Alkene Reactions: Simmons Reaction - Cyclopropane Formation

When alkenes are reacted with diiodomethane and a zinc-copper couple, the alkene is formed into a cyclopropane molecule. This reaction is known as the Simmons–Smith reaction and it preserves the original configuration of the molecule as well as adding stereochemistry.

Reaction

alkene ch2i2 reaction

The reaction mechanism is depicted below:

Mechanism:

alkene ch2i2 reaction mechanism

In the first step, the zinc-copper couple inserts itself into the diiodomethane, forming a methylene free radical that will later react with the alkene.

In the second step, the iodomethylzinc iodide, (ICH2ZnI) reacts with the alkene bond in a concerted step, binding the carbon atom from the iodomethylzinc iodide and kicking off the Zn-I and I atoms, forming the final cyclopropane product.

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