Ramberg-Bäcklund Reaction

Professor Dave Explains
30 Sept 202205:24
EducationalLearning
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TLDRThe Ramberg-Bäcklund reaction, first introduced in 1940, is a unique olefin synthesis starting from alpha-halo sulfones. Using a strong base, it extrudes SO2, forming a mixture of E and Z isomers, with a preference for Z in simple cases. The reaction is favored for synthesizing highly substituted and strained olefins, difficult through other methods. The mechanism involves carbanion formation, intramolecular SN2-like reaction to form a thiirane dioxide, followed by a cheletropic extrusion of SO2 to yield the olefin. Despite modest yields, it's valuable for synthesizing challenging compounds like cyclobutenes, with modifications enhancing its versatility and applications in synthesis.

Takeaways
  • 🔬 The Ramberg-Bäcklund reaction is a unique olefination method introduced by Swedish chemists Ludwig Ramberg and Birger Bäcklund in 1940.
  • 🌟 It begins with alpha-halo sulfones and involves the extrusion of SO2 using a strong base to form olefins, typically yielding a mixture of E and Z isomers.
  • 💡 The reaction is advantageous for synthesizing highly substituted olefins, including strained ones, which are challenging to produce through intermolecular processes.
  • 🛠️ The mechanism starts with the formation of a carbanion facilitated by the electron-withdrawing sulfone group, similar to the stabilization provided by a carbonyl group.
  • 🔄 Common bases like hydroxide or tert-butoxide ions can be used to form the carbanion, leading to an SN2-like intramolecular reaction and the formation of a thiirane dioxide.
  • 🔬 Thiirane dioxides are strained three-membered rings with two carbons and a sulfur, which are prone to fragmentation.
  • 🔀 The extrusion of SO2 is a cheletropic reaction, a subset of pericyclic reactions, where two bonds are formed or broken to the same atom, conserving the stereochemistry.
  • 📉 The reaction does not always yield pure E or Z isomers; simple alpha-halo sulfones tend to produce a mixture with the Z isomer predominating.
  • ⚗️ E-olefins can be accessed using excess base, allowing the thiirane dioxide to equilibrate to the more stable E isomer before fragmentation.
  • 🔍 There are various methods to prepare the required alpha-halo sulfone starting material, including alpha-halogenation of a sulfide with N-chlorosuccinimide followed by oxidation.
  • 🔄 The Ramberg-Bäcklund reaction is particularly useful in cyclic systems where the Z-geometry is ensured by the excessive strain of the hypothetical E-olefin.
  • 🌐 The reaction has seen many modifications and finds widespread use in synthesis, including the epoxy-Ramberg-Bäcklund variant leading to allylic alcohols with good E selectivity.
Q & A

  • What is the Ramberg-Bäcklund reaction?

    -The Ramberg-Bäcklund reaction is a chemical process that involves the conversion of alpha-halo sulfones into olefins through the extrusion of SO2 using a strong base, resulting in a mixture of E and Z isomers, unless the sulfone is cyclic.

  • Who first described the Ramberg-Bäcklund reaction?

    -The reaction was first described by Swedish chemist Ludwig Ramberg and his student Birger Bäcklund in 1940.

  • What is the typical leaving group in the Ramberg-Bäcklund reaction?

    -The typical leaving group in the Ramberg-Bäcklund reaction is a halogen, most commonly chlorine or bromine, which is part of the alpha-halo sulfone.

  • Why are alpha-halo sulfones advantageous in olefin synthesis?

    -Alpha-halo sulfones are advantageous because they are available through a variety of methods and can be used to synthesize highly substituted olefins, especially strained ones, which are more challenging to produce through intermolecular processes.

  • What is the role of the sulfone group in the Ramberg-Bäcklund reaction?

    -The sulfone group is highly electron-withdrawing and stabilizes a negative charge on the adjacent carbon, facilitating the formation of a carbanion, similar to the role of a carbonyl group.

  • What is the first step in the mechanism of the Ramberg-Bäcklund reaction?

