Ylide

From Free net encyclopedia

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. 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.

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 obeserved in certain phosphonium ylides Template:Ref in scheme 1 Template:Ref.

Image:Phosphoniumylide rearrangement.png

[edit]

References

  1. Template:Note [3,3]-Rearrangements of Phosphonium Ylides Marcelle L. Ferguson, Todd D. Senecal, Todd M. Groendyke, and Anna K. Mapp J. Am. Chem. Soc.; 2006; 128(14) pp 4576 - 4577 Abstract
  2. Template:NoteScheme 1. Reaction sequence: reaction of allyl alcohol with 2-chloro-5,5-dimethyl-1,3,2-dioxaphosphorinane to the phosphite ester is followed by metal carbene addition (from ethyl diazoacetate and ClFeTPP) to the ylide and then followed by rearrangement reaction (in blue) to the phosphonate. ja:イリド

Retrieved from "http://www.netipedia.com/index.php/Ylide"