30.7 Sigmatropic Rearrangements
2 min read•may 7, 2024
Sigmatropic rearrangements are like molecular acrobatics. These reactions involve a sigma bond doing a fancy flip across a chain of double bonds, changing the molecule's structure while keeping its overall connectivity intact.
These rearrangements come in different flavors, classified by how far the sigma bond moves. Understanding the rules behind them helps predict how molecules will change shape under heat or light, making them useful tools for chemists.
Sigmatropic Rearrangements
Concept of sigmatropic rearrangements
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- Involve concerted migration of a across a conjugated while π system remains intact allowing for σ bond migration
- σ bond breaks at original location and forms at new location resulting in structural change while π system adjusts to accommodate new bonding arrangement
- Number of π and σ bonds remains constant throughout rearrangement maintaining overall connectivity of molecule (1,3-pentadiene, 1,5-hexadiene)
- Sigmatropic rearrangements are a type of pericyclic reaction, characterized by a
Notation for sigmatropic rearrangements
- Classified using where i represents number of atoms σ bond migrates over from starting point and j represents number of atoms σ bond migrates over to reach final position
- : σ bond migrates over one atom from starting point and five atoms to reach final position (1,3-pentadiene)
- : σ bond migrates over three atoms from starting point and three atoms to reach final position ( of 1,5-hexadiene)
- Sum of i and j determines number of atoms involved in rearrangement
- For , i + j must be even number ()
- For , i + j must be odd number ()
Suprafacial vs antarafacial sigmatropic modes
- Suprafacial sigmatropic rearrangements: migrating σ bond remains on same face of π system throughout rearrangement
- retained when i + j = (thermal reactions, Hückel topology)
- Stereochemistry inverted when i + j = (photochemical reactions, Möbius topology)
- Antarafacial sigmatropic rearrangements: migrating σ bond starts on one face of π system and ends on opposite face
- Stereochemistry inverted when i + j = (thermal reactions, Hückel topology)
- Stereochemistry retained when i + j = (photochemical reactions, Möbius topology)
- predict stereochemical outcome of sigmatropic rearrangements based on number of atoms involved (i + j) and reaction conditions (thermal or photochemical)
- Thermal reactions follow Hückel topology
- Photochemical reactions follow Möbius topology
Orbital symmetry considerations
- plays a crucial role in determining the feasibility and stereochemical outcome of sigmatropic rearrangements
- The conservation of orbital symmetry governs the allowed pathways for these reactions
- Thermal and photochemical reactions differ in their orbital symmetry requirements, leading to distinct reaction outcomes
- Understanding orbital symmetry helps predict the stereochemistry of sigmatropic rearrangements and explains why certain reactions are favored under specific conditions
Key Terms to Review (20)
[1,5] Sigmatropic Rearrangement: A [1,5] sigmatropic rearrangement is a type of pericyclic reaction in organic chemistry where a substituent migrates from one position to another position five atoms away, resulting in the formation of a new carbon-carbon bond.
[3,3] Sigmatropic Rearrangement: A [3,3] sigmatropic rearrangement is a type of pericyclic reaction in organic chemistry where a substituent migrates from one atom to a neighboring atom through a cyclic transition state involving a 3-membered ring. This rearrangement is a key concept in understanding the reactivity and mechanisms of certain organic transformations.
[i,j] Notation: The [i,j] notation is a way to represent a specific element within a matrix or array. The square brackets enclose two integer values, where 'i' represents the row index and 'j' represents the column index, allowing for precise identification and manipulation of individual elements in a larger data structure.
Antarafacial Sigmatropic Rearrangement: An antarafacial sigmatropic rearrangement is a type of pericyclic reaction where a substituent migrates from one position to another within a molecule, with the new substituent position being on the opposite face of the molecule relative to the original position.
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.
Cope rearrangement: The Cope rearrangement is a thermal, pericyclic reaction involving the 3,3-sigmatropic rearrangement of 1,5-dienes to form isomeric 1,5-dienes. It occurs without the aid of catalysts and involves a concerted movement of electrons and atomic positions within the molecule.
Cope Rearrangement: The Cope rearrangement is a sigmatropic rearrangement reaction in organic chemistry, where a [3,3]-shift of substituents occurs in a cyclic or acyclic system containing a conjugated diene. This rearrangement is a powerful tool for the synthesis of complex organic molecules.
Hückel Topology: Hückel topology is a concept in organic chemistry that describes the arrangement and connectivity of atoms in cyclic, conjugated systems. It is particularly relevant in the context of sigmatropic rearrangements, which involve the migration of substituents within a molecule.
Möbius Topology: Möbius topology refers to the unique properties of the Möbius strip, a one-sided, non-orientable surface that can be formed by twisting a strip of paper and joining the ends. This topology has important implications in the study of sigmatropic rearrangements, a class of pericyclic reactions involving the migration of substituents within a cyclic system.
Orbital Symmetry: Orbital symmetry is a fundamental concept in organic chemistry that describes the spatial arrangement and interaction of molecular orbitals involved in pericyclic reactions, such as electrocyclic reactions, cycloadditions, and sigmatropic rearrangements. It helps predict the stereochemical outcomes and feasibility of these concerted reactions.
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.
Photochemical Reactions: Photochemical reactions are chemical transformations that are initiated or driven by the absorption of light energy. These light-induced reactions involve the excitation of molecules, leading to the formation of new products or the rearrangement of existing molecules.
Sigmatropic Rearrangement: A sigmatropic rearrangement is a pericyclic reaction in which a $\sigma$-bond migrates from one position in a molecule to another, typically accompanied by the formation of a new $\sigma$-bond and the breaking of an existing $\sigma$-bond.
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 Sigmatropic Rearrangement: A suprafacial sigmatropic rearrangement is a pericyclic reaction in which a $\sigma$ bond migrates from one atom to another within the same molecule, with the migrating group and the new bond forming on the same side (suprafacial) of the molecular plane.
Thermal Reactions: Thermal reactions are chemical reactions that are driven by the input of heat energy. These reactions involve the breaking and forming of chemical bonds, often leading to the transformation of reactants into different products. Thermal reactions are a fundamental concept in organic chemistry, particularly in the context of sigmatropic rearrangements.
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.
π System: A π system, also known as a pi system, is a type of chemical bonding arrangement found in organic molecules where electrons are delocalized across multiple atoms, typically in conjugated systems. This delocalization of electrons allows for the stabilization of the molecule and influences its chemical reactivity and properties.
σ Bond: A σ bond is a type of covalent chemical bond formed by the head-on overlap of atomic orbitals, resulting in an increased electron density along the internuclear axis. This type of bond is essential in the understanding of organic chemistry concepts such as benzyne and sigmatropic rearrangements.