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Pericyclic Reactions: Aromaticity of Transition States

Aromaticity of Transition States: Summary

If extension of the definition of aromaticity is applied to nonplanar systems, s orbitals and Möbius aromatic compounds, the rules can be summarized in one sentence:

Selection Rules for Pericyclic Reactions
Pericyclic reactions preferrably pass through aromatic transition states.
  • Hückel systems exhibit an even number of sign inversions including zero in the cyclic delocalized basis set, regardless how the signs are chosen. They are aromatic with 4n+2 electrons.
  • Möbius systems exhibit an odd number of sign inversions in the cyclic delocalized basis set, regardless how the signs are chosen. They are aromatic with 4n electrons.
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