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Pericyclic Reactions

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The question this page answers: How do we classify cyclic concerted reactions?

Deeper reading: Clayden 2e: Chapter 34 Pages 877–908 and Chapter 35 Pages 909–930 — see our chapter-by-chapter practice map for Clayden.

The four classes of pericyclic reactions

What do pericyclic reactions have in common?

Pericyclic reactions are concerted and go through cyclic transition states.

This class of reactions encompasses a variety of transformations in which the reacting orbital(s) are overlapped in a continuous cycle. Pericyclic reactions can be categorized by the change in # of σ and π bonds:

NameBond Changes
SigmaPi
Cycloaddition (and Cycloreversion)+2 (–2)–2 (+2)
Electrocyclic Ring-Closing (and Ring-Opening)+1 (–1)–1 (+1)
Sigmatropic Reaction00
Ene Reaction+1–1

Cycloadditions and cycloreversions

How do two π systems become a ring?

Cycloaddition reactions result in the formation of a new ring from two π systems. Cycloreversion reactions break apart a ring to form two π systems.

Here is a [4 + 2] cycloaddition:

Here is a [4 + 2] cycloaddition:

Here is a [2 + 2] cycloreversion:

Here is a [2 + 2] cycloreversion:

Covered in depth: Cycloadditions and Diels-Alder Reactions.

Electrocyclic reactions

What does an electrocyclic reaction look like?

Electrocyclic reactions can either form a ring or open a ring.

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:

Covered in depth: Electrocyclic Reactions.

Sigmatropic reactions

Which pericyclic reactions are rearrangements?

Sigmatropic reactions are also recognized as rearrangement reactions.

Here is a [1, 5] sigmatropic H-shift:

Here is a [1, 5] sigmatropic H-shift:

Covered in depth: Sigmatropic Reactions.

The ene reaction

What reacts in an ene reaction?

In the ene reaction, a π bond reacts with an allylic hydrogen.

Here is an ene reaction:

Here is an ene reaction:

Stereospecificity and the Woodward-Hoffmann rules

What controls whether a pericyclic reaction is allowed?

Pericyclic reactions proceed with stereospecificity that depends on: 1) the # of π electrons 2) the reaction conditions.

The Woodward-Hoffmann Selection Rules, which derive from analyses of orbital symmetry, are applied to determine the nature of the cyclization process, which depends on whether the π systems are reacting antarafacially or suprafacially:

The Woodward-Hoffmann Selection Rules, which derive from analyses of orbital symmetry, are applied to determine the nature of the cyclization process,

A given pericyclic process will only be allowed under either thermal conditions or photochemical conditions, not both. See the other notes for more information.

The dedicated guides: Cycloadditions and Diels-Alder Reactions, Electrocyclic Reactions, and Sigmatropic Reactions.

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