What is the protocol for Wittig reaction to convert an aldehyde to an alkene, including ylide preparation from triphenylphosphonium salt, base choice, solvent, and how to identify E/Z selectivity?
Research Activity · 4 steps
Research will be limited to:
- Reaction: Wittig
- Reactant1: Aldehyde
- Product: Alkene
- Reagent: Ylide
- Aspect: E/Z selectivity
Strategy: The user is looking for a comprehensive technical protocol for the Wittig reaction, specifically focusing on the transformation of aldehydes to alkenes. The request requires details on ylide preparation, the choice of base and solvent, and how these factors influence E/Z selectivity.
My research strategy will focus on:
1. Identifying standard protocols for both stabilized and non-stabilized ylides.
2. Determining the specific base/solvent combinations that favor Z-alkenes (standard Wittig) vs. E-alkenes (Schlosser modification or stabilized ylides).
3. Extracting step-by-step experimental procedures including temperature and addition order.
Queries:
- Wittig reaction protocol aldehyde to alkene E/Z selectivity factors base solvent
- Schlosser modification Wittig reaction E-selective protocol aldehyde
- Z-selective Wittig reaction conditions non-stabilized ylide aldehyde base solvent
Found 3 websites. Summarized content:
[PDF] 1. The Wittig Reaction
The provided document discusses the Wittig reaction with a focus on E/Z selectivity. Here's a summary of the relevant information:
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Mechanism & Stereoselectivity: The Wittig reaction proceeds via a four-centered transition state to form an oxaphosphetane directly, not through a betaine intermediate. The Z:E ratio of alkenes is determined by the cis:trans ratio of oxaphosphetanes (kinetic control). The stereochemistry is established in the transition state leading to the oxaphosphetane.
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Ylide Stabilization & Selectivity:
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Non-stabilized ylides: Favor the trans-alkene (more specifically, the trans-oxaphosphetane) due to dominant 1,2-steric interactions in a planar transition state.
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Stabilized ylides: (carbonyl-stabilized) Favor the E-alkene due to a more planar transition state and the thermodynamic advantage for the trans-disubstituted oxaphosphetane. 1,3-interactions are relieved.
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Salt Effects (Schlosser Modification): Lithium halides catalyze oxaphosphetane ring-opening and stabilize the betaine intermediate. Excess salt promotes equilibration leading to the E-alkene. To maintain Z-selectivity, the reaction should be performed with low salt concentrations.
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Other Wittig Variants: Mentions Horner-Wittig (using phosphine oxides), Horner-Wadsworth-Emmons (using phosphonates) which often lead to the E-alkene due to thermodynamic control.
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Z-Selective Wittig Reactions: Mentions examples with corresponding references (Corey, Weinshenker, Schaaf, Huber, J. Am. Chem. Soc. 1969, 91, 5675; Martin, Soler, Betancort, Martin, J. Org. Chem. 1997, 62, 1570).
Wittig reaction - Wikipedia
The Wittig reaction is used to convert aldehydes and ketones to alkenes using a triphenyl phosphonium ylide (Wittig reagent). The choice of ylide substituents influences the E/Z selectivity of the resulting alkene.
- Unstabilized ylides (R3 = alkyl, H): Favor the formation of the Z-alkene with moderate to high selectivity. If the reaction is performed in dimethylformamide in the presence of lithium iodide or sodium iodide, the product is almost exclusively the Z-isomer.
- Stabilized ylides (R3 = ester or ketone): Favor the formation of the E-alkene with high selectivity.
- Semistabilized ylides (R3 = aryl): Generally give poor E/Z selectivity.
Schlosser Modification: The main limitation of the traditional Wittig reaction is that the reaction proceeds mainly via the erythro betaine intermediate, which leads to the Z-alkene. The erythro betaine can be converted to the threo betaine using phenyllithium at low temperature. This modification affords the E-alkene.
Solvents: Typical solvents for Wittig reactions include THF and diethyl ether.
Lithium effects: The presence of lithium salts can have a significant effect on the stereochemical outcome, potentially leading to equilibration of intermediates and altering the E/Z ratio. Reactions under lithium-free conditions are generally under kinetic control.
Alkenes from Aldehydes and Ketones - Wittig Reaction
Here's a summary of the relevant information from the provided website content regarding Wittig reactions using aldehydes to form alkenes, focusing on ylide preparation, base, solvent, and E/Z selectivity:
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Reaction Overview: The Wittig reaction converts an aldehyde or ketone and a phosphonium ylide (Wittig reagent) into an alkene and triphenylphosphine oxide.
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Ylide Preparation: Wittig reagents are typically made from a phosphonium salt, which is generated by reacting triphenylphosphine with an alkyl halide. The resulting salt is then deprotonated with a strong base like n-butyllithium.
[Ph3P+CH2R]X− + C4H9Li → Ph3P=CHR + LiX + C4H10 -
E/Z Selectivity:
- Unstabilized ylides generally lead to predominantly the Z-alkene product.
