A reaction in which a cyclopropane ring is produced from an alkene. It uses the Simmons–Smith reagent, which was originally diiodomethane (CH2I2) with a Zn/Cu couple. Usually, diethyl zinc is used rather than Zn/Cu. The mechanism involves the formation of H2C(I)(ZnI) and carbene transfer from the zinc to the double bond of the alkene. This reaction is named after Howard Ensign Simmons and Ronald Smith, who discovered it in 1958–59. There has been a considerable amount of work on extensions of the Simmons–Smith reaction.