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Electrocyclic Reactions

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The question this page answers: What are the characteristics of electrocyclic reactions?

Deeper reading: Clayden 2e: Chapter 35 Pages 922–930 — see our chapter-by-chapter practice map for Clayden.

Part of the Pericyclic Reactions overview.

Counting electrons: 4π and 6π processes

How do we name an electrocyclic reaction?

Electrocyclic reactions are described by the number of electrons involved in the process.

Here is a 4π electrocyclic ring-opening:

Here is a 4π electrocyclic ring-opening:

Here is a 6π electrocyclic ring-closing:

Here is a 6π electrocyclic ring-closing:

Conrotatory vs. disrotatory motion

Which way do the ends of the π system rotate?

The Woodward-Hoffmann Rules tell us that electrocyclic reactions take place stereospecifically.

The rules describe whether the two atoms at the ends of the π system rotate in the same direction (conrotatory) or in opposite directions (disrotatory).

The rules describe whether the two atoms at the ends of the π system rotate in the same direction (conrotatory) or in opposite directions (disrotatory

Different π MOs are required under different reaction conditions in order to have the same phasing required for orbital overlap. Here are the Woodward-Hoffmann Rules for allowed electrocyclic reactions:

Different π MOs are required under different reaction conditions in order to have the same phasing required for orbital overlap. Here are the Woodward

Torquoselectivity

When does the direction of rotation matter?

Electrocyclic reactions can potentially display torquoselectivity between the two possible rotations for a given process.

Torquoselectivity is usually unimportant in electrocyclic ring-closing reactions because the possible products are enantiomers and are thus formed in equal amounts:

Torquoselectivity is usually unimportant in electrocyclic ring-closing reactions because the possible products are enantiomers and are thus formed in

In electrocyclic ring-opening reactions, however, sterics can favor one product over the other:

In electrocyclic ring-opening reactions, however, sterics can favor one product over the other:

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