Sigmatropic rearrangement is a pericyclic reaction in organic chemistry involving the migration of sigma bonds across pi systems, altering molecular structures without intermediates. These reactions are governed by the Woodward-Hoffmann rules and are crucial in synthetic chemistry for creating complex molecules, including pharmaceuticals and natural products. Understanding sigmatropic shifts, such as the Cope and Claisen rearrangements, is vital for chemists.
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Sigmatropic rearrangement is a type of pericyclic reaction in organic chemistry where a sigma bond and its associated substituents migrate across a pi system within a molecule, leading to the creation of new chemical bonds and potential modification of the molecule's properties
Bond Alteration
Sigmatropic rearrangement can significantly alter the molecular architecture of a molecule by creating new chemical bonds and modifying its physical and chemical properties
Orbital Symmetry
Sigmatropic rearrangements are subject to the principles of orbital symmetry, as outlined by the Woodward-Hoffmann rules, which dictate the allowed and forbidden reactions under thermal or photochemical conditions
The distribution of products in sigmatropic rearrangements can be influenced by temperature, making it an important factor to consider in these reactions
Sigmatropic rearrangements proceed through a concerted mechanism, where the bond-breaking and bond-forming steps occur in a single, continuous process without intermediates
[i, j] Notation
Sigmatropic rearrangements are denoted using the [i, j] notation, where 'i' and 'j' represent the number of atoms between the migrating group and the new bond formation site, respectively
Suprafacial and Antarafacial
The terms 'suprafacial' and 'antarafacial' are used to describe whether the migration of the sigma bond occurs on the same side or across opposite sides of the molecule, respectively, and are crucial for predicting the stereochemical outcomes of the reaction
Sigmatropic rearrangements involve a cyclic transition state that exhibits a high degree of symmetry, which is crucial for the conservation of orbital symmetry and the successful completion of the reaction
Claisen Rearrangement
The Claisen rearrangement is a prominent example of a sigmatropic shift in synthetic chemistry, transforming allyl phenyl ether into o-allylphenol
Cope Rearrangement
The Cope rearrangement is another important sigmatropic shift in synthetic chemistry, facilitating the interconversion of cis- and trans- isomers
Polymer Synthesis
Sigmatropic rearrangements play a role in industrial polymer synthesis, where they are used to create complex molecular structures
Biochemical Pathways
Sigmatropic rearrangements are fundamental to certain biochemical pathways, such as the conversion of squalene to lanosterol in steroid biosynthesis, which involves a series of [1,2]-sigmatropic shifts
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Nature of sigmatropic reactions
Concerted process, no intermediates, bond-breaking and bond-forming occur simultaneously.
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Sigmatropic reaction notation
[i, j] notation indicates atoms between migrating group and new bond formation site.
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Role of temperature in sigmatropic reactions
Affects product distribution by influencing reaction rate and orbital symmetries.
