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Pericyclic Reactions: Aromaticity of Transition States

Aromaticity of Transition States: Extension of the Definition of Aromaticity Applied to Pericylic Transition States

Planar, "conventional" aromatic compounds

Planar, "conventional" aromatic compounds

6 p Orbitals 6 Electrons

5 p Orbitals 6 Electrons

n p Orbitals n Electrons

In "normal" aromatic compounds p orbitals are oriented in a cyclic arrangement, parallel to each other and perpendicular to the plane of the ring. In nonaromatic transition states, the definition of aromaticity must be expanded to non-planar systems and must include s orbitals, for example

Aromatic and antiaromatic transition states

A set of overlapping orbitals in a cylic arrangement (red line) represents either an aromatic or an antiaromatic sytem. There is no difference to be discerned whether a p orbital is oriented in such a manner that interaction with a neighboring orbital only takes place with one orbital lobe of each (left example) or whether that conjugation (red line) continues through the origin of an p orbital. Furthermore, completion of a cyclic conjugated system from a π system can be achieved by the inclusion of an s orbital. Such unusual looking conjugated systems are rare in "normal" compounds but are the rule in transition states.

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