Structures and Reactivities of Ethylene Dication 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.