Methane superacids

The high acidity of superacids makes them extremely effective pro-tonating agents and catalysts. They also can activate a wide variety of extremely weakly basic compounds (nucleophiles) that previously could not be considered reactive in any practical way. Superacids such as fluoroantimonic or magic acid are capable of protonating not only TT-donor systems (aromatics, olefins, and acetylenes) but also what are called (T-donors, such as saturated hydrocarbons, including methane (CH4), the simplest parent saturated hydrocarbon. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Protonated methanes and their homologues and derivatives are experimentally indicated in superacidic chemistry by hydrogen-deuterium exchange experiments, as well as by core electron (ESCA) spectroscopy of their frozen matrixes. Some of their derivatives could even be isolated as crystalline compounds. In recent years, Schmidbaur has pre-... 

Protoiiation (and protolysis) of alkanes is readily achieved with superacids. The protonation of methane itself to CH5, as discussed earlier, takes place readily. 

The superacid-catalyzed electrophile oxygenation of saturated hydrocarbons, including methane with hydrogen peroxide (via H302 ) or ozone (via HOs ), allowed the efficient preparation of oxygenated derivatives. 

The finding that highly deactivated aromatics do not react with N02 salts is in accord with the finding that their greatly diminished TT-donor ability no longer snffices to polarize NOi. Similarly, (j-donor hydrocarbons such as methane (CH4) are not able to affect such polarization. Instead, the linear nitronium ion is activated by the superacid. Despite the fact that is a small, triatomic cation, the 11011-... 

Lower alkanes such as methane and ethane have been polycondensed ia superacid solutions at 50°C, yielding higher Hquid alkanes (73). The proposed mechanism for the oligocondensation of methane requires the involvement of protonated alkanes (pentacoordinated carbonium ions) and oxidative removal of hydrogen by the superacid system. 

Strong acids or superacid systems generate stable fluorinated carbocations [40, 42] Treatment of tetrafluorobenzbarrelene with arenesulfonyl chlorides in nitro-methane-lithium perchlorate yields a crystalline salt with a rearranged benzo barrelene skeleton [43] Ionization of polycyclic adducts of difluorocarbene and derivatives of bornadiene with antimony pentafluonde in fluorosulfonyl chloride yields stable cations [44, 45]... 

The most stable of all alkyl cations is the tert-butyl cation. Even the relatively stable tert-pentyl and fen-hexyl cations fragment at higher temperatures to produce the tert-butyl cation, as do all other alkyl cations with four or more carbons so far studied. Methane,ethane, and propane, treated with superacid, also yield ten-butyl cations as the main product (see 2-17). Even paraffin wax and polyethylene give the ten-butyl cation. Solid salts of frrf-butyl and rerf-pentyl cations (e.g., MeaC" SbFg ) have been prepared from superacid solutions and are stable below -20°C. ... 

In alkane condensations in superacid media it is assumed, albeit never observed in solution, that reversible methane or ethane protonation is the first step." Subsequent loss of dihydrogen, yielding the highly unstable methyl and ethyl cations, and reaction with excess alkane builds up higher hydrocarbons [Equation (6)]. 

The third alternative approach of methane conversion is via electrophilic reactions.77 The electrophilic conversion of methane is based on the feasibility of electrophilic reactions of single bonds and thus saturated hydrocarbons.90 Both C—H and C—C bonds can act as electron donors against strongly electrophilic reagents or superacids. Olah s studies showed that even methane is readily protonated or alkylated under these conditions. Methane with SbF5-containing superacids was found to undergo condensation to C2-C6 hydrocarbons at 50-60°C. 

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... 

Natural gas instead of pure methane can also be used in condensation reactions.91 When natural gas is dehydrogenated, the C2-C4 alkanes it contain are converted into olefins. The resulting methane-olefin mixture can then without separation be passed through a superacid catalyst, resulting in exothermic alkylative condensation ... 

