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Unlock your guides19.11 Nucleophilic Addition of Phosphorus Ylides: The Wittig Reaction
2 min read•may 7, 2024
The is a powerful tool for making carbon-carbon double bonds. It combines a phosphorus ylide with an aldehyde or ketone to create with specific structures. This reaction is crucial for building complex molecules in organic synthesis.
Understanding the Wittig reaction's mechanism and is key to predicting outcomes. It offers advantages over other alkene-forming reactions, making it a go-to method for creating diverse carbon frameworks in organic chemistry.
Wittig Reaction
Wittig reaction mechanism
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- of phosphorus ylide to carbonyl carbon of aldehyde or ketone forms
- Betaine intermediate cyclizes to give four-membered oxaphosphetane intermediate
- Oxaphosphetane decomposes through cycloelimination reaction yields alkene product and phosphine oxide byproduct
- Stereochemistry of alkene product depends on ylide structure and reaction conditions
- Unstabilized ylides (no conjugating groups) typically give (Z)-alkenes (cis)
- Stabilized ylides (conjugated with electron-withdrawing groups like carbonyl or cyano) typically give (E)-alkenes (trans)
- The reaction's is influenced by the stability of the ylide and its
Preparation of phosphorus ylides
- Alkyl halide (e.g., ) undergoes SN2 reaction with phosphine (typically ) forms
- Treatment of phosphonium salt with (, , ) deprotonates alpha carbon generates resonance-stabilized ylide
- Ylide is nucleophilic at alpha carbon due to negative charge and stabilizing effect of adjacent phosphonium group
- Base plays crucial role in generating ylide from phosphonium salt via deprotonation
- Ylides are important in organic synthesis
Wittig reaction vs other alkene syntheses
- Highly selective for synthesis of alkenes with defined stereochemistry and substitution patterns
- Can prepare highly substituted alkenes including
- Allows synthesis of and through sequential Wittig reactions
- Tolerates wide range of functional groups in both ylide and carbonyl compound
- Advantages over aldol condensation:
- Does not require strong acids or bases which can cause isomerization or rearrangement of sensitive substrates
- Aldol condensation may give mixtures of regioisomers with unsymmetrical
- Advantages over :
- Typically gives higher yields and more tolerant of steric hindrance
- Peterson olefination advantageous for synthesis of silyl-substituted alkenes
Carbonyl Chemistry and Related Reactions
- The Wittig reaction is a key transformation in
- It involves the conversion of carbonyl compounds to alkenes
- Related reactions include , which also form carbon-carbon double bonds
- Understanding of resonance structures is crucial for predicting reactivity and selectivity in these reactions
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