How does a Simmons-Smith reaction produce cyclopropanes?

A Simmons-Smith reaction produces cyclopropanes by reacting an alkene with iodomethane and a zinc-copper couple.

The Simmons-Smith reaction is a classic method for the synthesis of cyclopropanes, which are three-membered carbon rings. This reaction involves the use of an alkene, iodomethane (CH2I2), and a zinc-copper couple (Zn-Cu). The reaction is named after Howard Ensign Simmons, Jr. and Ronald D. Smith who first reported it in 1958.

The mechanism of the Simmons-Smith reaction begins with the formation of a carbenoid species. This is achieved by the reaction of iodomethane with the zinc-copper couple. The zinc atom inserts itself into the carbon-iodine bond of iodomethane, resulting in a zinc-carbenoid complex. This complex is highly reactive and behaves similarly to a carbene, a species with a neutral carbon atom with only six valence electrons.

The next step is the reaction of the zinc-carbenoid complex with the alkene. The complex approaches the alkene in a manner that allows for simultaneous bond formation with both carbon atoms of the alkene. This results in the formation of a three-membered ring, a cyclopropane, with the zinc atom attached to one of the carbon atoms.

The final step is the removal of the zinc atom, which is achieved by reaction with another molecule of iodomethane. This results in the formation of a new zinc-carbenoid complex, which can then react with another molecule of alkene, and the release of the cyclopropane product.

The Simmons-Smith reaction is a powerful tool in organic synthesis because it allows for the formation of cyclopropanes from alkenes in a single step. The reaction is stereospecific, meaning that the stereochemistry of the alkene is preserved in the cyclopropane product. This makes it a valuable method for the synthesis of complex molecules with defined stereochemistry.