Question

Treatment of a cyclic ketone with diazomethane is a method for accomplishing a ring-expansion reaction. For example, treatment of cyclohexanone with diazomethane yields cycloheptanone. Propose a mechanism.

Short answer

The mechanism involves formation of a diazoketone, Wolff rearrangement, and ring expansion to cycloheptanone.

Step-by-step solution

Generate a Diazomethane Carbenoid

Diazomethane, a compound with the formula CH₂N₂, decomposes to form a highly reactive methylene carbenoid (:CH₂). This happens as one of the nitrogen atoms breaks its bond with the carbon, with the elimination of nitrogen gas (N₂).

Formation of Cyclohexanone Diazoketone

The reactive methylene carbenoid attacks the carbonyl carbon of cyclohexanone. This results in the formation of a new bond between the carbenoid and the carbonyl carbon, generating a 7-membered ring structure known as a diazoketone.

Wolff Rearrangement

In the next step, under thermal or photochemical conditions, the diazoketone undergoes a Wolff rearrangement. During this rearrangement, the diazo group loses nitrogen gas to form a ketene intermediate.

Ring Expansion to Cycloheptanone

The ketene intermediate quickly reacts to lose the remaining reactive group and forms a new carbon-carbon bond, expanding the ring from six to seven members to yield cycloheptanone.

Key concepts

Cyclic Ketone

Cyclic ketones are a unique class of ketones where the carbonyl group, consisting of a carbon double-bonded to oxygen, is part of a carbon ring. This means that their structure forms a closed loop or a 'ring'. The size of the ring, which ranges from three to more than seven carbon atoms, significantly influences both the chemical properties and reactivity of the cyclic ketone.
Cyclohexanone, in particular, is a common six-membered cyclic ketone often used in chemical reactions to explore ring expansion.

• The structure and stability of the ring affect its reactivity; smaller rings tend to be more strained and thus more reactive.
• Cyclic ketones often serve as starting materials for various chemical transformations, such as ring expansions or contractions.

Understanding the basic properties of cyclic ketones is crucial when studying reactions such as the one with diazomethane for ring expansion.

Diazomethane

Diazomethane, with the chemical formula CH₂N₂, is a highly reactive compound used in organic chemistry for its interesting properties and reactions. It consists of a carbon atom bonded to two nitrogen atoms. Known for its role in introducing carbenes into reactions, diazomethane is extremely versatile.

• Carbenes are uncharged carbon species with two non-bonded electrons, making them highly reactive.
• Diazomethane can decompose to form methylene (:CH₂), an important carbenoid, by releasing nitrogen gas.

In reactions, diazomethane's ability to generate methylene carbenoid allows it to facilitate ring-expansion processes by forming new bonds with other molecules.

Wolff Rearrangement

The Wolff rearrangement is a pivotal transformation in organic chemistry, particularly known for its application in ring-expansion reactions. This rearrangement involves the conversion of a diazoketone to a ketene intermediate, primarily under heat or light.
This process is characterized by the loss of nitrogen gas from the diazo group.

• The ketene intermediate formed is highly reactive and transient, quickly undergoing further reactions.
• Wolff rearrangement is crucial in converting smaller ring structures into larger ones, making it valuable for synthesizing more complex molecules.

Understanding this rearrangement helps explain how cyclic structures can be expanded and tailored for specific chemical applications.

Ketene Intermediate

In the context of ring expansion reactions, the ketene intermediate is a fleeting yet important species. Ketene is characterized by a carbon atom double-bonded to both an oxygen and another carbon atom with a group on one side that is often very reactive.

• This structure enables the ketene to readily form new carbon-carbon bonds, facilitating ring expansion.
• It is a key player in the transformation steps following the Wolff rearrangement.

Despite its transient nature, the effective harnessing of the ketene intermediate is essential for achieving successful ring expansions, like conversion from cyclohexanone to cycloheptanone.