic acid at low temperature (Scheme 5.25). They... 

Prakash, Olah, and co-workers256 have prepared Mosher s acid analogs by the hydroxyalkylation of substituted benzenes with ethyl trifluoropyruvate [Eq. (5.95)]. Deactivated aromatics (fluorobenzene, chlorobenzene) required the use of excess triflic acid indicative of superelectrophilic activation.3 5 In contrast to these observations, Shudo and co-workers257 reported the formation gem-diphenyl-substituted ketones in the alkylation of benzene with 1,2-dicarbonyl compounds [Eq. (5.96)]. In weak acidic medium (6% trifluoroacetic acid-94% triflic acid), practically no reaction takes place. With increasing acidity the reaction accelerates and complete conversion is achieved in pure triflic acid, indicating the involvement of diprotonated intermediates. 

Olah et al.450 have recently described a new, highly efficient superelectrophilic formylation-rearrangementofisoalkanes. Branched ketones are formed in high yields and with high selectivity with no detectable branched acids (Koch products) in the presence of moderately strong superacids such as HF-BF3 or triflic acid-BF3. Carbonylation of isobutane under such conditions gives isopropyl methyl ketone in high yield [Eq. (5.164)] The transformation was interpreted with the involvement of... 

Knorr cyclization of a range of iV-susbtituted butyramides in triflic acid yields 4-methyl-17/-quinolin-2-one derivatives704 [Eq. (5.257)]. Suggested intermediates of the transformation directly observed by low-temperature multinuclear NMR spectroscopy (HSO3F SbF5 S02C1E, 40°C) are distonic superelectrophiles formed by diprotonation of the two carbonyl oxygen atoms. 

Diprotonated, superelectrophilic intermediates were suggested to be involved in both conversions. Considering protonated aldehydes, benzal-dehyde gives a carboxonium ion that is significantly resonance stabilized and thus unreactive towards aromatic substrates such as o-dichlorobenzene or nitrobenzene. Pyridinecarboxaldehydes, however, show much higher electrophilic reactivities due to their ability to form via TV-protonation the superelectrophile (5, eq 8).10 A similar situation is seen in the hydroxyalkylation reactions of acetyl-substituted arenes. Acetophenone is fully protonated in excess triflic acid, but the resulting carboxonium ion (6) is... 

Although electrophilic reactions involving dications with deactivated arenes may suggest the formation of superelectrophilic intermediates, there are a number of well-known examples of monocationic electrophiles that are capable of reacting with benzene or with deactivated aromatic compounds. For example, 2,2,2-trifluoroacetophenone condenses with benzene in triflic acid (eq 12).13 A similar activation is likely involved in the H2SO4 catalyzed reaction of chloral (or its hydrate) with chlorobenzene giving DDT (eq 13). 

Superacidic FSO3H (fluorosulfonic acid, Ho — 15) has also been used in some studies involving superelectrophilic activation. However, due to its tendency for sulfonation and oxidation, this acid has found only limited use in synthetic conversions involving superelectrophiles. Fluorosulfonic acid has been shown effective to activate nitronium salts in their reactions with weak nucleophiles, and again it was suggested that the protosolvated species (6) is involved in the reactions.28 Both fluorosulfonic acid and triflic acid have been reported to give the diprotonated species (14) from 3-arylindenones (eq 12) 29... 

Like the trialkyloxonium superelectrophiles, the salts of trimethyl sul-fonium (CH3)sS+, selenonium (CEL Se"1", and telluronium (CE Te4" ions have also been shown by Laali et al. to undergo superelectrophilic activation.41 These onium salts methylate toluene in FSChH-SbFj, but with the weaker Bronsted superacid CF3SO3H (triflic acid, //q —14.1), no methylation takes place (eq 12). 

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. ... 

Carboxonium ions are highly stabilized by strong oxygen participation and therefore are much less reactive compared to alkyl cations. However, under the superelectrophilic solvation by triflic acid, the Friedel-Crafts-t)q)e reactions still can occur via a protosolvated reactive intermediate. For example, 1-phenyl-2-propen-l-ones can be readily transformed into 1-indanones in good yields through triflic acid-catalyzed reaction (eq SO). ... 

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