19.11 Nucleophilic Addition of Phosphorus Ylides: The Wittig Reaction

Wittig Reaction

Wittig Reaction Mechanism

The Wittig reaction converts a carbonyl group (C=O) into an alkene (C=C) by reacting an aldehyde or ketone with a phosphorus ylide. The driving force behind the entire reaction is the formation of a very strong P=O bond in the triphenylphosphine oxide byproduct.

Here's how the mechanism proceeds:

1. Nucleophilic addition: The ylide carbon (which carries a negative charge) attacks the electrophilic carbonyl carbon. This forms a dipolar intermediate called a betaine, with opposite charges on carbon and oxygen.
2. Ring closure: The negatively charged oxygen attacks the positively charged phosphorus, forming a four-membered ring called an oxaphosphetane.
3. Cycloelimination: The oxaphosphetane breaks apart in a concerted [2+2] cycloreversion. The C–P and C–O bonds break simultaneously, producing the alkene and triphenylphosphine oxide ($Ph_3P=O$).

The stereochemistry of the product alkene depends on the type of ylide used:

• Unstabilized ylides (no electron-withdrawing groups on the ylide carbon) tend to give (Z)-alkenes (cis). These ylides are more reactive, and the reaction proceeds through a kinetically controlled pathway.
• Stabilized ylides (conjugated with electron-withdrawing groups like $-COOR$, $-COR$, or $-CN$) tend to give (E)-alkenes (trans). The reaction is thermodynamically controlled, favoring the more stable trans oxaphosphetane intermediate.

This stereoselectivity is one of the Wittig reaction's most useful features: you can often predict and control the alkene geometry just by choosing the right ylide.

Preparation of Phosphorus Ylides

Making the ylide is a two-step process:

1. Form the phosphonium salt. An alkyl halide reacts with triphenylphosphine ($Ph_3P$) in an $S_N2$ reaction. The phosphorus acts as the nucleophile, displacing the halide. Because this is $S_N2$, it works best with primary and methyl halides (secondary halides are slower, and tertiary halides won't work).

$Ph_3P + R{-}CH_2{-}Br \rightarrow [Ph_3P{-}CH_2R]^+Br^-$

1. Deprotonate to form the ylide. A strong base removes a proton from the carbon adjacent to phosphorus. Common bases include n-butyllithium (n-BuLi), sodium hydride (NaH), or sodium ethoxide (NaOEt). The choice of base depends on the acidity of that proton.

$[Ph_3P{-}CH_2R]^+Br^- \xrightarrow{\text{base}} Ph_3P=CHR$

The resulting ylide has two important resonance contributors: one with a C–P double bond and a neutral carbon, and one with a carbanion and a positive phosphorus. The carbon bearing the negative charge is the nucleophilic site that attacks the carbonyl.

Wittig Reaction vs. Other Alkene Syntheses

The Wittig reaction has several advantages that make it widely used:

• Positional selectivity. The new C=C double bond forms exactly where the C=O was. There's no ambiguity about where the alkene ends up, unlike elimination reactions where regiochemistry can be an issue.
• Functional group tolerance. Both the ylide and the carbonyl compound can carry a wide range of functional groups (esters, ethers, protected amines) without interfering.
• Access to substituted alkenes. You can prepare mono-, di-, tri-, and even tetrasubstituted alkenes by choosing the right ylide and carbonyl compound.

Compared to aldol condensation followed by dehydration:

• The Wittig avoids the harsh acidic or basic conditions that can cause side reactions like epimerization or rearrangement.
• Unsymmetrical ketones in aldol reactions can give mixtures of regioisomeric enolates, leading to product mixtures. The Wittig doesn't have this problem.

Compared to Peterson olefination (which uses $\alpha$-silyl carbanions instead of phosphorus ylides):

• The Wittig generally gives higher yields and handles sterically demanding substrates better.
• Peterson olefination has its own niche when silyl-substituted alkenes are the target.

Carbonyl Chemistry Context

The Wittig reaction fits into the broader theme of nucleophilic addition to carbonyls that runs through this entire unit. The ylide carbon is the nucleophile, and the carbonyl carbon is the electrophile. What makes the Wittig special is that the initial addition product doesn't stop at an alcohol; instead, the strong thermodynamic preference for the P=O bond pushes the reaction all the way to an alkene. Recognizing this pattern helps connect the Wittig to other carbonyl reactions you've studied, from Grignard additions to hydride reductions.