In alkane-alkene alkylation systems it is always the Jt-donor alkene that is alkylated by carbocations formed in the system. In the absence of excess alkenes (i.e., under superacidic conditions), however, the cr-donor alkanes themselves are alkylated. Even methane or ethane, when used in excess, are alkylated by ethylene to give propane and n-butane, respectively ... 

Oxidative carbocationic condensation of methane under superacidic conditions was first achieved in the late 1960s. Olah and coworkers352-355 observed that when methane was introduced into SbF5-containing superacids, C4 and higher alkyl cations were obtained. As the initial reaction [Eq. (3.41)] is endothermic, it became apparent that the superacid oxidatively helped to remove H2 (as HF) ... 

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... 

It is also significant that the direct oxidative condensation of methane to higher hydrocarbons takes place in the presence of S8 over superacidic catalysts, such as TaF5 and the like.357... 

The superacid-catalyzed reaction is thus alkylation of the methane with the ethyl cation. 

Superacids were shown to have the ability to effect the protolytic ionization of a bonds to form carbocations even in the presence of benzene.190 The formed car-bocations then alkylate benzene to form alkylbenzenes. The alkylation reaction of benzene with Ci—C5 alkanes (methane, ethane, propane, butane, isobutane, isopentane) are accompanied by the usual acid-catalyzed side reactions (isomerization, disproportionation). Oxidative removal of hydrogen by SbF5 is the driving force of the reaction ... 

The preparation of acetic acid represents a special case. Olah and coworkers as well as Hogeveen and coworkers have demonstrated that CO can react with methane under superacidic conditions, giving the acetyl cation and by subsequent quenching acetic acid or its derivatives (see Section 7.2.3). Monosubstituted methanes, such as methyl alcohol (or dimethyl ether), can be carbonylated to acetic acid.115 Similarly, methyl halides undergo acid-catalyzed carbonylation.115,116 Whereas the acid-catalyzed reactions can be considered as analogs of the Koch reaction, an efficient Rh-catalyzed carbonylation of methyl alcohol in the presence of iodine (thus in situ forming methyl iodide) was developed by Monsanto and became the dominant industrial process (see Section 7.2.4). 

Even methane, the least reactive alkane, was shown to undergo carboxylation under superacidic conditions.115,176 The formation of carboxylated products (acetic acid and methyl acetate) from methane was first observed by Hogeveen and coworkers by trapping methyl cation formed in SbF5 (60°C, 50 atm CO pressure) followed by quenching with H20 or MeOH. The intermediate methylcarboxonium ion (CH3CO+) and CH3CH2CO+ formed in a similar reaction of ethane were identified by NMR spectroscopy.176,177... 

In contrast to the radical processes described before, high selectivity is characteristic of electrophilic oxidation of methane.61 It reacts, for instance, with H2O2 in superacidic media to give methanol.1,62 The reaction is best explained by electrophilic insertion of the hydrogen peroxonium ion (H3Oj) into the methane C—H bond ... 

When methane is reacted with ozone in superacidic media,61,67 formaldehyde is directly formed through a pathway that is considered attack by +03H into a C—H bond, followed by cleavage of H2O2 to give very reactive methyloxenium ion (2), which instantly rearranges to protonated formaldehyde ... 

It should be mentioned that with superacidic electrophilic oxygenation of methane either to methanol (with protonated hydrogen peroxide) or to formaldehyde (with protonated ozone), the products formed are indeed the protonated products (CH3OH2 and CH2=OH+, respectively), which are protected from further electrophilic oxygenation, which happens only too readily in conventional oxidations. 

Electrophilic chlorination and bromination of methane over supported noble metals9-11 (Pt on A1203, Pd on BaSC ) and solid superacid catalysts9-13 (e.g., TaOF3 on alumina, Nafion-H, zeolites, SbF5-graphite, sulfated zirconia) have been studied (see Section 3.4.2). Monosubstitution with selectivities better than... 

Mota and co-workers have investigated the nature of superacid electrophilic species in HF-SbF5 by density functional theory63 and measured the ability of the system to protonate light alkanes (methane, ethane, propane, and isobutane).64... 

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