Carbocations cyclohexyl

It is believed that this process involves migration through a pentacoordinate protonated cyclopropane in which an alkyl group acts as a bridge in an electron-deficient carbocation structure. The cyclohexyl- methylcyclopentyl rearrangement is postulated to occur by rearrangement between two such structures. 

Both acetolyses were considered to proceed by way of a rate-determining formation of a carbocation. The rate of ionization of the ewdo-brosylate was considered normal, because its reactivity was comparable to that of cyclohexyl brosylate. Elaborating on a suggestion made earlier concerning rearrangement of camphene Itydrochloride, Winstein proposed that ionization of the ero-brosylate was assisted by the C(l)—C(6) bonding electrons and led directly to the formation of a nonclassical ion as an intermediate. 

Aluminum chloride, used either as a stoichiometric reagent or as a catalyst with gaseous hydrogen chloride, may be used to promote silane reductions of secondary alkyl alcohols that otherwise resist reduction by the action of weaker acids.136 For example, cyclohexanol is not reduced by organosilicon hydrides in the presence of trifluoroacetic acid in dichloromethane, presumably because of the relative instability and difficult formation of the secondary cyclohexyl carbocation. By contrast, treatment of cyclohexanol with an excess of hydrogen chloride gas in the presence of a three-to-four-fold excess of triethylsilane and 1.5 equivalents of aluminum chloride in anhydrous dichloromethane produces 70% of cyclohexane and 7% of methylcyclopentane after a reaction time of 3.5 hours at... 

Similar isomerization occurs in the presence of silica-alumina-thoria.104 As it might be expected, this reaction is similar to the isomerization of cyclohexane to methylcyclopentane. Both processes involve the same intermediate cyclohexyl car-bocation, which is formed, however, in different reactions. It may be formed from cyclohexane by hydride ion transfer, or by protonation of cyclohexene. Bicyclic alkenes undergo complex interconversions via carbocations over acidic catalysts.105... 

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... 

Kresge and Tobin48 also studied the hydrolysis of vinyl ethers and found a rate ratio of 130 between methyl vinyl ether and ethyl cA-trimethylsilylvinyl ether, corresponding to a stabilization of the /J-silyl carbocation of 2.9 kcal mol-1. In this case the small rate acceleration (compared to the cyclohexyl systems studied by Lambert) can be attributed to the unfavourable dihedral angle. The dihedral angle in the vinyl ether is 90° (24), and on protonation it drops to 60° (25), whereas maximum hyperconjugative interaction requires a dihedral angle of 0°. 

The unimolecular step in the reaction of cyclohexanol with hydrogen bromide to give cyclohexyl bromide is the dissociation of the oxonium ion to a carbocation. 

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. 

The synthetic equivalent for the cyclohexyl carbocation is cyclohexyl bromide. Thus, cyclohexanol can be prepared by the reaction of cyclohexyl bromide with hydroxide ion. 

The rate of substitution of the endo-brosylate 2,18 was considered normal, since its reactivity is comparable to that of cyclohexyl brosylate. Ionization of the exo-brosylate 2.17 is assisted by the neighbouring C1-C6 bonding electrons participation with the expulsion of the leaving group. The non-classical carbocation 2.20 is formed as an intermediate in which positive charge residing on Cl is delocalized on C2 as well (Scheme 2.15). 

However, not everyone was convinced by the existence of the non-classical carbocation. H. C. Brown 1977 pointed out that the norbornyl compounds are compared with cyclopentyl rather than with cyclohexyl analogues, 2.21 (eclipsing strain), and in such a comparison the endo-isomev is abnormally slow, the exo-isomer being only 14 times faster than cyclopentyl analogues. He also pointed out that the formation of racemic product is due to two rapidly equilibrating classical carbocation species (Scheme 2.17). The interconversion of enantiomeric classical carbocation species must be very rapid on the reaction timescale. 

