- N- and C-attacks on aromatics of phenylnitrenium ion generated from N-phenylhydroxylamine in the presence of trifluoroacetic acid containing polyphosphoric acid or trifluoroacetic anhydride
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A phenylnitrenium ion formed from N-phenylhydroxylamine in the presence of trifluoroacetic acid (TFA) containing polyphosphoric acid (PPA) interacts with the counterion -O2CCF3 and the unshared electron-pair of H2O, showing Hammett's ρ values -5.2 and -4.0 for N- and C-attacks on aromatics PhX (X = H, Me, Et, Ph and Cl), respectively. The ratio N-/C-attack for this nitrenium ion is lower than for the nitrenium ion interacting with only the counterion; the latter nitrenium ion is generated by the use of trifluoroacetic anhydride (TFAA) instead of PPA or by reaction of phenyl azide with aromatics in TFA. The ratio for the former nitrenium ion is affected by the aromatic substituent X:X = Me > X = Et > X = Ph > X = H > X = OMe at 20 and 50°C. The order for X = Cl is between those of X = H and X = Ph at 20°C, but highest at 50°C. The ratio is increased by higher reaction temperature and by decreased concentration of TFA. The results are demonstrated from the mechanistic viewpoint for the nitrenium ions.
- Takeuchi, Hiroshi,Taniguchi, Tomohito,Ueda, Takahiro
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p. 295 - 300
(2007/10/03)
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- Acid-Catalyzed Reactions of N-Arylhydroxylamines and Related Compounds with Benzene. Iminium-Benzenium Ions
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N-Arylhydroxylamines react with benzene in the presence of trifluoroacetic acid (TFA) at room temperature to give diphenylamines.When TFA was replaced by a strong acid, trifluoromethanesulfonic acid (TFSA), the major products were aminobiphenyls.The nature of the reaction was explored by reactions of 4-substituted phenylhydroxylamines and dialkylaniline N-oxides with benzene.Thus, it was demonstrated that the reactive intermediates are onium-benzenium dications which are trapped by benzene to give aminobiphenyls by a mechanism similar to the Friedel-Krafts alkylation.Further evidence for the proposed reaction mechanism was the observation that nitrosobenzene and azoxybenzene reacted with benzene to give analogous products in the presence of the stronger acid.
- Shudo, Koichi,Ohta, Toshiharu,Okamoto, Toshihiko
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p. 645 - 653
(2007/10/02)
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