C-H bond protolysis

The reverse reaction of carbenium ions with molecular hydrogen, can be considered as alkylation of H2 through the same pentacoordinate carbonium ions that are involved in C—H bond protolysis. Indeed, this reaction is responsible for the long used (but not explained) role of H2 in suppressing hydrocracking in acid-catalyzed... 

As shown in the case of ethane, the formation of methane is about 15 times higher than that of hydrogen, the byproduct of C—H bond protolysis. The reason can be that H2 readily quenches carbenium ions to their hydrocarbons. Thus ethane... 

Under strongly acidic conditions C-H bond protolysis is not the only pathway by which hydrocarbons are heterolytically cleaved. Carbon-carbon bonds can also be cleaved by protolysis involving pentacoordinate ions [Eq. (6.16)]. 

The first one, (A), includes (b) insertion of CO into the Pd-S bond (c) insertion of the C C triple bond of the enyne into the Pd-C(0)SR bond whereby Pd binds to the terminal carbon and the RSC(O) group to the internal carbon, and (d) C-H bond-forming reductive elimination or protolysis by the thiol to form 29 (Scheme 7-7). 

These reactions could proceed either via (1) insertion of the alkyne or the allene into the M-Se bonds, and (2) C-H bond-forming reductive elimination (or protolysis by selenol) or via (1) insertion of the alkyne or the aUene into the M-H bond, and (2) C-Se bond-forming reductive elimination. 

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 ... 

Similar mechanisms account for the formation of the same products from 2,2,3,3-tetramethylbutane. Besides these predominant pathways, other reactions such as insertion of protonated ozone into the C—C bond and protolysis of the C—H bond may also take place. 

The reaction path depicted in Scheme 5.14 involves Wagner-Meerwein shifts of the methyl group prior to cyclization followed by hydride shift to a number of cationic intermediates. The second scheme (Scheme 5.15) depicts ring closure before methyl migration. The first step involves protolysis of the C—H bond next to the methyl-bearing carbon. The corresponding ion can then rearrange by a 1,2-methyl shift and yield 1,16-dimethyldodecahedrane 28 by hydride abstraction from a hydride donor. 

This reaction is much faster than the carbon-carbon cleavage in neopentane, despite the initial formation of secondary carbenium ions. Norbomane is also cleaved in a fast reaction, yielding substituted cyclopentyl ions. Thus, protonation of alkanes induces cleavage of the molecule by two competitive ways (i) protolysis of a C—H bond followed by /3-scission of the carbenium ions and (ii) direct protolysis of a C—C bond yielding a lower-molecular-weight alkane and a lower-molecular-weight carbenium ion. 

The formation of C6 and C7 acids along with some ketones was reported in the reaction of isopentane, along with methylcyclopentane and cyclohexane with CO in HF-SbF5 at ambient temperatures and atmospheric pressure.406 Yoneda et al.407 have also found that other alkanes can be carboxylated as well with CO in HF-SbF5. Tertiary carbenium ions, which are produced by protolysis of C—H bonds of branched alkanes in HF-SbF5, undergo skeletal isomerization and disproportionation before reacting with CO. Formation of the tertiary carboxylic acids in the... 

Since the early 1960s, superacids have been known to react with saturated hydrocarbons to yield carbocations, even at low temperature [41]. This discovery initiated extensive studies devoted to electrophilic reactions and conversions of saturated hydrocarbons. Thus, the use of superacidic activation of alkanes to their related carbocations allowed the preparation of alkanecarboxylic acids from alkanes themselves with CO. In this respect, Yoneda et al. have found that alkanes can be directly carboxylated with CO in an HF-SbFs superacid system [42]. Tertiary carbenium ions formed by protolysis of C-H bonds of branched alkanes in HF-SbFs undergo skeletal isomerization and disproportionation prior to reacting with CO in the same acid system to form carboxylic acids after hydrolysis (eq. (9)). 

Sommer and coworkers have made important observations with respect to the activation of alkanes over sulfated zirconia, a new type of solid superacid. Whereas isotope exchange of small alkanes occurs with the involvement of the corresponding pentacoordinate ions, a classical carbenium ion-type mechanism was found to be operative for larger homologs (propane, isobutane). The exception is the isomerization of n-butane over sulfated zirconia promoted by Pt and alumina, where the initiation step for isomerization was suggested to be the protolysis of the C-H bond. ... 

