14.5 Characteristics of the Diels–Alder Reaction
2 min read•may 7, 2024
The is a powerful tool for creating complex molecules. It combines a and to form a cyclohexene ring, with the reactants' features determining the product's structure. Understanding these characteristics is key to predicting and controlling reaction outcomes.
, strained cyclic dienophiles, and s- diene conformations boost reactivity. The reaction's stereochemistry is governed by the and maintains the starting materials' configuration. These principles help chemists design effective syntheses and predict product structures.
Diels-Alder Reaction Characteristics
Features of good dienophiles
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- Electron-withdrawing groups (EWGs) attached to the dienophile lower the energy facilitating the reaction ( , \ce{C#N}, )
- Conjugated dienophiles have EWGs conjugated with the double bond further lowering the LUMO energy making them more reactive
- Strained cyclic dienophiles () are more reactive as the ring strain is relieved upon forming the driving the reaction forward
Stereochemistry in Diels-Alder reactions
- Cis dienes lead to cis substitution patterns and dienes lead to trans substitution patterns in the product maintaining the stereochemistry of the diene
- Cis dienophiles lead to cis stereochemistry and trans dienophiles lead to trans stereochemistry in the product maintaining the stereochemistry of the dienophile
- Endo rule favors the transition state with the bulk of the dienophile oriented towards the diene leading to the endo product even if less thermodynamically stable than the
- Stereochemistry is retained as the reaction proceeds through a maintaining the stereochemistry of the reactants in the product
- The Diels-Alder reaction occurs via , where both new bonds form on the same face of the π system
Conformational requirements for dienes
- Dienes must adopt an to participate in the reaction allowing for proper orbital overlap with the dienophile
- 1,3-disubstituted dienes favor the due to steric hindrance making them less reactive in Diels-Alder reactions
- Cyclic dienes () are locked in the making them more reactive in Diels-Alder reactions
- Bulky substituents on the diene can hinder the approach of the dienophile leading to reduced reactivity or selectivity for the less hindered face of the diene
Theoretical Foundations and Related Reactions
- The Diels-Alder reaction is a type of pericyclic reaction, characterized by a cyclic transition state and concerted electron movement
- explains the reactivity and selectivity of Diels-Alder reactions by considering HOMO-LUMO interactions
- As a [4+2] , the Diels-Alder reaction combines a 4π electron system (diene) with a 2π electron system (dienophile)
- The reverse process, known as the , can occur under certain conditions, breaking down the cycloadduct into its component diene and dienophile
Key Terms to Review (28)
Carbonyl: The carbonyl group is a functional group consisting of a carbon atom double-bonded to an oxygen atom. It is a key structural feature in many organic compounds, including aldehydes, ketones, carboxylic acids, and esters, and plays a crucial role in their chemical reactivity and properties.
Cis: The term 'cis' refers to a spatial arrangement of atoms or groups in a molecule where two identical substituents are on the same side of a carbon-carbon double bond or a ring structure. This orientation is in contrast to the 'trans' configuration, where the identical substituents are on opposite sides.
Concerted Cyclic Transition State: A concerted cyclic transition state refers to the arrangement of atoms in the rate-determining step of a pericyclic reaction, such as the Diels-Alder reaction. It describes the simultaneous formation of new bonds and breaking of old bonds that occur in a cyclic manner during the reaction process.
Cycloaddition: Cycloaddition is a fundamental organic chemistry reaction in which two or more unsaturated molecules, or parts of the same molecule, combine to form a cyclic adduct. This process is a powerful tool for the synthesis of a wide range of carbocyclic and heterocyclic compounds, and it is particularly important in the context of alkene oxidation, carbene addition, the Diels-Alder reaction, and various thermal electrocyclic and cycloaddition reactions.
Cycloadduct: A cycloadduct is a product formed in a cycloaddition reaction, such as the Diels-Alder reaction, where two or more reactants combine to form a cyclic structure. It is a key intermediate in many organic synthesis pathways.
Cyclopentadiene: Cyclopentadiene is a cyclic, unsaturated hydrocarbon with the molecular formula C₅H₆. It is a key component in the Diels-Alder cycloaddition reaction and is also involved in the formation of aromatic ions and sigmatropic rearrangements.
Diels-Alder Reaction: The Diels-Alder reaction is a type of cycloaddition reaction in organic chemistry where a conjugated diene reacts with a dienophile to form a cyclic product. It is a powerful tool for the synthesis of complex cyclic compounds and is widely used in organic synthesis.
Diels–Alder cycloaddition reaction: The Diels–Alder cycloaddition reaction is a chemical process in organic chemistry where a conjugated diene reacts with a substituted alkene (dienophile) to form a six-membered ring. This reaction occurs through a single, concerted step without the formation of intermediates.
Diene: A diene is a hydrocarbon compound that contains two carbon-carbon double bonds. Dienes are important in the context of various organic chemistry topics, including electrophilic additions to conjugated dienes, the Diels-Alder reaction, cycloaddition reactions, and pericyclic reactions.
