The reaction of an alkene with diazomethane forms a...

The reaction of an alkene with diazomethane forms a cyclopropane ring. Propose a mechanism for the reaction. (Hint: It is a concerted reaction.)
Note: Diazomethane is a gas that must be handled with great care because it is both explosive and toxic.

Key Concepts

Cyclopropanation

Cyclopropanation is the chemical transformation where an alkene is converted into a cyclopropane ring by the formation of two new carbon–carbon bonds. This reaction is significant because the cyclopropane ring is a highly strained, three-membered ring that exhibits unique reactivity and properties, making it an important motif in synthetic organic chemistry.

Diazo Compound Reactivity

Diazo compounds, such as diazomethane, are characterized by a diazo group that can act as a source of carbenes or carbenoid species. Their reactivity is central to many transformations, including cyclopropanation, where they interact with electron-rich alkenes to generate new carbon–carbon bonds. The handling of diazo compounds requires caution due to their toxicity and explosive nature.

Concerted Reaction Mechanism

A concerted reaction mechanism involves the simultaneous making and breaking of bonds in a single, continuous step without any intermediates. In the context of cyclopropanation, the reaction between an alkene and diazomethane occurs via a single transition state where both new bonds are formed concurrently, leading directly to the cyclopropane product.

Transition State Concept

The concept of a transition state is central to understanding concerted reaction mechanisms. It refers to the high-energy, fleeting arrangement of atoms where old bonds are partially broken and new bonds are partially formed. In the cyclopropanation reaction, the transition state involves the alkene and diazomethane aligned in such a way that the simultaneous bond formation creates the three-membered ring structure.

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