Diprotonated dications

It was shown in earlier studies that dihydropyrene 1 and its 2,7-di-tBu derivative 3 were both monoprotonated with FSO3H/SO2CIF at C-3. Diprotonation of 3 gave the 3,6-diprotonated cation selectively, that of 1 produced primarily the 3,6-diprotonated dication. When IH+ was heated, it was oxidized to 2H+ (Fig. 3). This process was explained by equilibrium deprotonation at higher temperature and stepwise oxidation. 

Protonation of methylhalonium ions to form diprotonated dications CH3XH22+ have been calculated (B3LYP/6-31-G level)348 to occur primarily on the C—H bond to form a pentacoordinated carbon with 2e-3c bond. The halogen atoms in dications, as expected, carry more charge than in the corresponding monocations and the dications are less stable than the monocations (49.5-13.Okcal mol-1). 

CF3S03H2+B(0S02CF3)4 ] to give triphenylmethane (Scheme 5.28). Experimental evidence supports the involvement diprotonated benzaldehyde in the reaction. Calculations showed (MP2/6-31G7/MP2/6-31G +ZPE level) that the reactive intermediate is the 0,C(aromatic)-diprotonated dication 62 and not the 0,0-diprotonated 63 dication, which is less stable by 20.6kcalmol 1. 

Deprotonation of porphyrins is possible with strong bases (e.g., sodium hydride) in pyridine or other polar, nonprotic solvents (not shown) (Falk, 1964). The corresponding sodium salts are formed, which produce similar two-banded visible spectra as the diprotonated dications. Little is known about the dianion chemistry. They cannot, however, be oxidized to dehydroporphyrins with only 16 7C-electrons in the inner conjugation pathway. Such a porphyrin would be antiaromatic and appear as a triplet state. It has not been detected so far. 

More recently, Olah (1993) postulated that the equilibrium between the methanediazonium ion (2.4) and the methylenediazenium ion (2.5) in superacid media is not a deprotonation-reprotonation process via diazomethane as shown in (2-2), but involves the diprotonated dication (H3C —I =I -H). So far, however, there is no experimental evidence nor a theoretical basis available for this dication. 

Carbon monoxide, hydrogen cyanide, and nitriles also react with aromatic compounds in the presence of strong acids or Friedel-Crafts catalysts to introduce formyl or acyl substituents. The active electrophiles are believed to be dications resulting from diprotonation of CO, HCN, or the nitrile.64 The general outlines of the mechanisms of these reactions are given below. 

Protonated amides and carboxylic acids have also been shown to activate adjacent electrophilic centers. Although protonated carboxylic acids and amides are not typically considered stable cationic groups, in superacidic media they can be readily generated and many have been observed by spectroscopic studies.16 As an example of electrophilic activation by a protonated carboxylic acid, P-phenylcinnamic acid (35) is diprotonated in super- acid to give the dication (36, observable by NMR) which then reacts with benzene and gives the indanone (37) in good yield (eq 12).17 It is known that the 1,1-diphenylethyl... 

Ohwada, T. Yamagata, N. Shudo, K. Friedel-Crafts-Type Reactions Involving Di- and Tricationic Species. Onium-Allyl Dications and 0,0-Diprotonated aci-Nito Species Bearing a Protonated Carbonyl Group. J. Am. Chem. Soc. 1991,113,1364-1373. 

The mono- and diprotonated carbocations and the two-electron oxidation dications derived from parent pyrene and its nonalternant isomers azupyrene (dicyclo-penta[e/, /]heptalene) (DCPH) 72 and dicyclohepta[eJ,g /z]pentalene (DCHP) 73 were studied at the B3LYP/6-31G(d) level (Fig. 29). The most likely site(s) for mono-and diprotonation were determined based on relative arenium ion energies and the structures of the energetically most favored carbocations were determined by... 

Several 1,4-disubstituted derivatives of the dication, 63, were successfully prepared. The 1,4-diphenyl, 1,4-dimethyl, 1,4-dicyclopropyl-substituted derivatives of carbodication 63 (65, 66, and 67) are exceptionally stable. The 1,4-dimethyl-1,4-cyclohexyl dication, devoid of the adjacent cyclopropyl groups, could not be prepared. The carbocationic center in all these dications are somewhat shielded as compared to their monocations. The C NMR chemical shifts of the carbocationic centers for the dications 65, 66, and 67 are 235.4, 293.4, and 260.8 ppm, respectively. The diprotonated ann -tricyclo(5.1.0.0 )octa-2,6-dione, 68, may be treated as a dicarboxonium ion, instead of dihydroxy dicarbenium ion, since the cabronyl carbon is shielded by only 25.2 ppm, much smaller than that observed for the protonated cyclohexanedione (34 ppm). The para carbons of the phenyl substituents in carbodication 65 are relatively shielded by about 5 ppm from that of the parent l,4-diphenylcyclohexane-l,4-diyl dication showing relatively less delocalization of the charge into the aromatic rings. 

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... 

An X-ray crystal structure of annulene-dione (64) indicates an anti,anti configuration between the methylene and sulfur bridges.102 Diprotonation gives highly localized positive charges in the dication (65), mainly due to unfavourable p-orbital overlap. 

Diprotonation of (97) with H2SO4 gives a species which is protonated at both carbonyl groups, not the delocalized annulene dication, presumably for steric reasons.140 Oxidation dications of perylenes, e.g. (98), are [4n + 2] species with diamagnetic ring currents, whereas the dibcnzo[ /. // ] derivative [two more benzene rings at the top and... 

Lammertsma and Cerfontain411 have obtained the cyclopropyldicarbinyl dication 197 by diprotonating the l,6-methano[10]annulene in much stronger Magic Acid at 60°C [Eq. (3.49)]. If the same protonation was carried out at — 120°C, they were able to obtain the previously discussed monocation 106.284... 

Lammertsma412 has also obtained dication 198 by the protonation of hexahydro-pyrene in Magic Acid solution. This is the first example of a p,p -diprotonated naphthalene derivative. Koptyug and co-workers270,413 have studied a,a -diproto-nated hexa- and octamethylnaphthalenes 199 under superacidic conditions. 

Protonation and diprotonation of formic acid and acetic acid have also been studied theoretically (HF and MP2/6-312G levels).553,558 Both cations can be visualized as the donor-acceptor complex of H20 and diprotonated carbon monoxide (HCOH2+) and protonated acetyl cation (CHaCOH23"), respectively. Both dications 277 and 278 are characterized by a long C-OH2 bond and a shorter C-OH bond, suggesting that the latter retains substantial double-bond character. 

The structure of alkenoyl cations (unsaturated acylium ions) was studied by Olah et al.658 by NMR spectroscopic methods. They found only a limited contribution from structure 343b and a substantial contribution of the delocalized ketene-like structure 343c, which is due to the ability of the Jt-electrons of the double bond to stabilize the positive charge. Substitution at the (3-carbon increases further the importance of 343c relative to 343a. Diprotonation of propenoyl and isopentenoyl cations studied theoretically [ab initio GIAO-CCSD(T) method]659 has been shown to result in the formation of dication 344 (tertiary carbenium-acylium dication) that is, the positive charges are localized primarily on CO and the (3-carbon. 

Olah and coworkers have used ab initio methods (MP2/6-311+G level) to calculate the structures of diprotonated superelectrophilic3-5 formaldehyde and thioformaldehyde dications.666 In both cases, the heteroatom-diprotonated structure... 

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