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30.5: Anchimeric Assistance by Other Neighboring Groups

  • Page ID
    44004
  • 5. Anchimeric Assistance by Other Neighboring Groups

    The ability of the pi-electrons in a suitably oriented, neighboring benzene ring to facilitate C-X ionization, where X is a halogen or a sulfonate ester, was described in the previous section. Other aromatic rings, such as naphthalene, furan and thiophene, may function in a similar manner, as may the pi-electrons of double and triple bonds. The following diagram shows three examples of neighboring double bond interaction, the first being one of the most striking cases of anchimeric assistance on record. The use of dashed lines to show charge delocalization is a common practice. The text box below the diagram provides additional commentary concerning these examples.

    Neighboring
    Group

    Double Bonds

    Triple Bonds

    Sulfur Atoms

    Oxygen Atoms

    Nitrogen Atoms

    Examples of other neighboring group perturbations, including non-bonding electron pair assistance by neighboring sulfur, oxygen and nitrogen atoms will be displayed above by clicking the appropriate button under the diagram. The text box commentary will change to suit the examples. In most of the cases involving heteroatom assistance, an "onium" intermediate is formed, in which the heteroatom is charged. Adjacent halogen atoms may also stabilize carbocations, as noted earlier with respect to trans-anti additions to cyclic alkenes. Functional rearrangement by way of halonium intermediates has also been reported. For example, a chloroform solution of the diaxial 2-bromo-3-chlorosteroid, shown on the left below, spontaneously rearranges to the more stable diequatorial 2-chloro-3-bromo isomer drawn on the right. The rearrangement is reversible and proceeds by way of the cyclic bromonium ion written in brackets.