Superacids bond cleavage

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. 

Small Alkanes with More than Two Carbon Atoms. Whereas methane and ethane show a very similar behavior towards superacidic media, alkanes with more than two carbon atoms undergo a more complex reaction scheme in which C—H and C—C bond cleavage compete with reversible protonation. In order to follow the initial steps, it is very useful to run the reaction in presence of carbon monoxide, which reacts rapidly with the initially formed carbenium ions yielding stable oxocarbenium ions unable to activate alkanes by hydride transfer.53... 

Tetramethyl-2,4-pentanediol forms the oxonium dication 230 in superacid, and dehydration is followed by carbon- carbon bond cleavage. Although such cleavage reactions can occur with monocationic onium ions, in this case the cleavage reaction is likely an indication of the super-electrophilic nature of the dicationic intermediate(s). 

The characteristic transformation of branched alkanes with ozone in superacidic media is C-C bond cleavage. When a stream of oxygen containing 1-5% ozone was passed through a solution of isobutane in FSO3F-SbF5-SO2ClF solution held at -78°C, the H and C NMR spectra of the resultant solution were consistent with the formation of dimethylmethylcarboxonium ion 67 [Eq. (6.52)] in 45% yield. Similar treatment of isopentane, 2,3-dimethylbutanc, and 2,2,3-trimethylbutane resulted in the formation of related carboxonium ions as the major products. 

When the dry ozonation technique is employed (dry silica gel with a preadsorbed organic substrate is saturated with ozone at -78°C then warmed up to room temperature), ozone exhibits an enhanced reactivity, presumably due to the slightly acidic nature of silica gel. Regioselectivities and stereoselectivities under these conditions are very similar to those in superacidic liquid-phase ozonation. Straight-chain alkanes are oxidized at the secondary carbon to yield a mixture of alcohols and ketones, Branched acyclic alkanes give tertiary alcohols and yield ketones via C-C bond cleavage, Insertion of ozone into the C-H and C-C a bonds was evoked to account for the observations (Scheme 6.27). 

The described alkane hydrocarboxylations show a number of important features. In particular, very high product yields (up to 95% based on alkane) can be attained [12-15, 18-22], especially considering the exceptional inertness of saturated hydrocarbons and the fact that such reactions involve C-H bond cleavage, C-C bond formation, and proceed in an acid-solvent-free H20/MeCN medium and at very mild temperatures (50-60 °C). Besides, these hydrocarboxylation reactions contrast with most of the state-of-the-art processes [1, 2] for the relatively mild transformations of alkanes that require the use of strongly acidic reaction media, such as concentrated trifluoroacetic or sulfuric acid, or a superacid. 

The superacid-catalyzed cracking of hydrocarbons (a significant practical application) involves not only formation of trivalent carbo-cationic sites leading to subsequent /3-cleavage but also direct C-C bond protolysis. 

It is postulated that the facile cleavage of the complex is due to the polarization of the carbon-nitrogen double bond. At pH 7, where these investigations were carried out, the spontaneous decomposition of the Schiff base is very slow, while the metal ion-catalyzed reaction has a half life of a few minutes. Since the hydrolysis of Schiff bases is catalyzed by hydrogen ion, the metal ion catalyst can be postulated to be a superacid catalyst present in neutral solution (17, 18). 

Protolytic reactions of saturated hydrocarbons in superacid media21 were interpreted by Olah as proceeding through the protonation (protolysis) of the covalent C—H and C—C single bonds. The reactivity is due to the electron donor ability of the <7 bonds via two-electron, three-center bond formation. Protolysis of C—H bonds leads via five-coordinate carbocations with subsequent cleavage of H2 to trivalent ions, which then themselves can further react in a similar fashion ... 

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 ... 

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 ... 

The rearrangement of some resin acids 226,227, and 228 in superacidic HSO3F and HSO3CI media has also been studied.836,837 Jacquesy and et al.838,839 have developed a novel isomerization of pregnan-3,20-diones 229 to mixture of isomers that also contains 13a isomers. The reaction is proposed to occur through the cleavage of C(13)-C(17) bond (Scheme 5.83). 

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