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

The reaction mechanism is depicted below:

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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