Oxaphosphetane Formation

Definition

Oxaphosphetane formation is a key step in the Wittig reaction, a widely used method for the synthesis of alkenes from carbonyl compounds and phosphorus ylides. This four-membered cyclic intermediate is formed through the nucleophilic addition of a phosphorus ylide to a carbonyl group, and its subsequent rearrangement is crucial for the overall success of the Wittig reaction.

5 Must Know Facts For Your Next Test

1. The formation of the oxaphosphetane intermediate is the key step in the Wittig reaction, as it determines the stereochemistry of the final alkene product.
2. The oxaphosphetane is formed through the nucleophilic addition of the phosphorus ylide to the carbonyl group, creating a tetrahedral intermediate.
3. The oxaphosphetane can exist in two possible stereoisomeric forms, the betaine and the spirane, which can lead to the formation of either the $E$- or $Z$-alkene product, respectively.
4. The stability and reactivity of the oxaphosphetane intermediate are influenced by factors such as the nature of the substituents on the phosphorus ylide and the carbonyl compound, as well as the reaction conditions.
5. The rearrangement of the oxaphosphetane to the final alkene product is a key step in the Wittig reaction, and it is believed to occur through a concerted, four-membered ring-opening mechanism.

Review Questions

• Explain the role of the oxaphosphetane intermediate in the Wittig reaction and how it determines the stereochemistry of the final alkene product.
  • The formation of the oxaphosphetane intermediate is the key step in the Wittig reaction, as it directly determines the stereochemistry of the final alkene product. The nucleophilic addition of the phosphorus ylide to the carbonyl group creates a tetrahedral oxaphosphetane intermediate, which can exist in two possible stereoisomeric forms: the betaine and the spirane. The betaine form leads to the formation of the $E$-alkene product, while the spirane form leads to the $Z$-alkene product. The stability and reactivity of the oxaphosphetane intermediate are influenced by various factors, such as the substituents on the phosphorus ylide and the carbonyl compound, as well as the reaction conditions.
• Describe the mechanism by which the oxaphosphetane intermediate rearranges to form the final alkene product in the Wittig reaction.
  • The rearrangement of the oxaphosphetane intermediate to the final alkene product is a crucial step in the Wittig reaction. This rearrangement is believed to occur through a concerted, four-membered ring-opening mechanism. In this process, the oxaphosphetane undergoes a ring-opening reaction, during which the carbon-oxygen bond is cleaved, and the carbon-carbon double bond is formed. This results in the release of the phosphine oxide byproduct and the formation of the desired alkene product. The specific stereochemistry of the final alkene is determined by the stereochemistry of the oxaphosphetane intermediate, as discussed in the previous question.
• Analyze the factors that can influence the stability and reactivity of the oxaphosphetane intermediate in the Wittig reaction, and discuss how these factors can impact the overall outcome of the reaction.
  • The stability and reactivity of the oxaphosphetane intermediate in the Wittig reaction are influenced by a variety of factors, which can ultimately impact the outcome of the overall reaction. The nature of the substituents on the phosphorus ylide and the carbonyl compound, such as their steric and electronic properties, can affect the stability of the oxaphosphetane and the ease of its rearrangement to the final alkene product. Additionally, the reaction conditions, such as temperature, solvent, and the presence of any additives, can also influence the stability and reactivity of the oxaphosphetane. For example, more sterically hindered or electronically deactivated substituents may lead to a less stable oxaphosphetane, potentially favoring the formation of undesired side products. Conversely, reaction conditions that promote the rapid rearrangement of the oxaphosphetane can enhance the yield and selectivity of the desired alkene product. Understanding and controlling these factors is crucial for optimizing the Wittig reaction and achieving the desired stereochemical outcome.

Related terms

Phosphorus Ylide: A phosphorus ylide is a highly reactive species containing a positively charged phosphorus atom and a negatively charged carbon atom, which can act as a nucleophile in the Wittig reaction.

Carbonyl Compound: A carbonyl compound is a molecule containing a carbon-oxygen double bond, such as an aldehyde or a ketone, which can undergo nucleophilic addition with a phosphorus ylide in the Wittig reaction.

Alkene: An alkene is an unsaturated hydrocarbon containing a carbon-carbon double bond, which is the desired product of the Wittig reaction.