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Master Organic Chemistry Reaction Guide

Electrocyclic Reactions

Description: Pi systems containing 2 or more conjugated pi bonds can undergo ring closure under thermal (heat) or photochemical (light) conditions. This is called electrocyclic ring closure. The reverse reaction (electrocyclic ring opening) is also possible.

1-description of electrocylic ring opening and closure

Notes: The stereochemistry of the ring opening/closure will depend on the number of pi electrons as well as whether heat ( Δ) or light (hν ) is used.

See these posts for more theoretical background on electrocyclic reactions.

Examples:

2-examples of electrocyclic ring opening and closure

Notes: The first example is a 4pi electrocyclic ring closure under photochemical conditions. The second example is a 4pi electrocyclic ring opening. The third example is a 4 pi electrocyclic ring opening. (Note that this creates a new diene, which can be used for Diels-Alder reactions).

The fourth example is a thermal electrocyclic ring closure. The fifth example is a photochemical 6pi electrocyclic ring opening. The last example is two consecutive electrocyclic reactions – an 8pi thermal electrocyclic ring closure (conrotatory) followed by a 6 pi thermal electrocyclic ring closure (disrotatory).

18-table of conditions - thermal and photochemical conrotatory and disrotatory

Mechanism: Electrocyclic reactions proceed through a cyclic, concerted transition state.

In the first example below, the C1-C2 pi bond breaks at the same time the C3-C4 sigma bond breaks, with simultaneous formation of the C2-C3 and C1-C4 pi bonds (Step 1, arrows A and B).

In the third example below, the C1-C2, C5-C6, and C3-C4 pi bonds all break at the same time, forming the C1-C6 sigma bond, the C4-C5 pi bond, and the C2-C3 pi bond.

3-mechanism for electrocyclic reaction