Highly stable cations

B. Triphenylmethyl (Trityl) Cation The highly stable triphenylmethyl cation (85) is found to undergo a wide variety of photoreactions depending on such variables as the nature of the solvent, the pH of the medium, and the presence or absence of... 

Besides the carbocations mentioned above, numerous highly stable carboca-tions have been isolated as the salts of inorganic anions. Azulene analogues of triphenylmethylium ion [ll ]-[20 ], [21 j, [22 " ] and [23 j trisubsti-tuted cyclopropenylium ions [l" ] and [24 ]-p6 ] cyclopropyl-substituted tropylium ions [27 ]-[30 ] tropylium ions annelated with bicyclic frameworks [31 ]-[36 ] and [37 ]-[39 ] fulvene-substituted cyclopropenylium [40 ] and tropylium [41 ] ions a tropylium ion condensed with acenaphthylene [42 ] and a cation containing a 147t periphery [43 ] have... 

In 1994, thiols were firstly used as stabilizers of gold nanoparticles [6a]. Thiols form monolayer on gold surface [18] and highly stable nanoparticles could be obtained. Purification of nanoparticles can be carried out, which makes chemical method of metal nanoparticles a real process for nanomaterial preparation. Various thiol derivatives have been used to functionalize metal nanoparticles [6b, 19]. Cationic and anionic thiol compounds were used to obtain hydrosols of metal nanoparticles. Quaternary ammonium-thiol compounds make the nanoparticle surface highly positively charged [20]. In such cases, cationic nanoparticles were densely adsorbed onto oppositely charged surfaces. DNA or other biomolecule-attached gold nanoparticles have been proposed for biosensors [21]. 

The highly stable cyclopropenium immonium cations 153 are formed from the ethoxy cyclopropenium cation 75 with primary and secondary aliphatic or aromatic amines42-119> ... 

Other factors influencing the rate of metal-ion transport across artificial membranes have been identified. As might be expected, such transport is dependent on the interplay of several factors. For example, as briefly mentioned already in Chapter 4, it is clear that the strength of complex-ation of the cation by the carrier must be neither too high nor too low if efficient transport is to be achieved. If the stability is low, then uptake of the metal ion from the source phase will be inhibited. Conversely, for those cases where highly stable complexes are formed, there will be a reluctance by the carrier to release the cation into the receiving phase. 

Tripheny(methyl cation (10 ) is effectively stabilized by electron donating substituents. B. W. Laursen et al. reported the highly stable carbocation 11+ with a pKr+ value of 19.7 (10). Recently, they reported a similar carbocation with a much higher p R+ value (23.7) (11). In our continuing efforts to prepare extremely stable carbocations, we have investigated the effect of introduction of electron donating substituents into each azulenyl group. 

Michael additions to benzotriazole-stabilized carbanions have been reviewed. review of the structural dependence of heterolytic bond dissociation energy of carbon-carbon a-bonds in hydrocarbons has summarized the synthesis and behaviour of molecules in which highly stable cationic and anionic hydrocarbon moieties have apparently been combined. 

When a tetraalkylammonium cation is used as a counterion in solvents of high polarity, such as AN or DME, the alkyl groups of the cation hinder the mutual approach of species with different charges. Ion pairs with the potassium cation are stable. This follows from a comparison of the polarographic behavior of the three isomeric dinitrobenzenes in the same solvent (DMF) using tetraethylammonium or potassium perchlorate as the carrier electrolyte (Todres 1970). The halfwave potentials corresponding to the conversion of p- and m-dinitrobenzenes into anion-radicals are independent of whether tetraethylammonium or potassium counterions are employed. The anion-radical is formed from o-dinitrobenzene at a potential that is less negative by almost 100 mV when... 

Finally, one example of trityl salt analogues in the phase-transfer catalysis is presented. The highly stable triazatriangulenium cations 62 [161, 162] were jnst recently introduced to the phase-transfer chemistry [163], Persistent to strongly basic and nncleophilic conditions, these salts revealed efficient catalytic activity in addition reactions (Scheme 64). Modification of the alkyl side chains on nitrogen allowed matching the fair hydro/lipophilicity with the optimised conditions in the alkylation, epoxidation, aziridination and cyclopropanation reactions. The results are comparable to those of tetrabutylammonium salts and in some cases showed even a better outcome. 

Ionic liquids are generally regarded as highly stable, and the widely used dial-kylimidazolium ionic liquids are indeed thermostable up to 300 °C [4]. The propensity of the [BF4] and [PF6] anions to hydrolyze with liberation of HF [37], which deactivates many enzymes, has already been mentioned. The [TfO] and [ Tf2N] anions, in contrast, are hydrolytically stable. Dialkylimidazolium cations have a tendency to deprotonate at C-2, with ylide (heterocarbene) formation. Such ylides are strong nucleophiles and have been used as transesterification catalysts, for example [38]. These could cause enzyme deactivation as well as background transesterification when formed in small amounts from anhydrous ionic liquids and basic buffer salts, for example. 

In contrast to highly stable and prolific fullerene anionic species, fullerene cations are rare. The first fullerene cation was prepared in 1996 by Reed and co-workers500 by single-electron oxidation of C76 to form radical cation C76 + isolated in solid form as the CBnH6Br6 salt [Eq. (3.56)]. The cation was identified in solution by a characteristic visible-near-infrared absorption (Amax = 780 nm), FT-IR and EPR spectroscopy. C60 + was generated in an analogous way later.501 Reed et al.501 also succeded in... 

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