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Pericyclic Reactions: Aromaticity of Transition States

Aromaticity of Transition States: Rules for Mechanism and Stereochemistry of Pericyclic Reactions

By extending the definition of aromaticity to nonplanar systems, s orbitals and aromatic compounds of the Möbius-type aromatics, the rules can be summarized in one sentence:

Selection rules for pericyclic reactions
Pericyclic reactions preferably proceed through aromatic transition states.

The following procedure explains the individual steps to determine whether a pericyclic reaction proceeds through either an aromatic or antiaromatic transition state:

  1. To begin the analysis, atomic orbitals participating in the pericyclic reaction have to be lined up (one per participating atom).
  2. Next, the cyclic overlap is drawn as a closed curve and the sign inversions are being counted. Zero or even sign inversions represent a Hückel system while odd inversions indicate a Möbius system.
  3. To determine whether the system in question is aromatic or antiaromatic, the number of electrons participating in the reaction, i.e., the delocalized electrons in the transition state, are being counted. The easiest way to determine the number of delocalized electrons consists of counting the number of arrows indicating the flow of electrons. The result, multiplied by two, gives the number of electrons that are required to formally formulate the mechanism.

To determine whether a transition state is aromatic (allowed) or antiaromatic (disallowed), one only must know

  • the topology of the transition state (Hückel or Möbius) and
  • the number of delocalized electrons (4n or 4n+2)

Summary

Tab.1
Cyclic conjugated sytems
Hückel systemsMöbius systems
Even number of sign inversionsOdd number of sign inversions
4n + 2 Electrons4n Electrons4n + 2 Electrons4n Electrons
AromaticAntiaromaticAntiaromaticAromatic
AllowedDisallowedDisallowedAllowed
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