Mitsunobu reaction mechanism reagents and stereochemical inversion of alcohols
Mitsunobu
The organic reaction which is responsible for transforming primary and secondary alcohols into ethers by treating them with diethyl azodicarboxylate (DEAD) or diisopropyl azodicarboxylate (DIAD) and triphenylphosphine is known as Mitsunobu reaction. The reaction requires a diazo carboxylate which is a compound having two carboxylate groups attached to nitrogen forming an azo bond(-N=N-). This reaction is named after Oyo Mitsunobu in the year 1967, who was a professor in Japan.
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Every reaction has a driving force and the driving force of this reaction is the formation of the P=O bond. The affinity of phosphorus towards oxygen is very high and that is why the driving force of this reaction is the formation of the molecule Ph3P=O.
Mitsunobu Reaction Procedure
Mitsunobu reaction follows Nucleophilic substitution reaction but the substitution reaction mechanism is not direct because alcohol(-OH) is not a good leaving group. Due to its bad leaving group characteristic, alcohol-containing hydrocarbon usually shows retention in the configuration of the final product as it has to go through SNi mechanism than the usual SN2 mechanism.
SN2 mechanism shows inversion in the configuration of the final product and the rate of the reaction depends on both the substrate and the nucleophile i.e. the order is 2.
Rate=k[substrate]1[Nucleophile]1
Order=1+1=2, therefore SN2 mechanism.
Mechanism of Mitsunobu Reaction
Step 1:
In the first step, the triphenylphosphine donates its electron to the nitrogen in the azodicarboxylate forming an anion.
The anion then attacks the acidic proton of the acid substrate forming a nitrogen-hydrogen bond in the dicarboxylate reagent and a zwitterion as an intermediate.
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Step 2:
In the second step, the nitrogen then donates its lone pair to the acidic proton of alcohol which leads to the formation of an oxonium ion which then attacks the triphenylphosphine because of the affinity of oxygen towards phosphorus.
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Step 3:
The product formed in the second step is attacked by the oxonium ion from behind which was formed due to the abstraction of an acidic proton from the acid substrate which leads to simultaneous removal of the molecule Ph3P=O and thus forming an ether, as the final product.
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Mitsunobu Reaction Conditions
The pKa value of the nucleophile is 12 or less than 12 for the reaction to take place successfully. The reason for the basic medium is to avoid the alkylation of azodicarboxylate. The overall reaction takes place in a neutral condition where the conditions aren’t too acidic or basic and the temperature can also be easily maintained as the reaction can be successfully carried out at 0°C to room temperature. The reaction uses standard non-polar solvents like THF and dichloromethane. It also uses polar solvents sometimes like DMF.
Intramolecular Mitsunobu Reaction
As the name suggests, this reaction does not take place between two substrates rather formed from the reaction in the substrate alone. The final product achieved from this reaction is cyclic. When the phenolic oximes are activated by the triphenylphosphine and DEAD (Diethyl azodicarboxylate) at a mild neutral condition at 0°C temperature, then we get the final cyclic product, a cyclic heteroatom consisting of both oxygen and nitrogen i.e. oxazoles.
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Advantages of Using DEAD
The advantages of using reagent DEAD or DIAD are-
(i) Their physical state is solid which helps in studying the reaction better.
(ii) Also, the polarity of the byproduct formed is completely different.
Driving Force of the Mitsunobu Reaction
As we know that alcohol functional group is a bad leaving group and instead of giving an inverted final product it retains the configuration of the substrate. Therefore, for the removal of –OH groups and to obtain an inverted product, a strong driving force is needed and that is achieved in the Mitsunobu reaction by the formation of the P=O bond because of its very strong affinity.
FAQs on Mitsunobu Reaction in Organic Synthesis
1. What is the Mitsunobu reaction?
The Mitsunobu reaction is a chemical reaction that converts a primary or secondary alcohol into an inverted substitution product using triphenylphosphine (PPh3) and an azodicarboxylate such as DEAD or DIAD. It allows an alcohol (R–OH) to react with an acidic nucleophile (H–Nu) to form R–Nu with inversion of configuration at a chiral center. This reaction is widely used in organic synthesis for forming esters, ethers, and other substituted products under mild conditions.
