14.4 The Diels–Alder Cycloaddition Reaction
2 min read•may 7, 2024
The Diels-Alder reaction is a powerful tool for creating six-membered rings. It combines a diene and , forming two new bonds in one step. This reaction is key for building complex molecules efficiently.
Understanding the Diels-Alder mechanism helps predict product structures and plan syntheses. It's a concerted process, meaning all bonds break and form at once, leading to specific stereochemistry in the final product.
The Diels-Alder Cycloaddition Reaction
Diels-Alder cycloaddition reaction
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- [4+2] between (4 electrons) and dienophile (alkene or alkyne with 2 electrons)
- Diene must adopt for proper orbital alignment
- Common dienes: , ,
- Dienophile typically electron-deficient alkene or alkyne
- Common dienophiles: , ethylene, acetylene
- Diene must adopt for proper orbital alignment
- Forms six-membered ring product ()
- Two new bonds formed between diene and dienophile
- Dienophile bond converted to bond in product
- Electrocyclic process thermally allowed with 4n+2 electrons (n = 0, 1, 2, etc.)
- Follows for
- Classified as a based on orbital symmetry considerations
Pericyclic vs polar and radical mechanisms
- Diels-Alder reaction proceeds via concerted, pericyclic mechanism
- All bond breaking and forming occurs simultaneously in single step
- No intermediates formed during reaction
- Does not involve ionic or radical species
- Polar reactions involve charged intermediates (carbocations, carbanions)
- Proceed in stepwise manner with distinct intermediates
- Radical reactions involve unpaired electron intermediates
- Also proceed in stepwise manner with distinct intermediates
- Pericyclic reactions occur in single concerted step
- with no intermediates
- Stereospecific and predictable stereochemical outcomes
Orbital overlap in bond formation
- Diels-Alder reaction involves overlap of diene 4 system with dienophile 2 system
- Diene interacts with dienophile ()
- Favored when diene electron-rich (high-energy HOMO) and dienophile electron-poor (low-energy LUMO)
- Leads to formation of two new bonds and conversion of dienophile bond to bond
- Diene interacts with dienophile ()
- Orbital overlap occurs through cyclic transition state
- Six atoms involved partially bonded in transition state
- electrons from diene and dienophile used to form new bonds
- Stereochemistry of diene and dienophile retained in product ()
- Cis dienophile gives cis ring junction, trans dienophile gives trans ring junction
- dictated by substituents
- Electron-withdrawing group on dienophile typically ends up to diene in product
- Classified as a , with both new bonds forming on the same face of the systems
Key Terms to Review (36)
[4+2] Cycloaddition: The [4+2] cycloaddition, also known as the Diels-Alder reaction, is a fundamental organic chemistry transformation where a conjugated diene (4 π electrons) and a dienophile (2 π electrons) combine to form a cyclohexene ring system. This pericyclic reaction is a powerful tool for the synthesis of complex cyclic compounds.
1,3-Butadiene: 1,3-Butadiene is a simple conjugated diene, composed of four carbon atoms with two carbon-carbon double bonds separated by a single carbon-carbon bond. This structural feature gives 1,3-butadiene unique chemical properties and reactivity that are important in various organic chemistry topics.
Concerted Mechanism: A concerted mechanism refers to a reaction that occurs in a single, continuous step without the formation of any discrete intermediates. In a concerted mechanism, the bonds that are being formed and broken happen simultaneously, leading to the product in a single, coordinated process.
Conjugated Diene: A conjugated diene is a type of organic compound that contains two carbon-carbon double bonds separated by a single carbon-carbon bond. This structural arrangement allows for the delocalization of electrons, leading to unique chemical properties and reactivity.
Cyclic Transition State: A cyclic transition state is a high-energy, ring-like molecular structure that forms temporarily during certain organic reactions, such as the Diels-Alder cycloaddition reaction and electrocyclic reactions. This transient state plays a crucial role in the mechanism and stereochemistry of these important transformations.
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.
Cycloaddition reaction: A cycloaddition reaction is a chemical process where two or more unsaturated molecules (or parts of the same molecule) combine to form a cyclic molecule. It involves the breaking and forming of new covalent bonds to create a ring structure without the loss of any atom.
Cyclohexene Derivative: A cyclohexene derivative is a cyclic organic compound that contains a six-membered ring with one carbon-carbon double bond. These compounds are closely related to the Diels-Alder cycloaddition reaction, which involves the formation of a cyclohexene ring from a diene and a dienophile.
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 Cycloaddition Reaction: The Diels-Alder cycloaddition reaction is a [4+2] cycloaddition reaction in organic chemistry where a conjugated diene and a dienophile combine to form a cyclic product. This pericyclic reaction is a powerful tool for the synthesis of complex cyclic organic compounds.
