Methylcyclopentyl cation

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 ring contraction of a cyclohexyl cation to a methylcyclopentyl cation is thermodynamically favorable but would require a substantial energy of activation if the rearrangement proceeded through a primary cyclopentylmethyl catiotL... 

Redman, E.W. Morton, T.H. Product-Determining Steps in Gas-Phase Breasted Acid-Base Reactions. D rotonation of 1-Methylcyclopentyl Cation by Amine Bases. J. Am. Chem. Soc. 1986,108, 5701-5708. 

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... 

Summary of Chemical Shift Parameters for the Trityl Cation 16, Phenylindanyl Cation 13, Cyclopentenyl Cation 8, Methylindanyl Cation 12, tert-butyl Cation 2, and Methylcyclopentyl Cation 17... 

Figure 24 reports 13C MAS spectra of the ferf-butyl cation (43) and the methylcyclopentyl cation 17 (45) on the solid metal halides A1C13 and AlBr3 the asymmetry parameters, CSAs, and isotropic shifts (Table III) are unambiguous for the species indicated. Repeated attempts in various laboratories to observe the ferf-butyl cation as a persistent species in a zeolite have thus far been unsuccessful. Detailed theoretical work will be required to determine whether or not the ferf-butyl cations are local minima (i.e., true intermediates) on typical reaction pathways in zeolites. The ease with which these cations form in true superacids (liquid or solid) should be contrasted with the history of negative observations in zeolites. 

Fig. 24. 75.4-MHz 13C MAS spectra showing the formation of the fert-butyl cation and the methylcyclopentyl cation on A1C13 and AlBr3. The methylcyclopentyl cation was synthesized by an intermolecular hydride transfer reaction as shown in the figure.

When cyclohexene is mixed with anhydrous triflic acid under a high pressure of carbon monoxide (120 atm) followed by the addition of benzene, cyclohexyl phenyl ketone and the isomeric cyclohexenyl cyclohexyl ketones are obtained with little isomerization of the initially formed cyclohexyl cation 22 to methylcyclopentyl cation 23 (Scheme 5.46).422... 

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). 

It is worthwhile to mention that not only cyclopentyl but also the cyclohexyl type precursors gives the 1-methylcyclopentyl cation 36. This indicates that the cyclopentyl cation has higher stability, which causes isomerization of the secondary cyclohexyl cation to the tertiary methylcyclopentyl ion. 

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... 

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). 

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]. 

All five products (boxed) come from rearranged carbocations. Rearrangement, which may occur simultaneously with ionization, can occur by hydride shift to the 3° methylcyclopentyl cation, or by ring expansion to the cyclohexyl cation. 

Entries 6-9 and 10-12 in Scheme 5.3 further illustrate the tendency for rearrangement to the most stable cation to occur. The tertiary 1-methylcyclopentyl cation is... 

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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