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.
- The species formed here immediately forms a 4-membered puckered ring transition state. The instability within this species means it is non-isolable and it rapidly collapses to form the kinetic product - a Z-alkene. Triphenylphosphine oxide is our major by-product. These results generally hold true for all unstable ylides (within reason). Unstable ylides are ones that can't move over different transitions (eg. alkyl or H). The second picture shows the puckering within the ring. If puckering had occurred in the reverse of this image then the phenyl group on the aldehyde would have sterically clashed with the phenyl groups of the -PPh3 side. The sterics at play here also demand that the majority of product be the Z-olefin I'm sorry about the poor graphics. Like all organic chemists, I fear technology :P Give me some shiny glasswear and a bench Hahah