Ylide

Definition

An ylide is a type of zwitterion, a molecule that contains both a positively and a negatively charged atom, typically carbon and a heteroatom like phosphorus or sulfur. Ylides are important intermediates in organic reactions, particularly in the Wittig reaction, which is used to form carbon-carbon double bonds.

5 Must Know Facts For Your Next Test

1. Ylides are highly reactive species that can participate in a variety of organic reactions, including the Wittig reaction, cycloadditions, and rearrangements.
2. The reactivity of ylides is due to the presence of the negatively charged atom, which can act as a nucleophile, and the positively charged atom, which can act as an electrophile.
3. Phosphonium ylides are the most commonly used ylides in organic synthesis, as they are relatively stable and can be easily prepared from phosphonium salts.
4. The Wittig reaction involves the reaction of an aldehyde or ketone with a phosphonium ylide to form an alkene, with the ylide being converted to a phosphine oxide byproduct.
5. Ylides can also be used in the synthesis of heterocyclic compounds, such as the formation of furans and pyrroles.

Review Questions

• Explain the structure and reactivity of ylides, and how they are used in the Wittig reaction.
  • Ylides are zwitterionic species containing a positively charged atom, typically phosphorus or sulfur, and a negatively charged atom, usually carbon. This unique structure gives ylides high reactivity, allowing them to act as both nucleophiles and electrophiles. The Wittig reaction is a key application of ylides, where a phosphonium ylide reacts with an aldehyde or ketone to form a carbon-carbon double bond, producing an alkene product and a phosphine oxide byproduct. The reactivity of the ylide is crucial for the success of the Wittig reaction, as it enables the formation of the new carbon-carbon bond.
• Discuss the factors that influence the stability and reactivity of different types of ylides, and how these factors impact their use in organic synthesis.
  • The stability and reactivity of ylides can vary depending on the nature of the positively and negatively charged atoms. Phosphonium ylides, for example, are generally more stable than sulfur-based ylides due to the higher electronegativity of phosphorus compared to sulfur. The substituents on the phosphorus atom can also affect the stability and reactivity of phosphonium ylides, with more electron-withdrawing groups typically increasing the reactivity. The choice of ylide in a synthetic scheme depends on the desired reactivity, as more reactive ylides may be better suited for certain transformations, while more stable ylides may be preferred for others. Understanding the factors that influence ylide stability and reactivity is crucial for effectively utilizing these versatile intermediates in organic synthesis.
• Evaluate the role of ylides in the synthesis of heterocyclic compounds, and explain how their unique properties can be exploited to access diverse ring systems.
  • In addition to their use in the Wittig reaction, ylides can also be employed in the synthesis of heterocyclic compounds. The nucleophilic and electrophilic character of ylides allows them to participate in cycloaddition reactions, leading to the formation of various heterocyclic rings. For example, the reaction of ylides with carbonyl compounds can result in the synthesis of furans, pyrroles, and other heterocyclic systems. The versatility of ylides in these transformations stems from their ability to form new carbon-carbon and carbon-heteroatom bonds, as well as their propensity to undergo rearrangements and cyclization reactions. By carefully selecting the ylide and the reaction partners, organic chemists can leverage the unique properties of these zwitterionic species to access a wide range of heterocyclic scaffolds, which are important structural motifs in many biologically active compounds and pharmaceutical drugs.

Related terms

Zwitterion: A molecule that contains both a positively and a negatively charged atom, typically a carbon atom bonded to a heteroatom like nitrogen, oxygen, or sulfur.

Wittig Reaction: An organic reaction that uses an ylide to form a carbon-carbon double bond, commonly used for the synthesis of alkenes.

Phosphonium Ylide: A type of ylide where the positively charged atom is a phosphorus atom, often used in the Wittig reaction.