Ylide

An ylide is a neutral molecule in which a heteroatom bearing a positive charge is adjacent to a carbon atom bearing a negative charge. This arrangement creates a dipolar, or zwitterionic, character despite an overall neutral molecule. Ylides are common in organic synthesis and are typically classified by the atom that carries the positive charge, most notably phosphorus and sulfur ylides.

Phosphorus ylides, also called phosphoranes, are exemplified by methylenetriphenylphosphorane, Ph3P+–CH2−, often drawn with a P=C bond in resonance. Sulfur ylides are sulfonium ylides with structure R2S+–CHR−; examples include ethylidene sulfonium derivatives. Nitrogen ylides (ammonium ylides) are less common but also described.

Preparation: Phosphorus ylides are typically generated by alkylation of triphenylphosphine to give a phosphonium salt, followed by deprotonation with a strong base to form the ylide. Sulfur ylides are produced similarly by alkylation of sulfide to give a sulfonium salt and subsequent deprotonation. Ylides can also be generated in situ from aldehydes and stabilized phosphorane reagents.

Reactions: The best-known use is the Wittig reaction, where a phosphorane ylide reacts with an aldehyde or ketone to form an alkene and triphenylphosphine oxide. This approach enables formation of defined alkenes. Sulfur and nitrogen ylides participate in epoxidations, cyclopropanations, and other rearrangements, depending on substitution and conditions. Stabilized ylides are generally less reactive than unstabilized ones.