Superacid transformations

As expected, superacids were found to be extremely effective in bringing about protolytic transformations of hydrocarbons. 

A fundamental difference exists between conventional acid-catalyzed and superacidic hydrocarbon chemistry. In the former, trivalent car-benium ions are always in equilibrium with olefins, which play the key role, whereas in the latter, hydrocarbon transformation can take place without the involvement of olefins through the intermediacy of five-coordinate carbocations. 

Brmnsted-Lewis Superacids. Conjugate Friedel-Crafts acids prepared from ptotic and Lewis acids, such as HCl—AlCl and HCl—GaCl ate, indeed, supetacids with an estimated value of —15 to —16 and ate effective catalysts in hydrocarbon transformation (217). 

However, the more important question of whether AN can be used in C,C-coupling reactions with nucleophiles remains open. It should be noted that Japanese researchers demonstrated in several studies that these transformations can be performed for benzene and certain electron-rich arenes (477). Just the same, this procedure requires severe conditions (the use of superacids at high... 

As mentioned above, persistent carbocation 9 underwent rearrangement into cation 10 which rearranged further into cation 11. To reveal general relations/factors governing cationic rearrangements in benzopentalene derivatives, the behavior of 5,5,10,10-tetramethyl-5,10-dihydroindeno[2,l-fl]indene (12) in superacids was studied (52). It had been expected that hydrocarbon 12 would transform into the long-lived 5,5,10,10-tetramethyl-4b,5,9b,10-tetrahydroindeno[2,l-a]inden-4b-yl cation (13). However, H and 13C NMR data showed that hydrocarbon 12 transformed firstly into isomeric ion 14 which transformed further into cation 15 (Scheme 11). 

Figure 13.3 Possible catalytic uses of acid and superacid solids in the selective transformation of C4-hydrocarbons by acid reactions.

Berkessel, A. and Thauer, R. K. (1995) On the mechanism of catalysis by a metal-free hydro-genase from methanogenic archaea Enzymic transformation of H2 without a metal and its analogy to the chemistry of alkanes in superacidic solution. Angew. Cbem., Int. Ed. Engl., 34, 2247-50. 

P205 was also reported to decompose methanol to mixtures of hydrocarbons with widely varying compositions.417 418 Polyphosphoric acid is unique in effecting the transformation of methanol under comparatively mild conditions (190-200°C)418 Superacidic TaF5 and related halide catalysts419 420 condense methanol to saturated hydrocarbons of the gasoline range at 300°C. 

Hydrocarbon formation from methyl chloride can be catalyzed by ZSM-5482 483 or bifunctional acid-base catalysts such as W03 on alumina.420,447 The reaction on ZSM-5 gives a product distribution (43.1% aliphatics and 57.1% aromatics at 369°C) that is very similar to that in the transformation of methanol, suggesting a similar reaction pathway in both reactions.483 W03 on A1203 gives 42.8% C2-C5 hydrocarbons at 327°C at 36% conversion.447 When using methyl bromide as the feed, conversions are comparable. However, in this case, HBr can be very readily air-oxidized to Br2 allowing a catalytic cycle to be operated. Since bromine is the oxidant, the reaction is economical. The one step oxidative condensation of methane to higher hydrocarbons was also achieved in the presence of chlorine or bromine over superacidic catalysts.357... 

Different catalysts bring about different types of isomerization of hydrocarbons. Acids are the best known and most important catalysts bringing about isomerization through a carbocationic process. Brpnsted and Lewis acids, acidic solids, and superacids are used in different applications. Base-catalyzed isomerizations of hydrocarbons are less frequent, with mainly alkenes undergoing such transformations. Acetylenes and allenes are also interconverted in base-catalyzed reactions. Metals with dehydrogenating-hydrogenating activity usually supported on oxides are also used to bring about isomerizations. Zeolites with shape-selective characteristics... 

Novel highly active aprotic superacid systems have been more recently found by Vol pin and coworkers to initiate rapid transformation of alkanes as well as C5—C6 cycloalkanes.43 The reaction, for example, of excess cycloalkanes without solvent... 

Alkylcyclohexanes, cycloheptane, and cyclooctane were also transformed into the esters of the corresponding tertiary cyclohexanecarboxylic acids in the presence of the superelectrophile CBr4—2AlBr3 in good yields.314 315 Such organic superacids generated from polyhalomethanes with aluminum halides are capable of promoting the selective and effective carboxylation of linear alkanes (propane, butane) and adamantane as well.316... 

