Question

What product would you obtain from a base-catalyzed Michael reaction of pentane-2,4-dione with each of the following \(\alpha, \beta\) -unsaturated acceptors? (a) Cyclohex-2-enone (b) Propenenitrile (c) Ethyl but-2-enoate

Short answer

Products are 5-(3-oxocyclohexyl)pentane-2,4-dione, 5-cyano-pentane-2,4-dione, and ethyl 5-oxohexanoate.

Step-by-step solution

Identify the Nucleophile and Electrophile

In the Michael reaction, the nucleophile is the deprotonated form of the dicarbonyl compound, here pentane-2,4-dione, which forms an enolate ion. The electrophile is the \(\alpha, \beta\)-unsaturated carbonyl or nitrile compound, capable of being attacked by the nucleophile due to its electron-deficient double bond. Identify cyclohex-2-enone, propenenitrile, and ethyl but-2-enoate as the electrophiles.

Formulate the Enolate Ion

Under basic conditions, a base will remove an acidic hydrogen from pentane-2,4-dione, usually from the methylene group between the carbonyl groups, to form an enolate ion. This species is resonance-stabilized and serves as the nucleophile.

Perform the Nucleophilic Attack

The enolate ion attacks the β-carbon of the \(\alpha, \beta\)-unsaturated carbonyl or nitrile, leading to the formation of a new carbon-carbon bond and creating a negatively charged intermediate. The double bond shifts to the α-carbon of the Michael acceptor.

Protonation and Product Formation

The negatively charged intermediate formed from the nucleophilic attack gets protonated, typically by the solvent or another available hydrogen source, leading to the formation of the stable Michael addition product.

Step 5A: Analyze Product for Cyclohex-2-enone

The product of the reaction between pentane-2,4-dione and cyclohex-2-enone is 5-(3-oxocyclohexyl)pentane-2,4-dione, where the enolate ion adds to the β-position of cyclohex-2-enone, forming a new six-membered ring structure.

Step 5B: Analyze Product for Propenenitrile

The product of the reaction between pentane-2,4-dione and propenenitrile is 5-cyano-pentane-2,4-dione, where the enolate ion adds to the β-position, with the nitrile group remaining intact.

Step 5C: Analyze Product for Ethyl but-2-enoate

The product of the reaction between pentane-2,4-dione and ethyl but-2-enoate is ethyl 5-oxohexanoate. The enolate ion adds to the conjugated double bond, forming an ester-functionalized product chain.

Key concepts

Nucleophile

A nucleophile is a chemical species that donates an electron pair to form a chemical bond in reaction processes. In the context of a Michael reaction, the nucleophile is the molecule that seeks out positively charged or electron-deficient species, called electrophiles.

In a base-catalyzed Michael reaction with pentane-2,4-dione, the nucleophile is actually an enolate ion. This enolate is formed when a base removes a proton from the dione, resulting in a molecule with a negatively charged oxygen that can share electron pairs readily.

Nucleophiles are essential players in organic chemistry since they drive the creation of new molecular structures by forming bonds with electrophiles. This is particularly important in forming stable organic products, such as those observed in product formation during Michael reactions.

Electrophile

Electrophiles are chemical species that accept electron pairs during reactions. In a Michael reaction, electrophiles are typically characterized by their electron-deficient double bonds.

The electrophiles in the given exercise include cyclohex-2-enone, propenenitrile, and ethyl but-2-enoate. These \(\alpha, \beta\)-unsaturated compounds have conjugated systems that make them highly reactive towards nucleophiles like the enolate ion of pentane-2,4-dione.

The reason these electrophiles are attractive targets for nucleophilic attack is due to the presence of polarized double bonds. For instance, in cyclohex-2-enone, the carbon of the carbonyl is partially positive, making it an ideal site for nucleophiles to attack.

Understanding the behavior of electrophiles helps predict how and where reactions, such as the formation of carbon-carbon bonds, may occur.

Enolate Ion

Enolate ions are key intermediates in many organic reactions, including the Michael reaction. They are formed when a base deprotonates an acidic hydrogen in a compound containing a carbonyl group, like pentane-2,4-dione.

The enolate ion is resonance-stabilized, meaning that its charge can be shared between the oxygen atom and the adjacent carbon atom. This delocalization of charge makes the enolate ion a stable and reactive nucleophile.

In a Michael reaction, the enolate ion formed from pentane-2,4-dione serves as the nucleophile, targeting the electrophile's \(\beta\)-carbon position to form a new carbon-carbon bond.

Carbon-Carbon Bond Formation

Carbon-carbon bond formation is a fundamental type of chemical reaction in organic chemistry. It is crucial for building complex organic molecules.

In the Michael reaction, carbon-carbon bonds are formed when the enolate ion (nucleophile) attacks the \(\beta\)-position of the electrophile, such as an \(\alpha, \beta\)-unsaturated carbonyl compound. This attack results in the linkage of two previously separate carbon systems, effectively expanding the molecular framework.

For example, when pentane-2,4-dione undergoes a Michael reaction with cyclohex-2-enone, a new bond is formed that incorporates the carbon atoms from the enolate and the electrophile, resulting in a stable compound with increased complexity.

The ability to reliably form carbon-carbon bonds is foundational for synthesizing new organic molecules in the laboratory and industry.