    -The first step in the mechanism is the formation of a carbanion through the action of a strong base, such as hydroxide or tert-butoxide ions.

  • What is a thiirane dioxide and how is it formed in the Ramberg-Bäcklund reaction?

    -A thiirane dioxide is a strained three-membered ring containing two carbons and a sulfur atom. It is formed through an SN2-like intramolecular reaction after the carbanion is formed.

  • What is a cheletropic reaction and how does it relate to the Ramberg-Bäcklund reaction?

    -A cheletropic reaction is a type of pericyclic reaction where two bonds are formed or broken involving the same atom. In the Ramberg-Bäcklund reaction, the sulfur atom loses two S-C bonds in a concerted manner, which is an example of a cheletropic reaction.

  • How can the Ramberg-Bäcklund reaction be used to access E-olefins?

    -E-olefins can be accessed by using an excess of base, which allows the thiirane dioxide to equilibrate to the more stable E isomer before fragmenting stereospecifically to the E-olefin.

  • What are some common methods for preparing alpha-halo sulfones, the starting material for the Ramberg-Bäcklund reaction?

    -Common methods include alpha-halogenation of a sulfide with N-chlorosuccinimide followed by oxidation, and alpha-chlorination of an alpha-lithiosulfone, among others.

  • Why is the Ramberg-Bäcklund reaction particularly useful for synthesizing cyclobutenes?

    -The Ramberg-Bäcklund reaction is particularly useful for synthesizing cyclobutenes because it can achieve this in cases where few other olefin syntheses are successful, despite sometimes modest to fair yields.

  • Can you provide an example of a modification of the Ramberg-Bäcklund reaction?

    -One example of a modification is the epoxy-Ramberg-Bäcklund reaction, where the leaving group is an epoxide, leading to the formation of allylic alcohols with good E selectivity.

Outlines
00:00
🧪 Ramberg-Bäcklund Reaction Overview

The Ramberg-Bäcklund reaction, first introduced by Swedish chemists Ludwig Ramberg and Birger Bäcklund in 1940, is a unique method for olefin synthesis starting from alpha-halo sulfones. This reaction is notable for its ability to produce highly substituted and strained olefins, which are challenging to synthesize through other methods. The process involves the formation of a carbanion using a strong base, followed by an intramolecular SN2-like reaction resulting in a thiirane dioxide intermediate. This intermediate undergoes a cheletropic reaction, leading to the extrusion of SO2 and the formation of olefins, typically as a mixture of E and Z isomers. The reaction mechanism is straightforward, and the stereochemistry of the thiirane dioxide is retained in the final olefin product.

