Ylide

An ylid or ylide (US) is a neutral molecule with positive and negative charges on adjacent atoms. They appear in organic chemistry as reagents or reactive intermediates. In one definition an ylide is

"A substance in which a carbanion is attached directly to a heteroatom carrying a substantial degree of positive charge and in which the positive charge is created by the sigma bonding of substituents to the heteroatoms" (A. William Johnson)

An ylide is accompanied to some extent by (and often depicted as) its double bonded resonance structure:

Wittig reagent resonance structures

The actual electron distribution in the bond depends on the entire molecular structure.

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:

R₁R₂C=N⁺R₃C^-R₄R₅

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 reasonance structure e.g.:

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• 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

Scheme 1. Phosphonium ylid rearrangement

Wittig reagents are found to react as nucleophiles in SN₂' substitution :

Wittig reagent in allylic rearrangement. 8% ene product not depicted

The initial addition reaction is followed by an elimination reaction.

References

de:Ylide