Addition to Carbn Heteromultiple bond
1) Phosphorus Ylide
Reaction of phosphonium ylide with aldehydes, ketones and lactols to give olefins.
The reaction is easy to carry out and proceeds under mild conditions.
It is valued in organic synthesis as a method for carbon–carbon bond formation, in which the position of the double bond is unambiguous.
The reaction generally leads to high yields of di- and trisubstituted alkenes from aldehydes and ketones but, because of steric effects, yields of tetrasubstituted alkenes from ketones are often poor.
Sluggish reactions can sometimes be forced by addition of hexamethylphosphoramide (HMPA).
1.1) Preparation of Phosphorus Ylide:
1.2) Reactions of Phosphorus Ylide
2) Olefin Geometry
2.1) With "non-stabilized" ylides the Wittig Reaction gives predominantly Z-olefins.
2.2) "Stabilized ylides" give predominantly E-olefins
Betaine formation is reversible and the reaction becomes under thermodynamic control to give the most stable product.
There is NO evidence for a betaine intermediate.
3) Phosphonate Modification (Horner-Wadsworth-Emmons) Reaction:
R is usually restricted to EWG such as CO2H, CO2Me, CN, SO2Ph etc. and the olefin geometry is usually E
4) Some Modification:
CF3CH2O- groups make the betaine less stable, giving more Z-olefin
5) Sulfur Ylide
Non-Stabilized Sulfur Ylide:
Methylation of dimethylsulfide with methyl iodide produces trimethylsulfoniurn iodide.
The positive charge on sulfur enhances the acidity of the methyl protons so that treatment of the sulfonium salt with a base converts it to dirnethylsulfonium methylide.
This "unstabilized" ylide should be used imrnediately after its preparation.
Reaction of the sulfur ylide with the carbonyl group of aldehydes, ketones, or enones forms a betaine intermediate, which decomposes by intramolecular displacement of Me2S by the oxyanion to yield the corresponding epoxide.
6) Stabilized Sulfur Ylide
Deprotonation of trimethylsulfoxonium iodide forms a sulfur ylide that is significantly more stable than dimethylsulfoniurn methylide and may be prepared and used at room temperature.
This "stabilized" ylide reacts with aldehydes and ketones to furnish epoxides.
The difference in reactivity between dimethylsulfonium methylide and dirnethyloxosulfonium methylide is apparent when considering their reactions with α-β-unsaturated ketones.
Whereas the "nonstabilized" ylide yields the epoxide, the "stabilized" ylide affords a cyclopropane via conjugate addition followed by ring closure and loss of dimethyl sulfoxide.