Wittig reaction
| Wittig reaction | |||||||||||
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| Named after | Georg Wittig | ||||||||||
| Reaction type | Coupling reaction | ||||||||||
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| Typical solvents | typically THF or diethyl ether | ||||||||||
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| March's Advanced Organic Chemistry | 16–44 (6th ed.) | ||||||||||
| Organic Chemistry Portal | wittig-reaction | ||||||||||
| RSC ontology ID |
RXNO:0000015 
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The Wittig reaction or Wittig olefination is a chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide (often called a Wittig reagent) to give an alkene and triphenylphosphine oxide.
The Wittig reaction was discovered in 1954 by Georg Wittig, for which he was awarded the Nobel Prize in Chemistry in 1979. It is widely used in organic synthesis for the preparation of alkenes. It should not be confused with the Wittig rearrangement.
Wittig reactions are most commonly used to couple aldehydes and ketones to singly substituted phosphine ylides. With unstabilised ylides this results in almost exclusively the Z-alkene product. In order to obtain the E-alkene, stabilised ylides are used or unstabilised ylides using the Schlosser modification of the Wittig reaction can be performed.
The steric bulk of the ylide 1 influences the stereochemical outcome of nucleophilic addition to give a predominance of the betaine 3 (cf. Bürgi–Dunitz angle). Note that for betaine 3 both R1 and R2 as well as PPh3+ and O− are positioned anti to one another.
Carbon-carbon bond rotation gives the betaine 4, which then forms the oxaphosphetane 5. Elimination gives the desired Z-alkene 7 and triphenylphosphine oxide 6. With simple Wittig reagents, the first step occurs easily with both aldehydes and ketones, and the decomposition of the betaine (to form 5) is the rate-determining step. However, with stabilised ylides (where R1 stabilises the negative charge) the first step is the slowest step, so the overall rate of alkene formation decreases and a bigger proportion of the alkene product is the E-isomer. This also explains why stabilised reagents fail to react well with sterically hindered ketones.
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