The Wittig reaction, also known as Wittig olefination, is a great way to turn aldehydes and ketones into alkenes.
The box-out method:
Before we get into the mechanism, let’s look at a really quick way to get the right answer on an exam. If you see an aldehyde or ketone and an ylide, you can actually use something called the box-out method to predict the product.
On the reactant side, we’ve got an aldehyde on the left and an ylide on the right; on the product side, we’ve got an alkene. There is a complicated mechanism, but let’s see how we can skip it to just figure out what that product is:
Basically, you can just draw a box around the carbonyl oxygen and the triphenylphosphine (Ph3P). From there, imagine joining the two double bonds together through a double bond.
The arrow-pushing mechanism:
The box-out method is great and all, but there’s nothing like a good mechanism to help understand exactly what’s going on. The first thing we need to do is get preparation going on the ylide. The best way to do it is to use a primary alkyl bromide (or other alkyl halide) but secondary will do.
Now we’ve got that triphenylphosphonium ion, we’re one step away from forming our ylide! All we need to do is add a strong base to form a carbanion. Notice below that the ylide is zwitterionic; that is, it’s got adjacent opposite charges. It’s stabilized by the resonance shown.
Great, now all that’s left is to react the ylide with the carbonyl. The ylide’s carbon is a pretty good nucleophile, and it can participate in nucleophilic addition. Let’s see how it’s done:
The carbonyl acts as an electrophile as the anionic carbon attacks it to form a betaine (pronounced beta-ene). The oxide attacks the cationic phosphorus to form an oxaphosphetane, which undergoes rearrangement to produce an alkene and phosphine oxide.
The Wittig doesn’t have selectivity for any particular stereochemistry without modification. It yields both the E-alkene and Z-alkene without preference. Modifications like the Horner-Wadsworth-Emmons and Schlosser preferentially form the E-alkene.