A ylid or ylide (US) is a neutral molecule with a positive and a negative charge on adjacent atoms. They appear in organic chemistry as reagents or reactive intermediates.
A ylide is accompanied to some extent by (and often depicted as) its double bonded resonance structure:
The actual electron distribution in the bond depends on the entire molecular structure.
- Certain nitrogen-based ylids also exist such as azomethine ylids with the general structure:
- These compounds can be envisioned as iminium cations placed next to a carbanion. The substituents R4, R5 are electron withdrawing groups. These ylids can be generated by condensation of an α-amino acid and an aldehyde or by thermal ring opening reaction of certain N-substituted aziridines. Stable carbenes also have a ylidic resonance structure e.g.:
- Iminophosphoranes (also called:phosphazides) with general structure R3P+-N-R are intermediates in the Staudinger reduction.
- The active form of Tebbe's reagent is often considered a titanium ylide. Like the Wittig reagent, it is able to replace the oxygen atom on carbonyl groups with a methylene group. Compared with the Wittig reagent, it has more functional group tolerance.
An important ylide reaction is of course the Wittig reaction but there are more. Many ylids are 1,3-dipoles and interact in 1,3-dipolar cycloadditions. For instance an azomethine ylid is a dipole in the Prato reaction with fullerenes.
Many ylids also react as olefins in rearrangement reactions such as a [3,3]-sigmatropic reaction observed in certain phosphonium ylids  
Wittig reagents are found to react as nucleophiles in SN2' substitution :
The initial addition reaction is followed by an elimination reaction.
- ^ Ferguson, Marcelle L.; Senecal, Todd D.; Groendyke, Todd M.; Mapp, Anna K. (2006). "[3,3]-Rearrangements of Phosphonium Ylides". Journal of the American Chemical Society 128 (14): 4576–4577. doi:10.1021/ja058746q.
- ^ (i) Reaction of allyl alcohol with 2-chloro-5,5-dimethyl-1,3,2-dioxaphosphorinane forms a phosphite ester. (ii) Metal carbene addition (from ethyl diazoacetate and ClFeTPP) forms an ylid. (iii) A rearrangement reaction (in blue) yields a phosphonate.
- ^ Facile SN2' Coupling Reactions of Wittig Reagents with Dimethyl Bromomethylfumarate: Synthesis of Enes, Dienes, and Related Natural Products Ramesh M. Patel and Narshinha P. Argade J. Org. Chem.; 2007; 72(13) pp 4900 - 4904; (Article) doi:10.1021/jo070728z