#olefination

Preparation of a phosphonium ylide for a Wittig olefination.

Triphenyl phosphonium ylides also called Wittig reagents. They are usually prepared from a phosphonium salt, and a strong base such as sodium hydroxide, potassium tert-butylate (as in this case, added on the second gif) or butyllithium.

What happenes here? This:

[Ph3P+CH2R]X− + C4H9Li → Ph3P=CHR + LiX + C4H10

The Wittig reaction  This reaction is essentially the conversion of a carbonyl group (C=O) into an alkene (C=C). It's arguably the most famous olefination reaction and relatively commonplace. It's major downfall tends to be it's poor atom economy: when you make tons of product, you have tons of waste. I personally don't like the hassle of separating my desired product from the Ph3PO.  The curly arrow mechanism above shows the attacks of electron pairs over the course of synthesis. -Triphenylphosphine(Ph3P) has a lone pair on the phosphorus atom which attacks the electron-poor carbon of my alkyl halide. Iodine is a relatively decent leaving group.

- ButylLithium(BuLi) is a strong base needed to extract a proton and make our new phosphorus ylide. The negative pole of this unstable ylide is then able to initiate back-side attack on the aldehyde (alkanal) and regain stability.