Carbonium ions cyclopropylmethyl

In contrast to classical tertiary and secondary cyclopropylmethyl cations (showing substantial charge delocalization into the cyclopropane ring but maintaining its identity), primary cyclopropylmethyl cations show completely o-delocalized nonclassical carbonium ion character (see Section 3.5.2.5). Also, some of the secondary cyclopropylmethyl cations undergo rapid degenerate equilibrium (see later discussion). 

An isomer of this carbonium ion has the primary carbon bearing the positive charge. This arrangement is called the cyclopropylmethyl carbon skeleton. If this carbonium ion is used, then it may rearrange to form the cyclobutyl carbonium ion. Suggest how this may occur. 

The solvolysis reactions of cyclopropylmethyl systems also provide evidence for the intermediacy of carbonium ions in nucleophilic substitution reactions. We have seen in Table 5.2 that the stabilizing effect a cyclopropyl group exerts on a carbonium ion is appreciable. A similar effect is evident on comparing rates of hydrolysis of cyclopropylmethyl compounds with model aliphatic compounds. The tertiary p-nitrobenzoates represented by the structure 1 had the relative rates indicated for hydrolysis in 80% aqueous dioxane at 60°C ... 

A ferrocenyl substituent attached to the carbonium ion centre of a cyclopropylmethyl cation provides stabilization of such a magnitude that homoallylic delocalization cannot be detected by spectroscopic methods, and quenching yields unrearranged products. Somewhat related is the observation of optically active bridgehead... 

Solvolysis studies with (357) and (358) show that the latter does not proceed by nitrogen loss and involvement of cyclopropylmethyl carbonium ion. The products from (357) are obtained from (358) only under photolytic conditions, and thus the involvement of diradicals in the ground-state reaction of (358) is also effectively eliminated (Scheme 53). ... 

In comparison with other experimental data, the rates of solvolysis of (496) and (497) allowed an estimation of 14kcalmol for the energy barrier to rotation about the bond which joins the cyclopropyl to the formal secondary carbonium ion of a cyclopropylmethyl cation. This is in good agreement with calculated values and other experimental results. Acid-catalysed solvolysis of the unsaturated compounds (498) gives a mixture of unrearranged products. The stereochemistry of the products exo.endo ca. 85 15) was only slightly affected by the solvent (acetic acid or methanol) and not at all by... 

The benzocyclobutenylmethyl tosylate (504) is the smallest member of a homologous series of benzocycloalkenylmethyl tosylates whose solvolyses have recently been reported. The products of hydrolysis and acetolysis are entirely rearranged and as expected it is the aryl group which migrates, almost certainly via the delocalized ion (505). Cyclopropylmethyl-oxa-carbonium... 

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