Wittig Reaction – conversion of ketones/aldehydes to alkenes
Description: The Wittig reaction is a useful way of forming alkenes from phosphorus ylides (“Wittig Reagents”) and aldehydes or ketones.
Notes: The Wittig reaction works for both aldehydes and ketones. The phosphorus species with the adjacent positive and negative charges is called an ylide. It can alternatively be drawn with a C–P double bond.
Notes: The Wittig reagent (ylide) in examples 1 and 2 is exactly the same, it’s just depicted differently. The Wittig can work for both aldehydes (example 1) and ketones (example 2). It’s also possible to use substituted Wittig reagents (examples 3 and 4). Where Z (“cis”) and E (“trans”) alkenes are possible, the Z alkene tend to be formed preferentially.
Mechanism: Formation of the Wittig reagent is performed first through SN2 of a phosphine at an alkyl halide, followed by deprotonation with strong base.
The Wittig reaction begins with the attack of the ylide carbon at the carbonyl carbon with concurrent formation of an O–P bond (Step 1, arrows A and B). The resulting four-membered ring (called an oxaphosphatane) then breaks down in a “reverse cycloaddition” to give the alkene and triphenylphosphine oxide. Some textbooks may also show addition (arrow A) happening first, followed by attack of oxygen on phosphorus (arrow B). This is also reasonable.
Notes: It’s also reasonable to draw the mechanism for the first step of the Wittig reaction with a P–C double bond.