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Cycloadditions and Diels-Alder Reactions

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

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

Part of the Pericyclic Reactions overview.

The [p + q] nomenclature

What do the numbers in [4 + 2] mean?

Cycloaddition/cycloreversion reactions are given a nomenclature of [p + q].

The q and p in [p + q] refer to the number of electrons in each of the reacting π systems:

The q and p in [p + q] refer to the number of electrons in each of the reacting π systems:

Selection rules for cycloadditions

Why heat for [4 + 2] but light for [2 + 2]?

The simplified Woodward-Hoffmann Rules says that: 1) [2 + 2] cycloadditions take place under photochemical reaction conditions. 2) [4 + 2] cycloadditions take place under thermal reaction conditions.

In more detail, for a [p + q] cycloaddition, these are the selection rules:

In more detail, for a [p + q] cycloaddition, these are the selection rules:

In general, it is more common for cycloadditions to take place between two π-systems that are both reacting suprafacially. Here is an analysis for a [4 + 2] cycloaddition, where we are looking at the HOMO of the diene and the LUMO of the monoene:

In general, it is more common for cycloadditions to take place between two π-systems that are both reacting suprafacially. Here is an analysis for a [

The Diels-Alder reaction

What reacts with what in a Diels-Alder reaction?

The Diels-Alder reaction refers specifically to [4 + 2] cycloadditions.

The cycloaddition reaction breaks two π and makes two σ bonds, and is exothermic by ~40 kcal/mol. In conventional Diels-Alder reactions, an electron-poor dienophile reacts with an electron-rich diene, which are analogous to electrophile and nucleophile, respectively:

The cycloaddition reaction breaks two π and makes two σ bonds, and is exothermic by ~40 kcal/mol. In conventional Diels-Alder reactions, an electron-p

The retro-Diels-Alder reaction

Can a Diels-Alder reaction run in reverse?

The Diels-Alder reaction is reversible in what is called the retro-Diels-Alder reaction.

At higher temperatures, entropy becomes important and the cycloreversion is favored:

At higher temperatures, entropy becomes important and the cycloreversion is favored:

Stereoselectivity: endo, exo, and the s-cis diene

Which product wins: endo or exo?

Diels-Alder reactions are stereoselective.

The endo product is favored over the exo product because the endo product is formed via a transition state that has stabilizing π-π interactions:

The endo product is favored over the exo product because the endo product is formed via a transition state that has stabilizing π-π interactions:

The diene must be in a s-cis conformation in order to react, which leads some dienes to be more difficult to conduct Diels-Alders reactions with:

The diene must be in a s-cis conformation in order to react, which leads some dienes to be more difficult to conduct Diels-Alders reactions with:

The groups at the ends of reacting π systems will end up predictably either cis or trans to each other, depending on whether the reaction is endo or exo:

The groups at the ends of reacting π systems will end up predictably either cis or trans to each other, depending on whether the reaction is endo or e

Electronic effects on reactivity

Which dienes and dienophiles react fastest?

The electronics of the diene and dienophile affect reactivity.

More electron-poor dienophiles are more reactive:

More electron-poor dienophiles are more reactive:

Inverse-demand Diels-Alders reactions are cases where the dienophile is electron-rich and the diene is electron-poor:

Inverse-demand Diels-Alders reactions are cases where the dienophile is electron-rich and the diene is electron-poor:

Regioselectivity: “ortho” and “para” products

Where do the substituents end up in the product?

Typical Diels-Alder reactions are regioselective and proceed to form “ortho” or “para” products.

The regioselectivity arises from electronic matching between reactants. 1-Substituted dienes react to yield “ortho” products, often exclusively:

The regioselectivity arises from electronic matching between reactants. 1-Substituted dienes react to yield “ortho” products, often exclusively:

2-Substituted dienes react to yield “para” products preferentially:

2-Substituted dienes react to yield “para” products preferentially:

The 1-substituent is more directive than the 2-substituent:

The 1-substituent is more directive than the 2-substituent:

Lewis acid catalysis

How do we make a Diels-Alder reaction faster and more selective?

The use of Lewis acid catalysts improves Diels-Alder reaction rates and regioselectivities.

Lewis acids enhance the electrophilicity of the dienophile, leading to improved regioselectivity and reaction rate:

Lewis acids enhance the electrophilicity of the dienophile, leading to improved regioselectivity and reaction rate:

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