Wittig olefination: the mechanism, step by step
The ylide adds to the carbonyl, the four-membered ring that results falls apart the other way, and the phosphorus leaves with the oxygen.
The pathway runs in 3 steps. What sits between them is a real structure: it is what the reaction passes through, and on a bad day it is what you isolate instead of the product.
Below is the sequence as it is taught. On the platform it is drawn on whatever structures you supply, with the curly arrows resolved to the actual atoms involved rather than to a general case with R groups on it.
The steps
The ylide carbon attacks the carbonyl
The carbon next to the phosphorus is electron-rich. It attacks the carbon of the C=O group, and the double bond's electrons move onto the oxygen.
Addition of the ylide carbanion into the carbonyl π*, giving a betaine with a negative oxygen and a positive phosphorus.
The oxygen closes onto the phosphorus
The negatively charged oxygen reaches round to the positively charged phosphorus and makes a bond, giving a four-membered ring.
Collapse of the betaine to the oxaphosphetane. For most ylides the ring forms directly rather than through a discrete betaine, and this drawing separates them for clarity.
The ring falls apart the other way
Two of the ring's bonds break: the one from carbon to oxygen, and the one from carbon to phosphorus. Nothing new is bonded here. The carbon-carbon bond that is already there becomes a double bond, and so does the phosphorus-oxygen bond.
Retro-[2+2] of the oxaphosphetane. Two ring sigma bonds break, C-O and C-P, and their electrons raise the order of the two bonds that remain: C-C to the alkene, P-O to the phosphine oxide. No bond is newly formed in this step. The driving force is the phosphorus-oxygen bond, among the strongest in organic chemistry, and it is what makes the whole sequence irreversible.
What this drawing assumes
- The betaine and the ring are drawn as separate species. For most ylides the ring forms in one step and the betaine is a teaching device rather than an observed intermediate.
- Which geometry of alkene results depends on the ylide and is decided in the step that forms the ring; this drawing does not claim one.
Other pathways this class runs by
- A stabilised ylide equilibrates before the ring closes and gives mostly the trans alkene; an unstabilised one closes faster than it equilibrates and gives mostly the cis.
Run it on your own structures
Enter the reactants and the product you expect. If the transformation is one the engine can perform and this pathway reaches that product from those structures, it is drawn on them — with the arrows on the right atoms and the intermediates you would actually pass through. If it does not reach the product, you are told that instead of being shown a drawing that does not apply.
Other mechanisms
Nucleophilic acyl substitution
The nucleophile adds to the carbonyl, the carbon holds four groups for a moment, and the leaving group is pushed out as the double bond comes back.
Bimolecular nucleophilic substitution
One step. The nucleophile comes in on the opposite side from the leaving group, and the carbon turns inside out as the exchange happens.
Imine formation
The amine adds to the carbonyl, the resulting alcohol-amine loses water, and a carbon–nitrogen double bond is left.
Diels-Alder cycloaddition
One step. Six electrons move round a ring at once, two new single bonds form at the ends and the double bond ends up in the middle.
Bimolecular elimination
One step. The base takes a hydrogen from one carbon while the leaving group departs from the next, and a double bond forms between them.
Unimolecular nucleophilic substitution
The leaving group goes first, on its own, leaving a flat carbon with a positive charge. Whatever is around then attacks it from either side.