1,2-Dicarbonyl compounds reacted with benzene in the presence of a strong acid, trifluoromethanesulfonic acid, to give gem-diphenylated ketones in high yields. The reaction was accelerated when the acidity of the medium was increased, supporting involvement of O,O-diprotonated 1,2-dicarbonyl species (i.e., 1,2-dihydroxyethylene dications) as the active electrophiles.
1,2-Dicarbonyl compounds such as 2,3-butanedione reacted with benzene in the presence of a strong acid, trifluoromethanesulfonic acid, to give gem-diphenylated ketones in high yield. The monoxime derivative of 1,2-diones also reacted with benzene in the acid, but slowly enough to allow discrimination of monophenylated and diphenylated oximes. The reactive electrophiles in these Friedel-Crafts reactions are considered to be O,O-diprotonated 1,2-dicarbonyl species and N,O-diprotonated ketoxime, i.e., related ethylene dications differentiated by a single different heteroatom substituent, a hydroxy or a hydroxyamino group. The reactivity depends on the electron-donating ability of substituents adjacent to the reaction center.
Kulp, Stuart S.; Romanelli, Anthony, Organic Preparations and Procedures International, 1992, vol. 24, # 1, p. 7 - 12