"Unlocking
the Sigma-Tropic Reactions: A Comprehensive Guide to Understanding the Origin, Types,
Naming System, and Mechanisms"
Sigmatropic reactions are a fascinating
area of organic chemistry that involves the migration of a sigma bond along a
conjugated system. This type of reaction is important in organic synthesis and
has been studied extensively for its usefulness in forming new carbon-carbon
bonds. In this article, we will explore the background of sigma-tropic
reactions, their naming system, and the different types of sigma-tropic
reactions.
Introduction:
“Molecular rearrangements in which a
σ-bonded atom or group, flanked by one or more
π-electron systems, shifts to a new location with a corresponding reorganization of the π-bonds” are
called Sigmatropic reactions. The total number of σ-bonds and π-bonds remain unchanged.
Or
In a sigmatropic rearrangement, a bond in the
reactant is broken, a new bond is formed, and the electrons rearrange.
The bond that
breaks is a bond to an allylic carbon.
It can be a bond between a carbon and a hydrogen, between a carbon and another carbon,
or between a carbon and an oxygen, nitrogen, or sulfur.
Background
of Sigma-tropic Reactions
The term "sigma-tropic" refers
to the movement of a sigma bond. This type of
reaction was first described in the early 1960s
by the chemist R. B. Woodward. Woodward was
awarded the Nobel Prize in Chemistry in 1965 for his pioneering work in the field
of organic synthesis, which included the discovery of
sigma-tropic reactions.
Sigma-tropic reactions are important in
organic chemistry because they allow chemists to form new carbon-carbon bonds
in a selective and efficient manner. They are also useful in the synthesis of complex natural products and pharmaceuticals.
Naming
System for Sigma-tropic Reactions:
Sigma-tropic reactions are named
according to the type of migration that occurs.
The naming system is based on the Greek letters alpha,
beta, and gamma, which refer to the position of the migrating sigma bond
relative to the conjugated system Or Double numbering
system (m,n or i, j) is used.
You have not encountered any numbering
systems like the one used to define a sigmatropic rearrangement before.
- Break the reactant's bond first in your mind.
- Then label the atoms to which it was linked with the number 1.
- After then, have a look at the product's new bond.
- The number of atoms in each of the pieces connecting the broken connection and the new bond should be recorded.
- The smaller number is stated first and the two numbers are enclosed in brackets.
Types
of Sigma-tropic Reactions:
There are several different types of
sigma-tropic reactions, each with its own unique mechanism and set of
conditions. The most common types of sigma-tropic reactions include:
- 1,3 Sigmatropic shifts
- 1,5 Sigmatropic shifts of H
- 1,7 Sigmatropic shifts of C
- 2,3 Sigmatropic shifts of C
- 3,3 Sigmatropic shifts of C
[1,3]-Sigmatropic
Rearrangement
In a [1,3]-sigmatropic rearrangement,
the migrating sigma bond moves between one carbon and three carbon. This type
of reaction is important in the synthesis of natural products and
pharmaceuticals and has been extensively studied for its usefulness in the
formation of carbon-carbon bonds.
[1,5]-Sigmatropic
Rearrangement
In a [1,5]-sigmatropic rearrangement,
the migrating sigma bond moves between one carbon and five carbon. This type of
reaction is commonly used in the synthesis of natural products and
pharmaceuticals.
[1,7]-Sigmatropic
Rearrangement
A [1,7]-sigmatropic rearrangement
involves the migration of a sigma bond from one end of a conjugated system to
the other, with the simultaneous formation of a new carbon-carbon bond. This
type of reaction is commonly used in the synthesis of natural products and
pharmaceuticals.
[2,3]-Sigmatropic
Rearrangement
The [2,3]-sigmatropic rearrangement involves the migration of a sigma bond from a carbon atom to a neighboring carbon atom, with the simultaneous formation of a new carbon-carbon bond. This type of reaction is commonly used in the synthesis of complex organic molecules.
[2,3] sigmatropic rearrangement of amine oxide is Meisenheimer rearrangement mentioned below.
[2,3] sigmatropic rearrangement of allyl-sulfoxide is Mislow Evan rearrangement given below. It bears the names of David A. Evans, who published further advances, and Kurt Mislow, who published the prototype reaction in 1966. In a 2,3-sigmatropic rearrangement, the reaction enables the synthesis of allylic alcohols from allylic sulfoxides.
[3,3]-Sigmatropic
Rearrangement
This type of sigma-tropic rearrangement
involves the migration of a sigma bond from one end of a conjugated system to
the other. The reaction is named [3,3] because the migrating bond moves from
three carbon to another three carbon.
Conclusion
Sigma-tropic reactions are an important
area of organic chemistry that have been extensively studied for their
usefulness in the formation of carbon-carbon bonds. They are useful in the
synthesis of complex natural products and pharmaceuticals, and the naming
system for sigma-tropic reactions is based on the position of the migrating
sigma bond relative to the conjugated system. There are several different types
of sigma-tropic reactions, each with its own unique mechanism and set of
conditions. The most common types of sigma-tropic reactions include the [3,3],
[1,5], and [2,3] sigmatropic rearrangements, but there are also several other
types that are worth exploring. Overall, sigma-tropic reactions are an
important tool in the arsenal of organic chemists, and they continue to be a
fascinating area of research in the field of organic synthesis.
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