The Simmons-Smith reagent is a chemical reagent used in organic chemistry to convert alkenes into cyclopropanes. It is typically prepared by reacting iodomethane with zinc in the presence of a copper(I) halide catalyst. This reagent is valued for its ability to add a three-membered ring to carbon compounds, which is useful in synthesizing complex molecules. The reaction mechanism involves the formation of a carbenoid species, which then inserts into the double bond of the alkene. The Simmons-Smith reaction is notable for its mild conditions and high regioselectivity, making it a popular choice for chemists working on cyclopropanation.