Dications with aromatic stabilization

Anti-aromatic 1,2-dithiins 179 display properties opposite to those of 1,4-dithiins 180, whose dications show aromatic stabilization. Unlike other antiaromatic compounds, the 1,2-dithiin derivatives, with eight jr-electrons (such as 181 and 182), appear in... 

Although diselenonium-, ditelluronium- and mixed sulfonium-selenonium dications can exhibit either oxidative or electrophilic properties in reactions with nucleophiles, substitution at the onium chalcogen atom is more typical.96 Owing to the increased stability of heavier dichalcogenium-dications, they react only with highly activated substrates such as aniline and tV,A-dimethylaniline, while no reaction is observed with phenol and diphenylamine.113 Reactions of ditelluronium dications with activated aromatics are also not known (Scheme 44).114... 

Chemical two-electron oxidation of the corresponding 1,2-dithiin resulted in the formation of a stable solution of dication 95 (no decomposition was observed at 18°C for months)256 [Eq. (4.73)]. 1H and 13C NMR spectra of the dication exhibited resonances at considerably lower fields compared with those of the neutral starting material suggesting aromatic stabilization. Calculated geometries (B3LYP/6-31G ) indicate a planar ring and calculated chemical shifts are in fair agreement with observed values. The related 1,4-dithiin dication was also characterized.257... 

Related classes of gitonic superelectrophiles are the previously mentioned protoacetyl dications and activated acyl cationic electrophiles. The acyl cations themselves have been extensively studied by theoretical and experimental methods,22 as they are intermediates in many Friedel-Crafts reactions. Several types of acyl cations have been directly observed by spectroscopic methods and even were characterized by X-ray crystal structure analysis. Acyl cations are relative weak electrophiles as they are effectively stabilized by resonance. They are capable of reacting with aromatics such as benzene and activated arenes, but do not generally react with weaker nucleophiles such as deactivated arenes or saturated alkanes. 

Fig. 4.23 Hiickel s rule says that cyclic n systems with An + 2 n electrons ( = 0, 1, 2,. .. An + 2 = 2, 6, 10,. ..) should be especially stable, since they have all bonding levels full and all antibonding levels empty. The special stability is usually equated with aromaticity. Shown here are the cyclopropenyl cation, the cyclobutadiene dication, the cyclopentadienyl anion, and benzene formal structures are given for these species - the actual molecules do not have single and double bonds, but rather electron delocalization makes all C/C bonds the same...

While the BP formed from diphenyloctatetraene is stable for several hours at room temperature, the p,p -N,N-dimethylamino substituted tetraene is stable for several days in solutions in contact with air. The formation of the P (- - ) and BP (-H +) charge states for a variety of substituted diphenylpolyenes is reviewed in Ref. 18, where the origin of the observed transitions is discussed in detail. Dianthracenylpolyenes form extremely stable bipolaronic dications that are stable for several months in solution or as optically transparent polycarbonate composites [7,12]. This dramatic stability enhancement has been rationalized on the basis of aromatic stabilization of the quinoid intermediate and is illustrated in Scheme 4. 

Trifluoromethanesulfonic (triflic) anhydride is commercially available or can be prepared easily by the reaction of triflic acid with phosphorus pentoxide [66] This moderately hygroscopic colorless liquid is a useful reagent for the preparation of various organic derivatives of triflic acid A large variety of organic ionic triflates can be prepared from triflic anhydride A recent example is the preparation of unusual oxo-bridged dicatiomc salts of different types [SS, 89, 90, 91, 92, 93] (equations 38-44) Stabilized dication ether salts of the Huckel aromatic system and some other systems (equations 38 and 39) can be prepared in one step by the... 

Our success in super-stabilization of cation 6 led us to the preparation of a higher homologue, that is, cyclooctatetraene (COT), fully annelated with BCO units 9 (9). As compared with a large number of studies on its radical anion or dianions, the studies on the cationic species of COT have been quite limited. There have been only one study by Olah and Paquette on the substituted COT dication (70), which is a typical 6n Hiickel aromatic system, and few sporadic studies on radical cations, which involve indirect spectral observations, such as electronic spectra in Freon matrix at low temperature (77,72) and constant-flow ESR study (13). 

Anodic oxidation in inert solvents is the most widespread method of cation-radical preparation, with the aim of investigating their stability and electron structure. However, saturated hydrocarbons cannot be oxidized in an accessible potential region. There is one exception for molecules with the weakened C—H bond, but this does not pertain to the cation-radical problem. Anodic oxidation of unsaturated hydrocarbons proceeds more easily. As usual, this oxidation is assumed to be a process including one-electron detachment from the n system with the cation-radical formation. This is the very first step of this oxidation. Certainly, the cation-radical formed is not inevitably stable. Under anodic reaction conditions, it can expel the second electron and give rise to a dication or lose a proton and form a neutral (free) radical. The latter can be either stable or complete its life at the expense of dimerization, fragmentation, etc. Nevertheless, electrochemical oxidation of aromatic hydrocarbons leads to cation-radicals, the nature of which is reliably established (Mann and Barnes 1970 Chapter 3). 

Removal of two electrons from the formal cyclic 87r-electron structures serves to produce potential Hiickel 4n + 2 aromatic systems. The loss of one electron to form a radical cation was referred to in Section 2.26.2.1, and removal of a second electron by electrochemical oxidation, leading to dicationic structures, has also been achieved for a wide range of unsaturated compounds with heteroatoms in the 1,4 positions (70ZC147, 73JA2375). The oxidations are discussed further in Section 2.26.3.1.5, where tabulated data are presented. An interesting feature is the stability of certain salts of the dications, some of which have been isolated. 

Since electrode measurements involve low substrate concentrations, reactive impurities have to be held to a very low level. The physical data and purification methods for several organic solvents used in electrode measurements have been summarized (Mann, 1969). But even when careful procedures for solvent and electrolyte purification are employed, residual impurities can have profound effects upon the electrode response. For example, the voltam-metric observation of dications (Hammerich and Parker, 1973, 1976) and dianions (Jensen and Parker, 1974, 1975a) of aromatic hydrocarbons has only been achieved during the last ten years. The stability of radical anions (Peover, 1967) and radical cations (Peover and White, 1967 Phelps et al., 1967 Marcoux et al., 1967) of aromatic compounds was demonstrated by cyclic voltammetry much earlier but the corresponding doubly charged ions were believed to be inherently unstable because of facile reactions with the solvents and supporting electrolytes. However, the effective removal of impurities from the electrolyte solutions extended the life-times of the dianions and dications so that reversible cyclic voltammograms could be observed at ambient temperatures even at very low sweep rates. 

Voltammetric oxidation of 1,4-dioxins and 1,4-dithiins in acetonitrile containing perchlorate yields a radical cation in the first one-electron oxidation and a dication in the second one-electron oxidation. Both compounds are typical r-electron-rich heterocycles, whereas the dication contains six 7r-electrons, and the relatively high stability of the dication is connected with this aromaticity [296]. 

It is difficult to choose a reference compound against which to characterize the stability of the dication. That it can be formed at all, however, is suggestive of special stabilization associated with the two-electron 7r-system. Aromaticity would also be predicted for the dianion of cyclobutadiene. There is some evidence that this species may have a finite existence. " Reaction of 3,4-dichlorocyclobutadiene with sodium naphthalenide followed in a few minutes by methanol-O-J gives a low yield of 3,4-di-J r nV -cyclobutene. As yet, however, no direct structural characterization of this species has been accomplished. 

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