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 skeletal isomerization of straight-chain paraffins is important for the enhancement of the octane numbers of light petroleum fractions. The isomerization of H-butane to isobutane has attracted much attention because isobutane is a feedstock for alkylation with olefins and MTBE synthesis. It is widely believed that the low-temperature transformation of n-alkanes can be catalyzed only by superacidic sites, and this reaction has often been used to test for the presence of these sites. 

The key of alkane transformation was assigned to the formation of CX3+-type cations that are electrophilic enough (probably due to a second complexation of A1X3), to abstract a hydride anion from linear and cycloalkanes. When these cations are generated in superacidic media, a protosolvation induces a superelectrophilic character, which was supported by Olah on the basis of high-level ab initio calculations 65 The generation of these cations was also used by various teams66,67 to initiate selective low temperature alkane activation. 

Superacids Immobilized on Solid Supports. The considerable success of Magic Acid and related superacids in solution chemistry and interest to extend the scope and utility of acid-catalyzed reactions, particularly hydrocarbon transformations, logically led to the attempts to adopt this chemistry to solid systems allowing heterogeneous catalytic processes. 

A great number of trialkyl(aryl) selenonium and telluronium ions are known, and their synthesis does not require the use of strong electrophilic alkylating or arylating agents.268,269 The synthesis and transformations of triorganotellurium ions have been treated in recent reviews.270,271 However, acidic selenonium and telluronium ions can be obtained only under superacidic conditions. 

The success of carbocation chemistry lies in the stabilization of carbocations in a medium of low nucleophilicity. Superelectrophiles, in turn, are reactive intermediates generated by further protonation (protosolvation). This second protonation increases electron deficiency, induces destabilization, and, consequently, results in a profound increase in reactivity. In particular, charge-charge repulsive interactions6 play a crucial role in the enhanced reactivity of dicationic and tricationic superelectrophilic intermediates. As various examples of acidity dependence studies show, without an appropriate acidity level, transformations may occur at much lower rate or even do not take place at all. In addition to numerous examples of superacid catalyzed reactions, various organic transformations, in which the involvement of superelectrophilic intermediates is invoked or superelectrophiles are de facto observed in the condensed state, are also included in this chapter. 

The Oxidative Pathway. For a long time, one of the difficulties in understanding the mechanism of the superacid-catalyzed transformations of alkanes was that no... 

In a subsequent study Devynck and co-workers81,82 studied the electrochemical oxidation of alkanes and alkenes in triflic acid monohydrate. The acidity of CF3SO3H H20 was found to be intermediate between that of aqueous acid media and superacidity. Alkanes undergo two-electron oxidation, whereas alkenes are protonated to yield carbenium ions in this medium. In addition to various transformations characteristic of carbenium ions [Eqs. (5.36)—(5.38)], they undergo a reversible disproportionation to give an alkane and an aldehyde [Eqs. (5.40)]. 

The AcBr-2AlX3 (X = Cl, Br) complexes display high activity in the alkylation of adamantane with alkanes to form poly alkylated adamantanes (Cn < C < C33) and bisadamantylalkanes (C23 < C < C50)119 [Eq. (5.70)]. The suggested pathway includes the 1-adamantyl cation and alkyl cations generated by hydride removal by the superacidic complexes. The 1-adamantyl cation then alkylates alkenes equilibrating with the alkyl cations. Various transformations may follow, resulting in the formation of additional products. 

Jouannetaud and co-workers229 have explored electrophilic trifluoromethylation under superacidic conditions of aniline derivatives229 and /V-heterocycles. Methyl-substituted anilines and substituted acetanilides [Eq. (5.85)] react with the CC13+ cation generated from CC14 in HF-SbF5 followed by fluorination to yield the corresponding trifluoromethyl derivatives. Under similar conditions, indolines are transformed to the 6-triluoromethyl derivatives, whereas substituted indoles yield 5-triluoromethyl derivatives.230... 

Akhrem and co-workers417 have successfully applied aprotic organic superacids in the carbonylation of a series of alkanes. Butane was transformed into isomeric carboxylic acids depending on the superelectrophilic reagent and isolated as the... 

Olah et al.450 have recently described a new, highly efficient superelectrophilic formylation-rearrangementofisoalkanes. Branched ketones are formed in high yields and with high selectivity with no detectable branched acids (Koch products) in the presence of moderately strong superacids such as HF-BF3 or triflic acid-BF3. Carbonylation of isobutane under such conditions gives isopropyl methyl ketone in high yield [Eq. (5.164)] The transformation was interpreted with the involvement of... 

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