Methylcyclopentyl ion

Tertiary cycloalkyl cations, such as the 1-methylcyclopent-l-yl cation 28, show high stability in strong acid solutions. This ion can be obtained from a variety of precursors (Figure 3.7).143,144 It is noteworthy to mention that not only cyclopentyl- but also cyclohexyl-type precursors give 1 -methylcyclopent-1 -yl cation 28. This indicates that the cyclopentyl cation has higher stability, which causes isomerization of the secondary cyclohexyl cation to the tertiary methylcyclopentyl ion. 

The thermodynamic ratio for the neutral hydrocarbon isomerization is very different as compared with the isomerization of the corresponding ions. The large energy difference (> 10 kcal mol-1) between secondary cyclohexyl cation 22 and the tertiary methylcyclopentyl ion 23 means that in the presence of excess superacid, only the latter can be observed [Eq. (5.47)]. 

However, in the case of long-chain alkylated cyclopentanes, the chain fission dominates the spectra. In fossil fuels the monoalkylated cyclohexanes are characterized by the m/z 83 fragment One should be aware of the problem that fragmentation of alkylated cycloalkanes could be followed by rearrangement of the ions formed hence m/z 83 could also be formed from the methylcyclopentyl ion. Of course we must bear in mind the fact that the molecular weight of monocyclic alkanes is the same as for mono-unsaturated alkenes and, when investigating mixtures, this point is of importance (see also the discussion on fossil fuels. Section VILA). 

Both methylcyclopentane and cyclohexane were found to give the methylcyclopentyl ion which is stable at low temperature, in excess superacid. When alkanes with seven or more carbon atoms were used, cleavage was observed with formation of the stable t-butyl cation. Even paraffin wax and polyethylene ultimately gave the t-butyl cation after complex fragmentation and ionization processes. 

Over a decade of research, we were able to show that practically all conceivable carbocations could be prepared under what became known as stable ion conditions using various very strong acid systems (see discussion of superacids) and low nucleophilicity solvents (SO2, SO2CIF, SO2F2, etc.). A variety of precursors could be used under appropriate conditions, as shown, for example, in the preparation of the methylcyclopentyl cation. 

The principal components of the trityl cation in zeolite HY are <5 = 282 ppm and <5j = 55 ppm. It is instructive to tabulate all of the 13C principal component data measured for free carbenium ions in zeolites as well as for a few carbenium ions characterized in other solid acid media (Table III). The zeolitic species, in addition to the trityl cation (119), are the substituted cyclopentenyl cation 8 (102), the phenylindanyl cation 13, and the methylindanyl cation 12 (113). Values for the rert-butyl cation 2 and methylcyclopentyl cation 17 (prepared on metal halides) (43, 45) are included for comparison. Note that the ordering of isotropic chemical shifts is reasonably consistent with one s intuition from resonance structures i.e., the more delocalized the positive charge, the smaller the isotropic shift. This effect is even more apparent in the magnitudes of the CSA. Since... 

The stereochemical consequences of front- and back-side displacements are different. With cyclic compounds, the two types of displacement lead to different products. For example, an SN2 reaction between cis-3-methylcyclopentyl chloride and hydroxide ion would give the cis alcohol by front-side approach but the trans alcohol by back-side approach. The actual product is the trans alcohol, from which we know that reaction occurs by backside displacement ... 

In an attempt to study the l-methylcyclopentyl-[70] to cyclohexyl-[71] cation interconversion, Olah et al. (1967) tried a number of cyclohexyl- and methylcyclopentyl-precursors under different superacidic conditions at —60°C. However, the only observed product was ion [70]. For the facile rearrangement of [71] to [70] Olah et al. favoured protonated cyclopropanes over primary carbocations as intermediates. 

These ions show close similarity with the corresponding parent ones both experimentally and theoretically. With the aim of studying rearrangements of the methylcyclopentyl cation type Saunders and Rosenfeld (1970) prepared a cation [113] from 1-methylcyclobutyl chloride in SbFj-... 

