Protonated iminium dications

Protonated iminium dications have also been studied by theoretical and gas-phase experimental studies.46 For the methyleniminium ion (95), the A-protonated species (96) was found to be a stable minimum. A second dicationic species was also located as a minimum, corresponding to the nonclassical structure 97, but it lies 27 kcal/mol above 96 on the potential energy surface (MP2/6-31G level). 

Okamoto and co-workers noted that N-phenylhydroxylamine gave predominately diphenylamine on treatment with benzene in TFA but mostly 4-aminobiphenyl and 2-aminobiphenyl in the stronger acid trifluoromethane-sulfonic acid (TFSA). Similar results were obtained if benzene was replaced by toluene or anisole. The authors suggested that the reaction in TFA proceeded through O-protonated hydroxylamine either via a direct Sn2 displacement on N by the aromatic nucleophile or via attack of the aromatic compound on the N of a nitrenium ion. In TFSA they favored a mechanism in which the diprotonated hydroxylamine lost water to generate an iminium-benzenium dication (11, Scheme 5), a protonated nitrenium ion. " This... 

As seen in the gitonic and vicinal systems, ammonium and related cationic centers may be components of superelectrophiles and reactive dications having the 1,3-dicationic structure. Several types of superelectrophilic aza-carbo dications have been studied in which protonated nitro groups are involved. For example, it was found that nitroethylene reacts with benzene in the presence of 10 equivalents of CF3SO3H to give deoxybenzoin oxime in 96% yield (eq 58).71 Since the reaction does not occur with only one equivalent of CF3SO3H, the formation of the /V./V -dihydroxy iminium-methyl dication 197 was proposed. In spectroscopic studies, the stable dication (199) can be directly observed by H and 13C NMR spectroscopy from solutions of l-nitro-2-methyl-l-propene (198) in CF3SO3H (eq 59). 

In a study of Troger s base derivatives 68 [R = H, Me RR = (CH2)3] in acid media (see Section II,B,3), Greenberg et al. found the dication of the dimethyl derivative (186, R = Me) to have nonidentical benzylic methylene groups and nonequivalent methyl NMR resonances. These results are consistent with the open iminium ion structure 69 (R = Me), and contrast with the results with Troger s base itself (68, R = H) and its spiro (cyclobutyl) derivatives 68 [RR = (CH2)3], in which the protonated closed structure 186 [RR = (CH2)3M] is favored (84JOC1127). 

The cation (26) was obtained by protonation of the corresponding azulene in dichloromethane. A tropylium ion-mediated a-cyanation of amines was described. The key step is a hydride transfer from the amine to the cation, resulting in cycloheptatriene and an iminium ion, the latter then reacting with cyanide to give the aminonitrile. The dehydrofropylium-Co2(CO)6 ion has been prepared as a BF4 salt. Various measures suggest that the ion is weakly aromatic, with about 25% of the aromaticity of the tropylium ion. Computational analysis of a number of annulenes predicts that the Mobius dication (CH)i4+ should be stable under persistent ion conditions. In particular, this dication is stable towards reactions such as cis-trans isomerization and electrocyclic rearrangement that limit the lifetime of other Mobius annulenes. 

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