3- cyclohexyl brosylate

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. 

The magnitude of the /-effect was reported by Shiner and coworkers, who studied the solvolyses of the three cyclohexyl brosylates 51 -5365. 

Shiner and coworkers were the first to study quantitatively the /-silyl effect in solvolysis90-95. They found a distinct kinetic acceleration of the solvolysis of cis-3-(trimethylsilyl)cyclohexyl brosylate 230 compared with the alkyl reference 231 in TFA [fc(230)A(231) = 462] which is equivalent to a lowering of the ionization barrier by... 

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). 

Problem 28.16 How do you account for the following relative rates of acetolysis of 2-substituted cyclohexyl brosylates In which cases is there evidence of a neighboring group effect ... 

Cis-3-(trimethylsilyl) cyclohexyl brosylate 48 solvolysed up to ninety times faster than the corresponding trans isomer 49 and the cis and trans f-butyl compounds. Whilst the cis compound gave mainly the substitution product with retention, the trans compound gave mainly products of elimination, as shown in equations 91 and 92. A large / ... 

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... 

Table 8.5 lists data for acetolysis of 2-substituted cyclohexyl brosylates having the general structure 25. The data confirm that a trans acetate reacts faster than does a cis acetate. The activation entropy data suggest a more ordered transition structure for the acetolysis of the trans isomer, consistent with the model of anchimeric assistance. A trans bromine is nearly as effective as a trans acetoxy, but a trans chloro group is far less effective—apparently due to the low stability of a chloronium ion in comparison with a bromonium ion. 

Similarly, both cis- and frflns-2-bromo-l-cyclohexyl arenesulfonates were converted to cyclohexene upon treatment with sodium iodide in propanol, although the trans isomers were more reactive. For example, trans-2-bromo-l-cyclohexyl tosylate was 62 times as reactive as the cis isomer, and trans-2-bromo-1 -cyclohexyl brosylate was 48 times eis reactive as its cis isomer. Again, the data suggested that the rate-limiting step in the case of the cis isomers is an Sisj2 substitution of iodide for bromide, followed by anti elimination of the trfl s-l-halo-2-iodocyclohexane. ... 

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... 

Relative Rate Constants and Activation Parameters for the Acetolysis of 2-Substituted Cyclohexyl Brosylates ... 

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