Ylide
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An ylide is a neutral molecule with positive and negative charges on adjacent atoms.
The most common ylides are phosphonium ylides, 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. Ylides can be 'stabilised' or 'non-stabilised'. Non-stabilised ylides react readily with both aldehydes and ketones whereas stabilised will only react with aldehydes.
Other common ylides include sulfonium ylides and sulfoxonium ylides, for instance the Corey-Chaykovsky reagent used in the preparation of epoxides or in the Stevens rearrangement. Certain nitrogen-based ylides also exist such as azomethine ylides 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 ylides can be generated by condensation of an α-amino acid and an aldehyde or by thermal ring opening reaction of certain N-substituted aziridines.
Iminophosphoranes (also called:phosphazides) with general structure R3P+-N-R are intermediates in the Stauding reduction.
Stable carbenes also have a ylidic reasonance structure e.g. R1R2N-C:-NR3</sup>R4 ↔ R1R2N+=C--NR3</sup>R4
Many ylides are 1,3-dipoles and interact in 1,3-dipolar cycloadditions. For instance an azomethine ylide is a dipole in the Prato reaction with fullerenes.
Many ylides also react as olefins in rearrangement reactions such as a [3,3]-sigmatropic reaction observed in certain phosphonium ylides <ref name=Ferguson>Ferguson, Marcelle L., Senecal, Todd D.; Groendyke, Todd M.; Mapp, Anna K. (2006). "[3,3]-Rearrangements of Phosphonium Ylides" (Abstract). Journal of the American Chemical Society 128 (14): 4576–4577. DOI:10.1021/ja058746q.</ref>
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