An 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.

An 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.

Preparation of a phosphonium ylide

An ylide can be prepared rather straightforwardly. Typically, a phosphine (e.g. triphenylphosphine) is allowed to react with an alkyl halide in a mechanism analogous to that of an S_N2 reaction. This forms an alkyltriphenylphosphonium salt which is then allowed to react with a strong base (in this case, dimsyl sodium) to form the ylide.

The salt products are not shown. Also, the product shown here is shown in the ylide form; however, it could also be shown as the phosphorane form in which the bond to phosphorus is a double bond with the methylene group. Due to an inductive effect, the trio of phenyl groups allows phosphorus to bear such a buildup of positive charge and shifts the negative charge to carbon, creating a reactive species.

Due to the S_N2 mechanism, a less sterically hindered alkyl halide reacts more favorably with triphenylphosphine than an alkyl halide with significant steric hindrance (such as tert-butyl bromide). Because of this, there will typically be one synthetic route in a synthesis involving such compounds that is more favorable than another.

Ylide types

• The most common ylids are phosphonium ylids, used in the Wittig reaction for double bond synthesis from carbonyl groups (C=O). The positive charge in these Wittig reagents is carried by a phosphorus atom with three phenyl substituents and one bond to a carbon bearing a negative charge and two substituents, commonly alkyl groups. Ylids can be 'stabilised' or 'non-stabilised'. Non-stabilised ylids react readily with both aldehydes and ketones whereas stabilised will only react with aldehydes.

• Other common ylids include sulfonium ylids and sulfoxonium ylids, for instance the Corey-Chaykovsky reagent used in the preparation of epoxides or in the Stevens rearrangement.

• 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 R₁, R₂ 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.

Ylide reactions

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 ^{[1] [2]}

Wittig reagents are found to react as nucleophiles in SN₂' substitution:^[3]

The initial addition reaction is followed by an elimination reaction.

References

1. ^ Ferguson, Marcelle L.; Senecal, Todd D.; Groendyke, Todd M.; Mapp, Anna K. (2006). "[3,3]-Rearrangements of Phosphonium Ylides". J. Am. Chem. Soc. 128 (14): 4576–4577. doi:10.1021/ja058746q. 
2. ^ (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.
3. ^ Ramesh M. Patel and Narshinha P. Argade (2007). "Facile SN2' Coupling Reactions of Wittig Reagents with Dimethyl Bromomethylfumarate: Synthesis of Enes, Dienes, and Related Natural Products". J. Org. Chem. 72 (13): 4900–4904. doi:10.1021/jo070728z.