Alkanes in superacids

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. 

Olah and coworkers have carried out reactions of alkanes in superacid solution (SbFs-HF or SbFs-FSOsH), leading to observable (by NMR) tertiary alkyl carbocations (17, 18, 19). This reaction is pictured as a protonation of the alkane to give a pentacoordinated intermediate which then loses H2. Thus, a plausible mechanism for the cyclodecyl -> 9-decalyl cation reaction could involve loss of the bridging hydrogen (as H ) to form decalin, and then reprotonation of the decalin to eventually form H2 + 9-decalyl cation. However, when the cyclodecyl cation 1 was prepared in SbFs-FSOsD solution, there was no evidence of H-D formation in the hydrogen gas that was produced. It was concluded therefore that the H2 gas must come directly from the carbocation hydrogens without any involvement by the superacid system. 

Remarkably, one H-ase found in methanogenic archaea, Methanobacterium thermoautotrophicum, does not contain transition metals at all 8 It catalyzes the reduction of a pterin compound by H2 and also produces a proton, as a step in methane formation from C02 and H2. One proposed mechanism is analogous to that of Olah33 for the reversible formation of carbocations and H2 from alkanes in superacid media, e.g., isobutane conversion [Eq. (10.7)]. However, the enzyme is... 

Berkessel, A., Thauer, R. K. (1995). On the Mechanism of Catalysis by aMetal-Free Hydrogenase from Methanogenic Archaea Enzymatic Transformation of without a Metal and Its Analogy to the Chemistry of Alkanes in Superacidic Solution. Angewandte Chemie International Edition, 34, 2247-2250. 

The initiating landmark paper is probably the thermodynamic study of Fabre et al. [83], who investigated the reactivity of alkanes in superacid medium... 

The mechanism can explain the formation of methane, ethane and hydrogen. The car-honium ion mechanism is the main pathway of the cracking of alkanes in superacid media. It was concluded that the carbonium ion mechanism predominates at high pressure, low hydrocarbon pressure and low conversion, and that the oppwsite applies to the hydride-transfer mechanism. 

Carboxoninm ions, for example, do not react with alkanes. However, in superacid solution acetaldehyde (or acetone), for example, readily reacts with isobutane involving diprotonated, highly reactive carbocationic species. 

Carbocations generated from alkanes using superacids react with carbon monoxide under mild conditions to form carboxyUc acid (188). In this process isomeric carboxyUc acids are produced as a mixture. However, when the reaction is mn with catalytic amounts of bromine (0.3 mmol eq) in HF-SbF solution, regio-selective carboxylation is obtained. / -Propane was converted almost exclusively to isobutyric acid under these conditions. 

In the 1970s, Brouwer and Kifflin reported the reactions of saturated hydrocarbons with aliphatic aldehydes and ketones in superacidic media. Analysis of the products from these reactions suggested that the protonated aldehydes and ketones (carboxonium ions) were reacting at the carbon-hydrogen o-bonds of the alkanes. This was a surprising observation because carboxonium... 

Olah, G. A. Germain, A. Lin, H. C. Forsyth, D. Electrophilic reactions at Single Bonds. XVIII.1 Indication of Protosolvated de facto Substituting Agents in the Reactions of Alkanes with Acetylium and Nitronium Ions in Superacidic Media. J. Am. Chem. Soc. 1975, 97, 2928-2929. 

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)]. 

General considerations on the mechanism of C3Hg reaction over HZSM-5 and Ga- HZSH-5. The products obtained from the reaction of C2 C5 alkanes over H-ZSM-5 zeolites were nicely interpreted (3-6) according to the classical carbenlum ion theory and the non-classical theory developed for reactions occurring in superacid media where an alkane is protonated to form the carbocation species. The general scheme proposed for propane reaction over H-ZSM-5 is ... 

Alkanes. A review in 1997 summarizes the electrophilic functionalization of alkanes in liquid superacids including carboxylation.311 When alkanes are treated with superacids in the presence of CO, the highly reactive carbocations are trapped,... 

H202 in superacids at —78°C converts simple straight-chain alkanes into primary alcohol (ethane), or secondary alcohols and ketones (propane, butane).1,62 89 9° Electrophilic hydroxylation of the secondary C—H bond by the incipient hydroxyl cation formed through the protolytic cleavage of hydroperoxo-nium ion accommodates these observations ... 

Carbon—carbon bond cleavage is also characteristic of branched saturated hydrocarbons reacting with ozone in superacid media.1 Depending on the structure of the reacting alkanes, different mechanisms can be operative. 

Estimates of the kinetics of methyl loss from energy-selected CztHg" species have been made by calculation.23 The hydride transfer from alkanes to carbenium ions in the gas phase is calculated to involve a species with a symmetric potential well, which is different from the situation in superacid or zeolite media.24 A correlation between the charge on a carbon and the in-plane tensor component of its 13 C chemical shift has been observed for a number of simple cationic and anionic species.25 High-level calculations... 

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. 

Alkylated carboxonium ions have also been prepared by direct electrophilic oxygenations of alkanes, alcohols, and so on, by ozone or hydrogen peroxide in superacidic media606 [Eq. (3.82)]. 

Alkyl- and aryloxenium ions (RO+) are similarly too reactive to be observed however, they may be involved in the oxidation of alkanes under superacidic... 

Results of protoly tic reactions of hydrocarbons in superacid media were interpreted by Olah as indication for the general electrophilic reactivity of covalent C H and C—C single bonds of alkanes and cycloalkanes. The reactivity is due to the tr-donor ability of a shared electron pair (of cr-bond) via two-electron, three-center bond formation. Consequently, the transition state of the reaction, is of three-center bound pentacoordinate carbonium ion nature [Eq. (5.5)]. 

These results are in agreement with the alkane behavior in superacid media and indicate the ease of oxidation of tertiary alkanes. However, high acidity levels are necessary for the oxidation of alkanes possessing only primary C—H bonds. 

Since ozone is a strong 1,3-dipole,635 or at least has a strong polarizability (even if a singlet biradical structure is also feasible), it is expected to be readily protonated in superacids, in manner analogous to its alkylation by alkylcarbenium ions. Protonated ozone HC>3+, once formed, should have a much higher affinity (i.e., be a more powerful electrophile) for cr-donor single bonds in alkanes than neutral ozone. 

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