Ylide
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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.
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[edit] Preparation of a phosphonium ylide
An ylide can be prepared rather straightforwardly. Typically, triphenylphosphine is allowed to react with an alkyl halide in a mechanism analogous to that of an SN2 reaction. This forms a alkyltriphenylphosphonium salt which is then allowed to react with a strong base 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 SN2 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.
[edit] 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.
[edit] 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.
[edit] References
- ^ 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:.
- ^ (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.
- ^ 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:.