Dienophile: A dienophile is a chemical species that is capable of undergoing a Diels-Alder cycloaddition reaction. It is an electrophilic component that reacts with a diene, the nucleophilic component, to form a cyclic product.
Electron-transport chain: The electron-transport chain is a series of protein complexes and small molecules within the mitochondrial membrane that transfer electrons, derived from nutrients, to molecular oxygen, creating a proton gradient used to produce ATP. This process is essential for aerobic respiration and plays a critical role in the body's energy production.
Electron-Withdrawing Groups: Electron-withdrawing groups are functional groups or substituents in a molecule that have a strong affinity for electrons, making them attractive to electrons. This property can significantly influence the reactivity, stability, and spectroscopic properties of the molecule.
Endo Rule: The endo rule is a principle that governs the stereochemistry of Diels-Alder cycloaddition reactions. It states that the preferred product of a Diels-Alder reaction is the endo isomer, where the substituents on the diene and dienophile are positioned on the same side of the newly formed ring.
Exo Product: In organic chemistry, the exo product refers to the stereochemical outcome of a Diels-Alder reaction where the newly formed substituents are positioned on the same side of the cyclic structure. This is in contrast to the endo product, where the substituents are positioned on opposite sides.
Frontier Molecular Orbital Theory: Frontier Molecular Orbital Theory is a model that describes the reactivity of organic molecules based on the behavior of their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). It provides a framework for understanding and predicting the outcomes of pericyclic reactions, such as the Diels-Alder cycloaddition reaction.
Lowest unoccupied molecular orbital (LUMO): The LUMO is the lowest energy molecular orbital that does not contain electrons but can accept them during chemical reactions or excitations. It plays a crucial role in determining the reactivity and properties of molecules, especially in conjugated systems analyzed by ultraviolet spectroscopy.
LUMO: LUMO, or Lowest Unoccupied Molecular Orbital, is a fundamental concept in molecular orbital theory that describes the energy level of the highest-energy orbital that is not occupied by electrons in the ground state of a molecule. The LUMO is crucial in understanding the stability and reactivity of conjugated systems, as well as the behavior of molecules in various photochemical and pericyclic reactions.
Nitrile: A nitrile is a functional group consisting of a carbon-nitrogen triple bond (C≡N). Nitriles are important in organic chemistry, with applications in the synthesis of various compounds and as precursors to other functional groups.
Nitro: The nitro group (NO2) is a functional group composed of a nitrogen atom double-bonded to two oxygen atoms. It is a key structural feature in organic chemistry, with significant implications in various reactions and properties of compounds.
Nitrogen rule: The Nitrogen Rule in organic chemistry is a guideline stating that organic compounds with an odd number of nitrogen atoms will have an odd molecular mass. This rule is useful for determining the possible presence of nitrogen in a compound based on its molecular ion peak in mass spectrometry.
Norbornene: Norbornene, also known as bicyclo[2.2.1]hept-2-ene, is a cyclic alkene compound that is commonly used in organic chemistry, particularly in the context of the Diels-Alder reaction and olefin metathesis polymerization. It is a strained, bicyclic hydrocarbon with a rigid structure that exhibits unique reactivity and applications.
Pericyclic Reactions: Pericyclic reactions are a class of organic reactions that involve the concerted rearrangement of pi-electrons within a cyclic transition state. These reactions are characterized by their unique mechanism, which allows for the formation or cleavage of cyclic structures through the simultaneous breaking and forming of chemical bonds.
Retro-Diels-Alder reaction: The retro-Diels-Alder reaction is the reverse of the Diels-Alder cycloaddition reaction, where a cyclic adduct is broken down into its diene and dienophile components. This process is particularly useful in organic synthesis for the selective cleavage of cyclic structures to access desired starting materials.
S-Cis conformation: In organic chemistry, the s-cis conformation describes the arrangement of atoms in a conjugated diene system where the diene is in a planar, extended conformation, allowing for optimal orbital overlap. This arrangement is crucial for the Diels–Alder reaction to occur as it facilitates the interaction between the diene and the dienophile.
S-cis Conformation: The s-cis conformation refers to the spatial arrangement of atoms in a molecule where two substituents are positioned on the same side of a carbon-carbon double bond. This structural feature is particularly relevant in the context of the Diels-Alder cycloaddition reaction and the stereochemistry of cycloadditions.
S-trans Conformation: The s-trans conformation is a specific arrangement of atoms in organic molecules where the substituents on adjacent carbon-carbon double bonds are oriented in a trans configuration. This structural feature is particularly relevant in the context of the Diels-Alder reaction, as it plays a crucial role in the stereochemistry and reactivity of the cycloaddition process.
Suprafacial Addition: Suprafacial addition is a type of cycloaddition reaction where the new bonds are formed on the same side of the reactants. This is a key concept in understanding the Diels-Alder cycloaddition reaction and its characteristics.
Trans: The term 'trans' refers to the spatial arrangement of atoms or functional groups in a molecule, particularly in the context of alkenes, Diels-Alder reactions, and steroids. It describes a configuration where the substituents are on opposite sides of a carbon-carbon double bond or a cyclic structure.