Oxonium dication

In contrast to the vicinal-oxonium dications, vicinal-sulfonium dications are well known dicationic systems. Many examples have been prepared and studied. As discussed in review articles,91 the disulfonium dications can be readily prepared by electrochemical or chemical oxidation of appropriate disulfides (eq 66), by reactions of mono-S-oxides with acidic reagents (eq 67),... 

Another novel class of oxonium dications involves the superacid-promoted ring opening reactions of oxazolines and related conversions.61 Oxazolines are well known for their ability to react with strong nucleophiles (i.e. alcohols) when protonated at the ring nitrogen 62 It was shown that the dicationic species (161 or 162) are capable of reacting with weak nucleophiles such as benzene (eq 52). 

There have been several distonic superelectrophiles described in the literature that are oxonium-centered dications. For example, a series of diols were solvated in FSOsH-SbFs-SC and the dicationic species were persistent at —80°C.77 Upon warming to 25°C, the bis-oxonium ions undergo rearrangement to the more stable carboxonium monocations (eq 77). Barring a concerted mechanism, the transformations are thought to involve the carbo-oxonium dications (i.e., 225) and concomitant hydride shifts. Interestingly, 2,5-hexanediol ionizes in superacid, and with warming the cyclic oxonium ion (229) is formed (eq 78). 

Tetramethyl-2,4-pentanediol forms the oxonium dication 230 in superacid, and dehydration is followed by carbon- carbon bond cleavage. Although such cleavage reactions can occur with monocationic onium ions, in this case the cleavage reaction is likely an indication of the super-electrophilic nature of the dicationic intermediate(s). 

These species were prepared from the appropriate ethers in FSCFH-SbFs-SO2 solution at — 60°C. Like the protonated diols, these oxonium dications (i.e., 235) were observed to undergo cleavage reactions leading to mono-cationic fragments or dehydration-rearrangement to give the oxonium ion 236 (eq 80). 

Another interesting oxonium-based dication arises from p-methoxy-benzene-diazonium ion in FSCbH-SbFs solution (eq 81).80 Dication 237 was observed by low-temperature 13C NMR spectroscopy. Interestingly, the o-methoxybenzenediazonium ion does not form the oxonium dication 238 (eq 82),... 

Of recent interest is the living cation of tetrahydrofuran. The oxonium ion with suitable counter ions is stable and exists in equilibrium with the monomer47. Bifunctional initiators were found to produce the polytetrahydrofuran dication. 

The dissolution of the adamanantane-2,6-dione in 1 4 SbFs FSO3H/SO2 gave the 2,6-dihydroxy-2,6-adamantanediyl dication (99). This dicarboxonium ion has more oxonium character than the carbenium character as shown by the much shielded absorption for the carbonyl carbon (8 C 247.7), relative to the monocarboxonium ion, 100(8 C 267.1). ... 

The two important resonance structures for the dication are the bis-oxonium structure (73a, a distonic superelectrophile) and the bis-carbenium structure (73b, a gitonic superelectrophile). Although it is understood that various factors should favor structure 73a (including... 

Several gitonic superelectrophiles have been reported having closely oriented oxonium and carboxonium ion centers, some of which may be considered 1,3-dications. A series of hydroxy-substituted carboxylic acids were studied in FSOsH-SbFs in solution and the oxonium-carbonium dications could be directly observed at low temperature.57 In the case of lactic acid, dication 147 is a persistent ion at — 80°C, but at temperatures above — 60°C, formation of the diprotonated lactide (148) is observed (eq 48). 

Although no mechanism is proposed for the dimerization of lactic acid in the FSOsH-SbFs solution, the process may very well involve the superelectrophile 147. Other oxonium-carbenium 1,3-dications have been suggested in superacid promoted pinacolone rearrangments of diprotonated aliphatic glycols and alkoxy alcohols (eqs 49-50).58... 

A number of studies have reported the formation of carboxonium-centered distonic dications. These systems include diprotonated diketones, diproto-nated dicarboxylic acids, their derivatives, and others. A wide variety of mixed dications have also been reported such as ammonium-carboxonium dications, oxonium-carboxonium dications, acyl-carboxonium dications, and other species. Many of these carboxonium-centered dications exhibit properties indicative of distonic superelectrophiles. 

In a related study, bromoanisoles were reacted with excess CHsF-SbFs and the dicationic product 250 was only formed with p -bromoanisole.82 NMR studies indicated that the dication is in equilibrium with the oxonium monocation 249 (eq 85). 

Oxonium salts alkylate diamides of dicarboxylic acids twice, provided the amide groups are separated by at least two carbon atoms,thus the bis(dimethylamides) of maleic acid or terephthalic acid have been converted to dications (69 Scheme 4) by means of oxonium salts. ... 

Whereas carotenoids in general are often considered by non-specialists as unstable compounds, they may easily be studied when particular precautions are made [1], This is also true for most subtypes with charged functional groups. The instability of carotenoid oxonium ions, radical cations, mono- and dications, however, represent special challenges. This review appears to be the first compilation on charged carotenoid species. 

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