Over the past few years, the debate over the origin of the p-silicon effect on carbocations has narrowed to one of the relative magnitudes of inductive and hyperconjugative factors. Theory and experiment are finally in agreement that hyperconjugation is by far the dominant factor—29 kcal/mol calculated to be from P-stabilization ( ) versus 9 kcal/mol from induction and polarization. The realization of these effects is dramatically revealed in the SnI solvolyses of the conformationally locked cyclohexyl trifluoroacetates (OTFA) (3-5), The relative solvolysis rates at 25 °C for compounds 3-5 are 1, 4 X 10, and 2.4 X 10, respectively. Compound 4 cannot attain the necessary anti-coplanar relationship of the Si-C and C-O bonds, which is present in 5 and required for full hyperconjugative interaction with the cation formed as the C-O bond suffers heterolysis. 

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. 

Both acetolyses were considered to proceed by way of a rate-determining formation of a carbocation. The rate of ionization of the cnrfo-brosylate was considered normal, since its reactivity was comparable to that of cyclohexyl brosylate. Winstein... 

Hyperconjugation has a profound effect on structure and stability of cyclohexyl cations. An elegant study combined theoretical results with experimental data to confirm that different hyperconjugative stabilization patterns lead to the formation of two equilibrating conformers of the 1-methyl-1-cyclohexyl cation where the carbocation p-orbital is oriented either pseudoaxiaUy or pseudoequatoriaUy. 

Furthermore, the rate constant for the reaction with the fra s-2-acetoxy compound was found to be nearly 10 greater than that of the cis isomer and five times greater than that of cyclohexyl tosylate itself. This evidence supported the view that the acetoxy group participates in the rate-limiting step of the reaction, not in a subsequent step after formation of the intermediate carbocation. The ionization of the tosylate is said therefore to be assisted... 

STRATEGY AND ANSWER We observe that this cyclohexyl bromide is tertiary, and therefore in methanol it should lose a bromide ion to form a tertiary carbocation. Because the carbocation is trigonal planar at the positive carbon, it can react with a solvent molecule (methanol) to form two products. 

The efficiency of oxidation of open-chain alkyl, cycloalkyl, and unsaturated alcohols in acetonitrile by 9-phenylxanthylium ion (PhXn+) was dependent on the alcohol stmc-tures. Structure-reactivity relationship was discussed with relation to formation of a carbocationic transition state (C +-OH). Kinetic isotope effects determined at a-D, p-D3, and OD positions for the reaction of 1-phenylethanol suggested a hydride-proton sequential transfer mechanism that involved a rate-limiting formation of the a-hydroxy carbocation intermediate. Unhindered secondary alkyl alcohols were selectively oxidized in the presence of primary and hindered secondary alkyl alcohols. Strained C(7)-C(ll) cycloalkyl alcohols reacted faster than cyclohexyl alcohol, whereas the strained C(5) and C(12) alcohols reacted slower. Aromatic alcohols were oxidized efficiently and selectively in the presence of aliphatic alcohols of comparable steric requirements. ... 

Not all carbocation rearrangements can be adequately accounted for by 1,2 shifts. For example, the ring contractionlof a cyclohexyl cation to a methylcyclopentyl... 

Brown s approach was to show that the properties ascribed to the nonclassical ion could be duplicated in systems involving classical carbocations. Since the rate enhancement of solvolysis of the exo isomer was an important part of the argument for (7-bond participation, he also analyzed the relative reactivities of various systems. He argued that the cxo-norbomyl system should be compared to the cyclopentyl system rather than to cyclohexyl brosylate. The torsional relationship of the leaving group and adjacent substituents is eclipsed in norbomyl sulfonates, and strain is relieved on ionization. Cyclohexyl brosylate, in contrast, is completely staggered in the ground state, and so no strain relief accompanies ionization. In the cyclopentyl... 

Cyclohexyl carbocation undergoes attack by nucleophiles. With this in mind,... 

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