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.
Diels–Alder reaction: The Diels–Alder reaction is a chemical reaction between a conjugated diene and a substituted alkene, known as the dienophile, to form a cyclic compound. This reaction is stereospecific and produces a six-membered ring through a concerted mechanism without the formation of intermediates.
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.
Endo Addition: Endo addition is a stereochemical outcome in the Diels-Alder cycloaddition reaction where the incoming dienophile adds to the diene from the same side as the existing substituents on the diene. This results in the formation of a bicyclic product with a fused ring system.
Exo Addition: Exo addition is a type of cycloaddition reaction in organic chemistry where the incoming group or substituent adds to the less substituted side of a cyclic diene in the Diels-Alder reaction, resulting in the formation of a cyclic product with the new substituent oriented in an exo configuration.
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.
Frontier Molecular Orbitals: Frontier molecular orbitals refer to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in a molecule. These orbitals play a crucial role in understanding and predicting the reactivity and behavior of molecules, particularly in the context of electrophilic additions to conjugated dienes, the Diels-Alder cycloaddition reaction, electrocyclic reactions, and photochemical electrocyclic reactions.
Furan: Furan is a heterocyclic organic compound consisting of a five-membered aromatic ring with one oxygen atom. It is an important structural unit in many natural and synthetic compounds and plays a key role in the Diels-Alder cycloaddition reaction.
HOMO: HOMO, or Highest Occupied Molecular Orbital, is a fundamental concept in molecular orbital theory that describes the highest energy level occupied by electrons in a molecule. This term is crucial in understanding the stability, reactivity, and spectroscopic properties of organic compounds, particularly in the context of conjugated systems, pericyclic reactions, and the chemistry of vision.
Homotopic: In the context of 1H NMR spectroscopy and proton equivalence, homotopic protons are those that can be interchanged by a symmetry operation without changing the molecule's overall spatial arrangement. These protons have identical chemical environments and therefore exhibit identical chemical shifts in NMR spectroscopy.
Kurt Alder: Kurt Alder was a German chemist who, along with Otto Diels, discovered the Diels-Alder cycloaddition reaction, a fundamental organic chemistry transformation that involves the addition of a diene and a dienophile to form a cyclohexene derivative. This reaction is a key tool in the synthesis of complex organic molecules and has had a significant impact on the field of organic chemistry.
Lewis Acid Catalyst: A Lewis acid catalyst is a type of catalyst that accepts a pair of electrons, facilitating chemical reactions by stabilizing reactive intermediates or transition states. These catalysts are particularly important in the context of the Diels-Alder cycloaddition reaction, where they can enhance the rate and selectivity of the transformation.
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.
Maleic Anhydride: Maleic anhydride is a cyclic organic compound with the chemical formula C$_{4}$H$_{2}$O$_{3}$. It is a versatile intermediate used in the synthesis of various chemicals and materials, and is particularly relevant in the context of the Diels-Alder cycloaddition reaction.
Otto Diels: Otto Diels was a German chemist who, along with his student Kurt Alder, discovered a powerful organic reaction known as the Diels-Alder cycloaddition. This reaction has become a fundamental tool in organic synthesis, allowing for the construction of complex cyclic compounds from simpler starting materials.
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.
Regioselectivity: Regioselectivity refers to the preference of a chemical reaction to occur at a specific site or region of a molecule, leading to the formation of one regioisomeric product over another. This concept is particularly important in the context of electrophilic addition reactions of alkenes, electrophilic aromatic substitution, and other organic transformations.
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.
Stereoselectivity: Stereoselectivity refers to the preference of a chemical reaction to form one stereoisomeric product over another. It is a crucial concept in organic chemistry that describes the ability of a reaction to control the spatial arrangement of atoms in the final product.
Stereospecificity: Stereospecificity refers to the ability of a chemical reaction to produce a specific stereoisomer or spatial arrangement of atoms in the product, based on the stereochemistry of the reactants. This concept is crucial in understanding the outcomes of various organic reactions, particularly those involving chiral molecules.
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.
Symmetry-Allowed Reaction: A symmetry-allowed reaction is a chemical transformation that follows the principles of orbital symmetry, as described by the Woodward-Hoffmann rules. These rules determine the allowed and forbidden pathways for pericyclic reactions, such as the Diels-Alder cycloaddition, based on the symmetry of the participating molecular orbitals.
Woodward-Hoffmann Rules: The Woodward-Hoffmann rules are a set of principles that describe the stereochemical outcomes of pericyclic reactions, such as electrocyclic reactions, cycloadditions, and sigmatropic rearrangements. These rules provide a framework for predicting the feasibility and stereochemistry of these types of organic reactions based on the topology of the molecular orbitals involved.