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Bonus Examples.

The final set of examples show how one can push the rules to their limits; these all constitute quantum predictions and remain to be discovered experimentally! This section is not examinable!
  1. Two (hypothetical) examples of cycloaddition with an even number of antarafacial components. The first is a π4a + π6a cycloaddition, again following a 4n+2 rule of aromaticity. 6a+4a

    This is a π2a + π2a + π2a + π2a photochemical (4n, even antarafacial = hν) addition and was contrived by Woodward and Hoffmann themselves to illustrate the point. blog

  2. A [1,5] migration of a methyl group can either go suprafacially with retention of configuration at the methyl (preserving a plane of symmetry) or antarafacially with inversion of configuration at the methyl (preserving an axis of symmetry). This last reaction corresponds to two antarafacial components. In such a small (six-membered) ring, the latter is calculated to be 23.4 kcal/mol (98 kJ/mol) higher in free energy than the suprafacial/retention 1,5 carbon antarafacial migration with inversion 1,5 Sigmatropic antarafacial with inversion But increasing the ring size to ten, as in a [1,9] methyl shift is predicted to result in a double-Möbius [1,9] antara/inversion mode being 15.2 kcal/mol lower in free energy than the more conventional Hückel [1,9] supra/retention route! In reality, lower energy [1,5] shifts are likely to compete with both the [1,9] shifts. 1,9 methyl antarrafacial shift with inversion
  3. A ten-electron [5,5] sigmatropic rearrangement with two antarafacial components (and following the 4n+2 rule) might be possible. Note the "figure-8, double S" shape. 5,5 sigmatropic with double antarafacial
  4. An example with four antarafacial components, two for methyl transfer with inversion, and two for antarafacial transfers across a chain. Its even possible to formulate reactions with six antarafacial components!. 5,5 Sigmatropic with antarafacial and inversion

The End (for real)

© Henry S. Rzepa, 1978-2014. Hide|show Toolbar.