Carbocations cyclohexyl, rearrangement

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. 

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. 

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. 

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

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