The ylide is an electrically neutral molecule that has a negative carbon with an unshared electron pair (usually a carbanion) adjacent to a positive heteroatom (typically nitrogen, phosphorus, or sulfur), where both the atoms have full octets (eight electrons in their valence shell).
The bond between the carbanion and positive heteroatom is best described as a polar covalent bond with significant ionic and double-bond character, depending on the ylide type.
Ylides are Zwitterions, meaning they carry both positive and negative charges.
The most common types of ylides are –
Phosphorus Ylides – Commonly used in the Wittig reaction to convert aldehydes or ketones into alkenes. Example: Ph₃P⁺–CH₂⁻ (Wittig reagent).
Sulfur Ylides – Less reactive, often used in cyclopropanation or epoxidation (e.g., Corey-Chaykovsky reagent). Example: (CH₃)₂S⁺–CH₂⁻).
Nitrogen Ylides – Less stable, azomethine ylide participates in 1,3-dipolar cycloadditions, often with alkenes or fullerenes. Example: CH₂⁻–N⁺(CH₃)=CH₂ (a simple azomethine ylide).
Nitrogen and Sulfur Ylides can also undergo [2,3]- or [1,2]-rearrangements (e.g., Stevens or Sommelet-Hauser).
The stability of the ylides can be increased by having electron-withdrawing groups on the carbanion. In case of the positive heteroatom, stability increases with stronger pπ-dπ bonding (phosphorus > sulfur > nitrogen) and electron-donating or withdrawing substituents.
Ylides are an important reagent to synthesize alkenes, natural products (e.g., vitamins, hormones), and complex molecules with stereocenters.