- To obtain the E-alkene, stabilized ylides are used, or the Schlosser modification is applied to unstabilized ylides. Stabilized ylides contain electron-withdrawing groups (e.g., Ph3P=CHCO2R or Ph3P=CHPh). These are more air-stable but less reactive and may require the Horner–Wadsworth–Emmons reaction for reactions with ketones.
- Performing the reaction in DMF with LiI or NaI favors the Z-isomer.
- The Schlosser modification converts the erythro betaine intermediate (leading to the Z-alkene) to the threo betaine using phenyllithium at low temperature, favoring the E-alkene.
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Limitations: Sterically hindered ketones react slowly and give poor yields, particularly with stabilized ylides. Aldehydes are labile and can oxidize or polymerize.
From the phosphonium salts, these reagent are formed more readily, requiring only NaOH, and they are usually more air-stable.
The Wittig reaction is a fundamental method for converting aldehydes into alkenes using a phosphonium ylide. The protocol varies significantly depending on whether you require the $Z$ (cis) or $E$ (trans) isomer.
1. Ylide Preparation from Phosphonium Salt
The Wittig reagent (ylide) is prepared in two steps:
1. Salt Formation: Triphenylphosphine ($\ce{PPh3}$) reacts with an alkyl halide (typically a primary or secondary bromide/iodide) via $\text{S}_{\text{N}}2$ substitution to form the phosphonium salt:
$$\ce{PPh3 + R-CH2X -> [Ph3P+-CH2R]X-}$$
2. Deprotonation: The salt is treated with a strong base to remove the $\alpha$-proton, generating the ylide:
$$\ce{[Ph3P+-CH2R]X- + Base -> Ph3P=CHR + Base-H + Salt}$$
2. Base and Solvent Selection
The choice of base and solvent depends on the stability of the ylide and the desired stereochemistry:
| Ylide Type | Substituent (R) | Typical Bases | Common Solvents |
|---|---|---|---|
| Non-stabilized | Alkyl, H | $n$-BuLi, NaH, $\ce{NaNH2}$ | THF, $\ce{Et2O}$, Benzene |
| Stabilized | $\ce{CO2R}$, $\ce{CN}$, $\ce{COR}$ | $\ce{NaOH}$, $\ce{NaOEt}$, $\ce{K2CO3}$ | $\ce{H2O}$, MeOH, $\ce{CH2Cl2}$ |
| Semi-stabilized | Aryl, Allyl | $\ce{NaOMe}$, $t$-BuOK | MeOH, EtOH, THF |
3. General Experimental Protocol
- Ylide Generation: Under an inert atmosphere ($\ce{N2}$ or Ar), dissolve the phosphonium salt in anhydrous THF. Cool to $0^\circ\text{C}$ or $-78^\circ\text{C}$ and add the base (e.g., $n$-BuLi) dropwise. The solution will typically turn a deep color (orange, red, or yellow). Stir for 30–60 minutes to ensure complete deprotonation.
- Aldehyde Addition: Add the aldehyde (neat or in THF) to the ylide solution. For non-stabilized ylides, this is often done at low temperature and then allowed to warm to room temperature.
- Workup: Quench with water or aqueous $\ce{NH4Cl}$. The byproduct, triphenylphosphine oxide ($\ce{Ph3PO}$), is often difficult to remove; it can be precipitated by adding hexane or removed via column chromatography.
4. Controlling $E/Z$ Selectivity
The stereochemical outcome is governed by the nature of the ylide and the reaction conditions:
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$Z$-Selectivity (Non-stabilized ylides):
- Conditions: Use "salt-free" conditions (e.g., using $\ce{NaNH2}$ or $\ce{KHMDS}$ as the base) in non-polar solvents like THF or $\ce{Et2O}$.
- Mechanism: Kinetic control favors the cis-oxaphosphetane intermediate due to minimized steric repulsions in the transition state.
- Reference: Wittig Reaction - Organic Chemistry Portal.
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$E$-Selectivity (Stabilized ylides):
- Conditions: Use ylides with electron-withdrawing groups. These reactions are often performed in protic solvents or $\ce{CH2Cl2}$.
- Mechanism: Thermodynamic control allows the intermediate to equilibrate to the more stable trans-oxaphosphetane.
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$E$-Selectivity (Schlosser Modification for non-stabilized ylides):
- Protocol: Generate the ylide with $n$-BuLi, add the aldehyde at $-78^\circ\text{C}$ to form the betaine-lithium salt complex, then add a second equivalent of a strong base (like PhLi) to deprotonate the betaine. Re-protonation with $t$-BuOH and warming leads to the $E$-alkene.
- Reference: Wittig Reaction - Wikipedia.
5. Identification of Selectivity
- NMR Spectroscopy: This is the primary method. In $^1\text{H}$ NMR, the coupling constant ($J$) between the vinylic protons is diagnostic:
- $Z$-alkenes: $J \approx 7\text{--}12\text{ Hz}$
- $E$-alkenes: $J \approx 14\text{--}18\text{ Hz}$
- Gas Chromatography (GC) or HPLC: Used to determine the $E/Z$ ratio by comparing peak areas, provided the isomers are separable.