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- Extended Pi systems with a vicinal sigma bond may rearrange by breaking
the sigma bond and forming a new sigma bond down the extended Pi system in a Pericyclic manner
with no net gain or loss of sigma or pi bonds.
- This is an intramolecular reaction.
- The reactions are classified by numbering from the broken sigma bond and
listing where the Pi systems rejoin in a new sigma bond, [n, m].
- The Cope and Claisen rearrangements are [3,3] Sigma-Tropic
rearrangements.
- If n + m is an even number, the stereochemistry of the newly formed
sigma bond follows n+m in the given table.
- Hence, apparent concerted [1,3] sigmatropic rearrangements like
enolizations are really step-wise acid or base catalyzed tautomerizations
but [1,7] reactions are physically possible.
- Supra-antara reactions may proceed by inversion of configuration at a
chiral center.
| Electrons [m+n]\Conditions |
Heat (not light) |
Light |
| 4n, n = 1, 2, ... (e.g. [1,3], [1,7]) |
supra-antara |
supra-supra |
| 4n+2, n = 0, 1, ... (e.g. [1,5],[3,3]) |
suprar-supra |
supra-antara |
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Sigmatropic Rearrangements