Superacids cation radical

As discussed, there are various methods of cation-radical generation. Every individual case needs its own appropriate method. A set of these methods is continuously being supplemented. For example, it was very difficult to prepare the cation-radicals of benzene derivatives with strong acceptor groups. However, some progress has been achieved, thanks to the use of fluorosulfonic acid, sometimes with addition of antimony pentafluoride, and lead dioxide (Rudenko 1994). As known, superacids stabilize cationic intermediates (including cation-radicals) and activate inorganic oxidants. The method mentioned is effective at -78°C. Meanwhile, -78°C is the boundary low temperature because the solution viscosity increases abruptly. This leads to the anisotropy of a sample and a sharp deterioration in the ESR spectrum quality. 

As was noted in Section 1.3.2.A, one-electron oxidation causes deprotonation of cation radicals. Cation radicals bearing protons that are (3 to a site of charge/spin density are superacids. Because of this feature, attention must be given to the distinction between cation radical and H-acid catalysis of cycloaddition. Bauld s group has elaborated a set of criteria that allow one to differentiate these mechanisms one from another (Reinolds et al. 1987). 

Radical cations that are produced by electrochemical oxidation are not stable in solvents with appreciable base character. This results because such radicals are subject to attack by available nucleophiles, and solvents that contain donor electron pairs are good nucleophiles. Cation radicals are most stable in solvents that are good Lewis acids and show negligible basic properties. Some of the solvent systems that have been employed to stabilize electrochemically produced cation radicals include nitromethane and nitrobenzene,21 dichloro-methane,22 trifluoroacetic acid-dichloromethane (1 9),23 nitromethane-AlCl3,24 and AlCl3-NaCl (1 l).25 Organic chemists should be familiar with the stabilization of carbonium ions by superacid media.26 These media usually contain fluorosulfuric acid, or mixtures of fluorosulfuric acid with antimony pen-tachloride and sulfur dioxide, and are potent solvents for the production and stabilization of organic cations. 

The last few years brought the rapid development of acid catalyst preparations especially the synthesis of solids with high acid strength - the so-called superacids. At the same time the investigations of acid catalysed reactions showed that apart from classical carbocations, pentacoordinated carbonium ions, cation-radicals and even radicals may be considered as possible transition states. The mechanism of non-classical intermediate formation has not been fully explained yet and needs further investigations. 

Electrochemical generation of fiillerene cation radicals has not been used for synthetic purposes yet. 50 cation radicals, formed by chemical oxidation in protic superacidic media, can be easily trapped with alcohols, aromatics, and x,y-diols to form alkoxylated or arylated fullerenes and difiilleroxyalkanes (earmuff ethers) respectively [92]. 

Polymerization of isobutylene, in contrast, is the most characteristic example of all acid-catalyzed hydrocarbon polymerizations. Despite its hindered double bond, isobutylene is extremely reactive under any acidic conditions, which makes it an ideal monomer for cationic polymerization. While other alkenes usually can polymerize by several different propagation mechanisms (cationic, anionic, free radical, coordination), polyisobutylene can be prepared only via cationic polymerization. Acid-catalyzed polymerization of isobutylene is, therefore, the most thoroughly studied case. Other suitable monomers undergoing cationic polymerization are substituted styrene derivatives and conjugated dienes. Superacid-catalyzed alkane selfcondensation (see Section 5.1.2) and polymerization of strained cycloalkanes are also possible.118... 

The classical structures of propan-1-yhum ion (1-propyl cation) are not minima on the potential energy surface of the C3Hy manifold. They are some 20kcalmol above 4, as determined computationally. This result is in fair agreement with gas-phase photoionization experiments on the w-propyl radical by Schultz,and Dyke and co-workers.These structures are transition states involved in the hydrogen scrambling of 4 in superacidic media. ° ... 

To our knowledge, 46 has never been observed in solution under stable conditions, even at low temperature. Pulse radiolysis " of benzyl chloride as well as flash photolysis ° of several derivatives in HHP have allowed the observation of the electronic absorption spectra of benzyl and its 4-methyl and 4-methoxy derivatives. The and NMR spectra of the 2,4,6-trimethylbenzyl cation and other more heavily substituted benzyl cations, however, have been studied at low temperature in superacid media. In the gas phase, cold benzyl radical has been probed by two-color, resonant two-photon ionization techniques, thus providing very accurate vibrational frequencies below 650 cm for the benzyl cation. Furthermore, the adiabatic ionization energy of benzyl radical and several isotopomers in the ground state were determined from their threshold photoionization spectra using resonant two-photon excitation and detection of electrons by pulsed field ionization. This information, combined with Af//° (CgH5CH2) from Ref. 212 leads to the value of Af//°m(46) reported in Table 9. 

It is demonstrated that sulfated zirconia does not act as a superacid, protorating paraffins, but activates the alkanes and other hydrocarbons via a radical cation type. 

The direct photolysis of onium salt-type initiators of the general structure (A — B) X results in the formation of Bronsted acids (superacids) of the structure H X ), as shown in Scheme 3.6. In this mechanism, radical cations... 

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