2. What reagents are used in the Mitsunobu reaction?
The Mitsunobu reaction uses triphenylphosphine (PPh3), an azodicarboxylate (such as diethyl azodicarboxylate, DEAD), an alcohol, and an acidic nucleophile. The typical components are:
- Alcohol (R–OH) – primary or secondary
- Acidic nucleophile (H–Nu) – such as a carboxylic acid or phenol
- PPh3 – activates the alcohol
- DEAD or DIAD – facilitates formation of the reactive intermediate
The reaction is usually performed in aprotic solvents like THF or dichloromethane.
3. How does the Mitsunobu reaction work?
The Mitsunobu reaction works through activation of the alcohol by PPh3 and azodicarboxylate, followed by an SN2-type nucleophilic substitution that inverts stereochemistry. The key steps are:
- Formation of a phosphonium intermediate from PPh3 and the azodicarboxylate.
- Activation of the alcohol to form an oxyphosphonium ion.
- Deprotonation of the acidic nucleophile (H–Nu).
- SN2 attack of Nu− on the activated carbon, causing inversion.
The byproducts are typically triphenylphosphine oxide (OPPh3) and a reduced hydrazine derivative.
4. Why does the Mitsunobu reaction cause inversion of configuration?
The Mitsunobu reaction causes inversion of configuration because it proceeds via an SN2 mechanism at the stereogenic carbon. In an SN2 reaction:
- The nucleophile attacks from the backside of the leaving group.
- The reaction occurs in a single concerted step.
- The stereochemistry flips (R → S or S → R).
Therefore, a chiral secondary alcohol will give a product with the opposite absolute configuration.
5. What types of nucleophiles can be used in the Mitsunobu reaction?
The Mitsunobu reaction requires an acidic nucleophile with a pKa typically below about 11. Common nucleophiles include:
- Carboxylic acids (forming esters)
- Phenols (forming aryl ethers)
- Imides and certain amides
- Thiols
Strongly basic or non-acidic nucleophiles are generally unsuitable because deprotonation is a key step in the reaction mechanism.
6. What is an example of a Mitsunobu reaction?
An example of the Mitsunobu reaction is the conversion of ethanol and acetic acid into ethyl acetate under Mitsunobu conditions. In general form:
- R–OH + R′–COOH + PPh3 + DEAD → R–O–CO–R′ + OPPh3 + reduced azodicarboxylate
For a chiral secondary alcohol, the resulting ester is formed with inversion at the stereocenter, which is a key feature in stereoselective organic synthesis.
7. What are the limitations of the Mitsunobu reaction?
The Mitsunobu reaction has limitations including steric hindrance and byproduct removal issues. Major limitations are:
- Tertiary alcohols do not react due to steric hindrance.
- Bulky secondary alcohols react slowly.
- Nucleophiles must be sufficiently acidic.
- Removal of triphenylphosphine oxide can be difficult.
These drawbacks can complicate large-scale or industrial applications.
8. What is the role of triphenylphosphine in the Mitsunobu reaction?
The role of triphenylphosphine (PPh3) in the Mitsunobu reaction is to activate the alcohol by forming an oxyphosphonium intermediate. Specifically:
- PPh3 reacts with the azodicarboxylate to form a reactive phosphonium species.
- This species converts the –OH group into a better leaving group.
- After substitution, PPh3 is oxidized to OPPh3.
Thus, PPh3 enables nucleophilic substitution under mild, neutral conditions.
9. What is the difference between the Mitsunobu reaction and an SN1 reaction?
The Mitsunobu reaction differs from an SN1 reaction because it proceeds by an SN2 mechanism with inversion, not through a carbocation intermediate. Key differences include:
- Mitsunobu (SN2): one-step, backside attack, inversion of configuration.
- SN1: two-step, carbocation intermediate, often racemization.
- Mitsunobu requires specific reagents (PPh3 and azodicarboxylate), while SN1 occurs under polar protic conditions.
Therefore, the Mitsunobu reaction is stereospecific, whereas SN1 reactions are not.
10. Why is the Mitsunobu reaction important in organic synthesis?
The Mitsunobu reaction is important in organic synthesis because it allows stereospecific inversion of alcohols and formation of C–O, C–N, and C–S bonds under mild conditions. Its significance includes:
- Precise control of stereochemistry in chiral molecules.
- Efficient synthesis of esters and ethers.
- Wide application in pharmaceutical and natural product synthesis.
It is especially valuable when inversion of a single stereocenter is required without harsh reagents.
