Cyclohexyl cations

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... 

Monosubstituted Alkenes. Simple unbranched terminal alkenes that have only alkyl substituents, such as 1-hexene,2031-octene,209 or ally Icy clohexane230 do not undergo reduction in the presence of organosilicon hydrides and strong acids, even under extreme conditions.1,2 For example, when 1-hexene is heated in a sealed ampoule at 140° for 10 hours with triethylsilane and excess trifluoroacetic acid, only a trace of hexane is detected.203 A somewhat surprising exception to this pattern is the formation of ethylcyclohexane in 20% yield upon treatment of vinylcyclohexane with trifluoroacetic acid and triethylsilane.230 Protonation of the terminal carbon is thought to initiate a 1,2-hydride shift that leads to the formation of the tertiary 1-ethyl-1-cyclohexyl cation.230... 

This bimolecular mechanism also applies to cycloalkanes which can be activated by intermolecular hydride transfer to small carbenium ions to form cyclohexyl cations prior to cracking. Alternately, the cyclohexyl cations can deprotonate and form cyclohexene. With two similar intermolecular hydride transfers an aromatic can also form [46]. 

Why does hydrochlorination of /-butylethylene not also proceed in part by a termolecular mechanism The apparent reason is shown in Table 7.3 The carbocation formed from /-butylethylene is more stable than the cyclohexyl cation, and therefore kn of Equation 7.8 is larger for /-butylethylene. Furthermore, /-butylethylene has a small k2 because of steric interference of the bulky t-butyl group in a termolecular transition state. Table 7.3 gives the estimated rate constants, klt k2, and k3 of Equation 7.8 for four olefins. The rate constant, kx, decreases with the ability of the substrate to stabilize a positive charge. The larger value of k2 for 1,2-dimethylcyclohexene than for cyclohexene means that the j8 carbon in the transition state of the Ad3 mechanism has some cationic character... 

The more negative pAR is preferred for two reasons. Firstly, as will be clear below, it is correlated better with the gas-phase stability of the isopropyl cation. Secondly, derivation of pAR 16.5 for the cyclohexyl cation from the rate constant for protonation of cyclohexene98 gives a value which, for a secondary alkyl cation, seems too close to that of the t-butyl cation (pAR = -16.4), even though the difference is increased by 2.1 log units if allowance is made for the more favorable geminal interactions of an OH... 

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)]. 

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... 

The species that attacks the benzene ring is cyclohexyl cation, formed by protonation of cyclohexene. 

The mechanism for the reaction of cyclohexyl cation with benzene is analogous to the general mechanism for electrophilic aromatic substitution. 

Stabilization by the bisected spirocyclopropyl group was employed in the successful preparation of the secondary cyclohexyl cation 101, which is otherwise unobtainable as a long-lived ion . Ion 101 was prepared by three routes, starting from 2-spirocyclopropyl-cyclohexanol, rra 5-bicyclo[4.2.0]octan- l-ol or bicyclo[4.1.0]hept-l-yl methanol and ShF / SOCIF at -78 °C. The bisected nature of the cyclopropyl group is indicated by a single absorption for the cyclopropyl methylene groups. The ion is stable up to -10 °C, where it rearranges to the bicyclo[3.3.0]oct-l-yl cation 69 (equation 60). 

Dimethyl-2-pentyl cation and 1,3-dimethyl-1-cyclohexyl cation 256... 

Dimethyl-2-hexyl cation and 1,4-dimethyl-1-cyclohexyl cation 257... 

When a cyclohexyl cation is produced in the solvolysis of a homoallylic system, formation of a three-membered ring can occur. 

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