Friedel-Crafts-type reactions involving di- and tricationic species. Onium-allyl dications and O,O-diprotonated aci-nitro species bearing a protonated carbonyl group

Article abstract of DOI:10.1021/ja00004a044

Stable carbocations do not react with nonactivated benzenes. For example, acetophenone does not react with benzene in the presence of trifluoromethanesulfonic acid (TFSA), while trifluoroacetophenone does do so under acidic conditions owing to activation of the electrophilicity of the hydroxycarbenium cation by the trifluoromethyl group. This and other studies suggest that an electron-withdrawing substituent on the cationic center increases the reactivity toward benzenes. In this paper, involvement of multiply positively charged (dicationic and tricationic) species, which have sufficient electrophilicity toward benzene, is demonstrated in the acid-catalyzed reactions of cinnamaldehyde and its derivatives and also in the acid-catalyzed reactions of nitromethanes. The species formed from cinnamaldehyde, cinnamaldimine, cinnamaldoxime, and their derivatives in TFSA or TFSA-SbF₅ have an adequate reactivity toward benzene. O-Protonated cinnamaldehyde and its derivatives, N-protonated cinnamaldimine, and N,N-dimethylcinnamaldiminium salt do not react with benzene. Since a strong acid catalyst is required for the reactions, participation of doubly protonated species, onium-allyl dications, is proposed. Ab initio calculations of (1) the donor-acceptor interaction energies of a neutral donor (such as water and ammonia) and a doubly charged allyl dication and (2) proton affinities demonstrated that the ammonium-allyl dication is more stable than the oxonium-allyl dication, in accordance with the experimental observation. Nitronic acids also react with benzene at the ipso position with respect to the nitro group to give the phenylated oximes in the presence of TFSA. The reaction with benzene is not catalyzed by trifluoroacetic acid, which is sufficiently acidic to monoprotonate a nitronic acid to the protonated aci-nitro form. The reaction requires a stronger acid, trifluoromethanesulfonic acid, suggesting intervention of the dication formed by O,O-diprotonation of acinitroalkanes rather than the monoprotonated aci-nitroalkane. As a result of further study on the phenylation reactions, we found a facile phenylation reaction of nitromethanes substituted with an electron-withdrawing group, catalyzed by TFSA, to give phenylated α-carbonyloximes in high yields. A triply positively charged cation, an O,O-diprotonated aci-nitro species bearing a protonated ethoxycarbonyl group, which can react with nonactivated benzene, is proposed to be an intermediate in this reaction.