Superacids, halogenation

Prior syntheses of long-lived perhalomethyl cations have been achieved by halide abstraction by use of either a strong Lewis acid (in superacidic or S02C1F solvent media) or Ag+ (vide supra), but no routes to such carbocations through oxidative removal of a halogen bound to carbon were known. The objectives of the current work have been to provide structural and spectroscopic data that, thus far, have been lacking for these systems, and to provide oxidative routes to... 

It is supposed that hydrogen bonds between halogen atoms of the superacid and the a-hydrogens of the oxime in the Lewis acid-oxime complex can play a role in the hydride transfer either by inter- or by intra-molecular processes. 

Any condensation of methane to ethane and subsequently to higher hydrocarbons must overcome the unfavorable thermodynamics. This can be achieved in condensation processes of oxidative nature, where hydrogen is removed by the oxidant. SbF5- or FS03H-eontaining superacid systems also act as oxidants. The oxidative condensation of methane was subsequently found to take place with more economical cooxidants such as halogens, oxygen, sulfur, or selenium 91... 

The usual way to achieve heterosubstitution of saturated hydrocarbons is by free-radical reactions. Halogenation, sulfochlorination, and nitration are among the most important transformations. Superacid-catalyzed electrophilic substitutions have also been developed. This clearly indicates that alkanes, once considered to be highly unreactive compounds (paraffins), can be readily functionalized not only in free-radical from but also via electrophilic activation. Electrophilic substitution, in turn, is the major transformation of aromatic hydrocarbons. 

Halogenation. Fluorination, chlorination, and bromination of alkanes catalyzed by superacids have been reported.1,2 Reactions may be carried out in the liquid phase, or in the gas phase over solid superacids or supported noble metal catalysts. High selectivity and relatively mild reaction conditions are the main features of these transformations. 

The halogenation of saturated aliphatic hydrocarbons is usually achieved by free radical processes.523 Ionic halogenation of alkanes has been reported under superacid catalysis. Olah and co-workers524,525 have carried out chlorination and chlorolysis of... 

Carbon-Halogen Vicinal-Dieations Trihalomethyl cations are shown to have enhanced reactivities in superacid solution, while poly-halomethanes in the presence of excess AlBr3 or AICI3 exhibit the properties of aprotic superacids.79 The trihalomethyl cations CX3+ (178, X=C1, Br, I) have been characterized by NMR and IR spectroscopy. The stability of these species is attributed to substantial resonance-stabilization by back-donation from the nonbonded electron pairs of the halogen atoms.22 Trihalomethyl cations are capable of hydride abstraction from alkanes and alkyl groups when the reactions are carried out in the presence of Bronsted or Lewis superacids (eq 46-48).80... 

Halohydrocarbons with labile bromine atom react even more easily. On the other hand, organic halides with less labile halogen atoms require more drastic reaction conditions, and sometimes even the application of pressure. In its entirety the reaction is a useful method for synthesis of organotrichlorogermanes. There are no analogous reactions in the chemistry of chloroform and trichlorosilane and the logical explanation of the phenomena can be found in the concept of HGeCl3 superacidity. 

The solid superacids of the second group which have a possibility of leaching or evaporating of halogen compounds seem to be environmentally undesirable as catalysts. These superacids and Nafion-H have been extensively reviewed [1-5]. H-ZSM-5 and heteropolyacids are discussed in Sections 2.3.3 and 2.1.7 of this handbook, respectively. Therefore, the superacids of the first group for which many papers have been contributed recently and the industrial application of which is promising, are mainly described here and the other superacids are dealt with only briefly. 

AlCU-CuSQj, AlCh-CuCb, etc with Ho values from -13 16 to -14 52 are active for the isomerization of alkanes Gattcrman Koch reaction, etc [2.4] These solid superacids have also some disadvantages because they contain halogen atoms... 

There has always been considerable interest in the halogen cations and it has often been postulated that the simple ions I+, Br+, and Cl+ are the reactive intermediates in aromatic halogenation reactions. No convincing evidence has ever been obtained for these cations however, the search for them has led to the discovery of the I cation and a number of other related halogen cations (Gillespie and Morton, 1970) which are stable in superacid media. The I cation can be conveniently prepared by the oxidation of I2 with S206F2 in solution in HSOsF. 

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