Whereas the cyclohexane-methylcyclopentane isomerization involves initial formation of the cyclohexyl (methylcyclopentyl) cation, that is, via protolysis of a C-H bond, it should be mentioned that in the acid-catalyzed isomerization of cyclohexane, up to 10% hexanes are also formed, and this is indicative of C-C bond protolysis (Scheme 6.10). 

A. Goeppert, J. Sommer, H/D exchange, protolysis and oxidation of isopentane and propane in Superacids, a-basicity and reactivity of C-H bonds, New J. Chem., 2002, 26, 1335-1339. 

The protolysis occurs following the direct protonation of the different conditions, protolysis of a C—H bond occurs yielding rearranged t-pentyl ion (t-amyl cation, equation 31). 

Other Six-membered Ring Syntheses.—Alkyl phenyl ketones possessing a branched alkyl chain of five or more carbon atoms undergo protolysis of a tertiary C—H bond on dissolving in the superacid HF-SbFs, producing carbonium ion intermediates which cyclize to 4,4-dialkyl-1-tetralones in good yield, e.g. (138)->(139) ... 

Protolysis can involve not only C—H but also C—C bonds [Eq. (1.23)] this explains why alkanes can be directly cleaved protolytically by superacids, which is of significance, for example, in hydrocleaving heavy oils, shale oil, tar-sand bitumens, and even coals ... 

Treating alkanes with superacids such as FS03H-SbF5, Olah and coworkers observed both C—H and C—C bond protolysis 303... 

Clearly these examples also are in line with what is happening in hydrocracking. In acid-catalyzed hydrocracking cleavage of the larger saturated hydrocarbon chains can take place via p cleavage induced by carbenium cations formed via C—H protolysis, and also via direct C—C bond protolysis (Scheme 11.13). 

Zeolites such as HZSM-5 were considered as superacids on the basis of the initial product distribution in accord with C-H and C-C bond protolysis when isoalkanes were reacted at 500°C (the Haag and Dessau mechanism).135 The reactivity was assigned to superacidic sites in the zeolite framework.136 The superacid character of other solid acids was claimed on the basis of Hammett indicator color change137,138 or on the basis of UV spectrophotometric measurements.139,140 In 2000, a special issue of Microporous and Mesoporous Materials141 was devoted to the superacid-type hydrocarbon chemistry taking place on solid acids as suggested by the late Werner Haag. 

Sigma-Basicity Reversible Protonation or Protolysis of C-H and C-C Bond... 

This direct oxidation generates itself a proton and is probably only a minor pathway under superacidic condition where, in the absence of the proton trap, protolysis of the C—H and C—C bond occurs very rapidly. The mechanism is most probably of electron transfer nature as suggested in Eq. (5.22) and (5.23)... 

Alkanes and Strong Solid Acids. Since the early reports by Nenizetscu and Dragan67 on alkane isomerization on wet aluminum chloride in 1933, all mechanistic studies have led to a general agreement on the carbenium-ion-type nature of the reaction intermediates involved in acid-catalyzed hydrocarbon conversions. In contrast with this statement, the nature of the initial step is still under discussion and a variety of suggestions can be found in the literature among which direct protolysis of C—H and C—C bonds, protonation of alkenes present as traces, and oxidative activation are the most often quoted.54,55... 

Hydrido alkyl species L M(H)(R) are particularly prone to elimination of R—H this thermodynamically favored reaction is the reverse of C—H activation (see Section 21-4) and explains why for a long time intermolecular C—H activation remained elusive. For example, the protolysis of (TMEDA)PtMe2 by HC1 does not lead to the direct electrophilic attack of H+ on the Pt—Me bond but gives thermally unstable hydrido alkyl (TMEDA)Pt(H)ClMe2 which undergoes reductive elimination via a coordinatively unsaturated 5-coordinate intermediate 97... 

A mixture of carboxonium ions are formed when 2,2-dimethylpropane (neopentane) is treated with ozone. " At -78°C, ethyldimethylcarboxonium ion 73 is formed exclusively [Eq. (6.53)]. At temperatures higher than -20°C dimethylmethylcarboxonium ion 67 becomes the predominant product [Eq. (6.54)]. Formation of 73 can be best explained by a reaction path that involves insertion of protonated ozone into the C-H a bond and the 72 transition state [Eq. (6.53)]. The most probable path of the formation of 67, in turn, is protolysis of the C-C bond through transition state 74 [Eq. (6.54)]. 

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