Mindmap
Keywords
💡Olefination reactions
Olefination reactions refer to a class of chemical reactions that result in the formation of olefins, which are hydrocarbons with at least one carbon-carbon double bond. In the context of the video, these reactions are the central theme, with the Ramberg-Bäcklund reaction being a unique method for olefin synthesis.
💡Ramberg-Bäcklund reaction
The Ramberg-Bäcklund reaction is a specific olefination reaction first described by Swedish chemists Ludwig Ramberg and Birger Bäcklund in 1940. It is highlighted in the video as a method to produce olefins from alpha-halo sulfones through the extrusion of SO2, resulting in a mixture of E and Z isomers.
💡Alpha-halo sulfones
Alpha-halo sulfones are the starting materials in the Ramberg-Bäcklund reaction, characterized by the presence of a halogen (commonly chlorine or bromine) attached to the alpha carbon of a sulfone group. The video explains that these compounds are versatile and can lead to the synthesis of highly substituted olefins.
💡E and Z isomers
E and Z isomers refer to the stereoisomers of olefins, where the E isomer has the higher priority groups on opposite sides of the double bond, and the Z isomer has them on the same side. The video mentions that the Ramberg-Bäcklund reaction typically yields a mixture of these isomers, unless the starting sulfone is cyclic.
💡Carbanion
A carbanion is a carbon-centered anion, which contains a negative charge on a carbon atom. In the video, the formation of a carbanion through the use of a strong base is the first step in the Ramberg-Bäcklund reaction, facilitated by the electron-withdrawing nature of the sulfone group.
💡SN2-like intramolecular reaction
The term SN2-like intramolecular reaction in the video describes a reaction mechanism where a carbanion, formed in an intramolecular process, displaces a leaving group (in this case, a halogen) through a nucleophilic substitution similar to the SN2 reaction, but occurring within the same molecule.
💡Thiirane dioxide
Thiirane dioxide is an intermediate in the Ramberg-Bäcklund reaction, characterized by a three-membered ring containing two carbon atoms and a sulfur atom, with the sulfur being in a fully oxidized state. The video explains that this strained ring is prone to fragmentation, leading to the extrusion of SO2.
💡Cheletropic reaction
A cheletropic reaction, as introduced in the video, is a type of pericyclic reaction where two bonds are formed or broken involving the same atom. In the context of the Ramberg-Bäcklund reaction, the硫 atom in thiirane dioxide loses two S-C bonds in a concerted manner, resulting in the formation of an olefin.
💡Stereochemistry
Stereochemistry is the aspect of chemistry concerned with the three-dimensional orientation of atoms in molecules. The video emphasizes that the stereochemistry of the thiirane dioxide intermediate is retained in the final olefin product due to the concerted nature of the cheletropic reaction.
💡Stereoselectivity
Stereoselectivity refers to the preference for the formation of one stereoisomer over another in a chemical reaction. The video notes that the Ramberg-Bäcklund reaction lacks complete stereoselectivity, often yielding a mixture of E and Z isomers, but mentions that E-olefins can be accessed using excess base.
💡Cyclic systems
Cyclic systems in the context of the video refer to ring-shaped molecular structures. The Ramberg-Bäcklund reaction is particularly useful in cyclic systems where the Z-geometry of the double bond is favored due to the excessive strain that would be present in the hypothetical E-isomer.
💡Epoxy-Ramberg-Bäcklund reaction
The epoxy-Ramberg-Bäcklund reaction is a variation of the original reaction mentioned in the video, where the leaving group is an epoxide instead of a halogen. This modification leads to the formation of allylic alcohols with good E selectivity, showcasing the versatility of the Ramberg-Bäcklund reaction.
Highlights

The Ramberg-Bäcklund reaction is a unique approach to olefin synthesis, first described in 1940.

It involves the use of alpha-halo sulfones and extrusion of SO2 with a strong base to produce olefins.

The reaction yields a mixture of E and Z isomers, with the exception of cyclic sulfones.

Alpha-halo sulfones are versatile, with availability through various methods.

Highly substituted olefins, including strained ones, can be synthesized, which is challenging through intermolecular processes.

The mechanism begins with the formation of a carbanion by a strong base, facilitated by the electron-withdrawing sulfone group.

Common bases like hydroxide or tert-butoxide ions are used to form the carbanion.

An SN2-like intramolecular reaction leads to the formation of a strained three-membered thiirane dioxide ring.

Thiirane dioxide rings are prone to fragment through a cheletropic reaction, extruding SO2.

The stereochemistry of the thiirane dioxide is retained in the final olefin product.

Pure E or Z isomers are challenging to obtain, with Z isomer predominating in simple alpha-halo sulfones.

E-olefins can be accessed using excess base, allowing for equilibration to the more stable E isomer.

There are multiple methods for synthesizing the required alpha-halo sulfone starting material.

The Ramberg-Bäcklund reaction is particularly useful in cyclic systems for synthesizing strained olefins.

The reaction has been modified for various leaving groups, expanding its versatility.

An example modification is the epoxy-Ramberg-Bäcklund reaction, leading to allylic alcohols with good E selectivity.

The Ramberg-Bäcklund reaction continues to find widespread use 80 years after its discovery, with ongoing creative applications.

Transcripts

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Olefin SynthesisChemical ReactionRamberg-BäcklundCheletropic ReactionThiirane DioxideStereochemistryChemistry TutorialHistorical ChemistrySulfur CompoundsSynthetic Method