30.5 Cycloaddition Reactions
2 min read•may 7, 2024
are fascinating transformations that create new rings by combining unsaturated molecules. These reactions follow specific rules based on orbital symmetry, leading to stereospecific products. Understanding cycloadditions is key to predicting and controlling the formation of complex ring systems.
The reaction and [2+2] cycloadditions are two important types with distinct characteristics. Diels-Alder reactions form cyclohexenes thermally, while [2+2] cycloadditions create cyclobutanes photochemically. These differences stem from orbital symmetry considerations, which govern reaction feasibility and selectivity.
Cycloaddition Reactions and Orbital Symmetry
Concept of cycloaddition reactions
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Organic chemistry 26: Diels-Alder cycloaddition View original
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Organic chemistry 26: Diels-Alder cycloaddition View original
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- Involve formation of new rings by combining two unsaturated molecules (alkenes, dienes) in a single step with no intermediates
- Characterized by the number of atoms from each molecule that form the new bonds ([4+2] Diels-Alder, [2+2] photochemical)
- proceeds through a cyclic transition state leading to stereospecific products
- Governed by orbital symmetry considerations described by the and Frontier Molecular Orbital (FMO) theory
- Classified as due to their cyclic transition state and concerted mechanism
Diels-Alder vs [2+2] cycloadditions
- Diels-Alder reactions ([4+2] cycloadditions)
- Involve a (4π electrons) and a (2π electrons) forming products under thermal conditions
- Favored by electron-rich dienes (high ) and electron-poor dienophiles (low ) leading to up to four new stereocenters
- Follow the favoring maximum secondary orbital overlap in the transition state (endo product)
- of the product is determined by the orientation of the reactants
- [2+2] Cycloadditions
- Involve two alkenes (2π electrons each) forming products under photochemical conditions (UV light)
- Stereospecific retaining the relative stereochemistry of the alkenes leading to up to two new stereocenters
- Thermally forbidden according to the Woodward-Hoffmann rules due to the symmetry mismatch of the HOMO and LUMO
Suprafacial vs antarafacial cycloadditions
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- New bonds form on the same face of the π system through same-side overlap of the p orbitals
- More common than antarafacial due to favorable orbital overlap and lower geometric constraints
- Allowed thermally when the total number of suprafacial components is (4q+2) and photochemically when (4r)
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- New bonds form on opposite faces of the π system through opposite-side overlap of the p orbitals
- Rare and limited to large ring systems or molecules with specific conformational requirements
- Allowed thermally when the total number of antarafacial components is (4r) and photochemically when (4q+2)
Molecular Orbital Considerations in Cycloadditions
- play a crucial role in determining the feasibility and selectivity of cycloaddition reactions
- The symmetry and energy of the frontier molecular orbitals (HOMO and LUMO) of the reactants determine whether the reaction is allowed or forbidden
- can alter the electronic state of molecules, enabling otherwise forbidden reactions by exciting electrons to higher energy orbitals
Key Terms to Review (22)
[2+2] Cycloaddition: [2+2] cycloaddition is a type of pericyclic reaction where two pi bonds (typically from alkenes or alkynes) combine to form a cyclobutane or cyclobutene ring. This reaction is a key concept in understanding the stereochemistry and reactivity of various organic transformations.
Antarafacial Cycloadditions: Antarafacial cycloadditions are a class of pericyclic reactions where the new bonds are formed on the opposite sides of a cyclic system. This type of cycloaddition reaction is an important concept in organic chemistry, particularly in the context of understanding the stereochemistry and regiochemistry of cyclic compound formation.
Anti stereochemistry: Anti stereochemistry describes the spatial arrangement in a chemical reaction where two substituents are positioned on opposite sides of a double bond or ring structure after the reaction. It is particularly relevant in the halogenation of alkenes, resulting in products where the added atoms are located across from each other.
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.
Cycloaddition Reactions: Cycloaddition reactions are a class of organic reactions where two or more unsaturated molecules, or parts of the same molecule, combine to form a cyclic product. These reactions are widely used in organic synthesis to construct important ring structures.
Cyclobutane: Cyclobutane is a cyclic alkane with four carbon atoms arranged in a square-shaped ring. It is the simplest cycloalkane with a four-membered ring and has a unique set of properties that make it an important topic in organic chemistry.
Cyclohexene: Cyclohexene is a cyclic alkene compound with the molecular formula C₆H₁₀. It is a versatile organic compound that plays an important role in various chemical reactions, including the addition of hydrogen bromide (HBr) and cycloaddition reactions.
Diels-Alder: The Diels-Alder reaction is a type of cycloaddition reaction in organic chemistry that involves the combination of a conjugated diene and a dienophile to form a cyclic product. It is a powerful tool for the synthesis of complex cyclic compounds and is widely used in the field of organic synthesis.
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.
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.
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.
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.
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.
Molecular Orbitals: Molecular orbitals are the wave functions that describe the behavior of electrons in a molecule. They are formed by the combination of atomic orbitals and play a crucial role in understanding the structure, bonding, and reactivity of chemical compounds.
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.
Photochemistry: Photochemistry is the study of chemical reactions that are initiated by the absorption of light energy. It explores how molecules undergo structural and electronic changes when exposed to light, leading to the formation of new compounds or the transformation of existing ones.
Stereochemistry: Stereochemistry is the study of the three-dimensional arrangement of atoms in molecules and how this arrangement affects the chemical and physical properties of the substance. It examines the spatial orientation of atoms and their relationship to one another, which is crucial in understanding many organic chemistry concepts.
Suprafacial Cycloadditions: Suprafacial cycloadditions are a class of pericyclic reactions where the new bonds are formed on the same side of the reacting π-system. This allows for the concerted formation of new cyclic structures from acyclic precursors.
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.