Photoheterolysis of benzylic chlorides [204] yielded results signifying that simple benzyl cations, such as cumyl and 1-phenylethyl cations, can exist in the solution as free ions radicals arising from a competing photohomolysis are also observed frequently. Haloalkyl-carbocations are studied by heterolysis of the corresponding dihalides in super acid media [205]. NMR chemical shifts are interpreted as evidence for an interaction between the vacant orbital of cationic center of the haloalkyl carbocations with a lone electron pair of the halogen atom. 3-chloro-l-methylcyclopentyl cation 73, thermally eliminates hydrogen chloride and yields l-methyl-2-cyclopentyl cation 74, a similar behavior reported for y-chloroalkyl carbocations [206] (Scheme 5). 

This argument cannot be applied to our examples when large amounts of polymers are formed. Besides, Deno and co-workers 41) have detected in concentrated sulfuric acid solutions of camphene and related compounds (borneol, fenchol) high concentrations of a mono-cyclic cyclohexenic carbonium ion, produced also by proton addition to l-methyl-3-isopropylidene-l-cyclohexene, a clear case of ring breaking in acid media. More recently, Olah and co-workers reported that in highly acidic systems such as HF or HSO3F and SbFs, cyclohexane produces, besides the expected cyclohexyl- and methylcyclopentyl carbonium ions, hexyl and isohexyl carbonium ions 42). 

In a ESCA study with Mateescu and Riemenschneider38 we also succeeded to observe the ESCA spectrum of the norbornyl cation and compared it with that of the 2-methylnorbomyl cation and other trivalent carbenium ions, such as the cyclopentyl and methylcyclopentyl cations. The Is electron spectrum of the norbornyl... 

The HF-SbFs system works well in the Gattermann-Koch formylatlon of arenes and the Koch carbonylation of alkanes [54]. For instance, biphenyl is diformylated in HF-SbFs-CO to afford 4,4 -diformylbiphenyl as a major isomer (Scheme 14.20). The carbonylation of alkanes with C5-C9 carbon atoms in the HF-SbFs-CO system affords mixtures of C3-C8 carboxylic acids after hydrolysis of the generated secondary carbenium ions [55]. Successive treatment of methylcyclopentane with CO in HF-SbF and with water produces cyclohexanecarboxylic acid as a major product (Scheme 14.21) [56]. It seems that a tertiary methylcyclopentyl cation readily isomerizes to the more stable cyclohexyl cation before being trapped by CO. Bicyclic a, -unsaturated ketones are functionahzed by HF-SbF or FSOsH-SbFs under a CO atmosphere to give saturated keto esters after methanolysis (Scheme 14.22) [57]. Alcohols with short carbon chains also react with CO in HF-SbFs to give the corresponding methyl esters [58]. y-Butyrolactones are carboxy-lated under the same conditions to afford 1,5-dicarboxyhc acids [59]. 

Apart from the two extreme categories of classical and nonclassical ions there is a large group of ions with intermediate types of structure. Thus, for a 2-methylnorbomyl ion which was earlier, on the basis of the NMR spectrum, assigned the structure of a partially cr-delocalized ion the value of A8 is 304ppm less than the analogous value for the typically tertiary ions — methylcyclopentyl (374 ppm), methylcyclohexyl (362 ppm), 2-methyl-2-bicyclo[2,2,2]octyl (385 ppm) and 2-methyl-2-bicyclo[3,2,l)octyl (355 ppm). 

Referring carbocations to classical or nonclassical on the basis of the value of AS ( C) it is also useful to compare these values for the secondary ion in question and for the corresponding methyl-substituted tertiary ion. The comparison of classical secondary and tertiary ions shows small differences (for isopropyl and tert-butyl the difference is —4 ppm, for cyclopentyl and methylcyclopentyl it is —10 ppm), for nonclassical ions these differences are far larger 2-norbornyl — 2-methylnorbomyl, — 129 2-bicyclo[2,l,l]hexyl — 2-methyl-2-bicyclo[2,l,l]hexyl, —97 ppm etc. 

Entries 6-9 and 10-12 illustrate the tendency for rearrangements to occur leading to the most stable cation in each particular system. The tertiary 1-methylcyclopentyl cation is the only ion observed from a variety of precursors containing five- and six-membered rings. The tertiary bicyclo[3.3.0]octyl cation is formed from all bicyclooctyl precursors. As previously mentioned, the tendency to rearrange to thermodynamically stable ions by multiple migrations is a consequence... 

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