Cyclizations, superelectrophilic products

Superelectrophilic intermediates were also proposed in the cyclization reaction of Q -(methoxycarbonyl)diphenylmethanol (37) with superacid (eq 24).25 The yield of cyclized product is found to increase dramatically with increasing acidity of the reaction medium. The conversion is believed... 

Several lines of evidence in the studies of Ohwada and Shudo support the involvement of these gitonic superelectrophiles. An analagous dicationic species (47) has been directly observed by 1H and 13C NMR at low temperature in both CFsSOsH-SbFs and CF3SO3H solutions. With dication 47, the NMR spectra show an equivalence of the aromatic rings suggesting a perpendicular relationship between the two cationic centers. The dications having the 4-methylphenyl substituents (61-62) have likewise been directly observed by NMR at low temperature, and upon warming the solutions, the expected cyclization products are formed (eqs 11-12). 

With the monocationic species, no fluorene cyclization is observed. However upon addition of CF3SO3H, the cyclization occurs almost quantitatively. This is consistent with formation of the protonated, dicationic intermediate (46) leading to the cyclization product (59). In this same study, it is noted that other stable monocationic 1,1-diarylethyl cations (i.e., the 1,1-diphenylethyl cation) do not readily form the fluorene ring system, indicating the importance of superelectrophilic activation. 

There have been two reports involving gitonic superelectrophiles composed of carboxonium ions and vinylic carbocations in a 1,3-relationship. In the reaction of 3-phenylpropynoic acid (65) with benzene in superacid the novel carboxonium-vinyl dication 66 is generated, followed by reaction with benzene and then cyclization (eq 22).26a Likewise, the unsaturated amide (67) gives the cyclization product in high yields (70-97%) in very strong acids (polyphosphoric acid, CF3SO3H, Nation SAC-13, or HUSY eq 23).30... 

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