Propination

Propination is a chemical reaction where an alkene is converted into a cyclopropane ring. This is typically achieved through the addition of a carbene or carbenoid species across the double bond of the alkene. Carbenes are neutral molecules containing a divalent carbon atom with only six valence electrons, making them highly reactive electrophiles. Carbenoids are species that behave similarly to carbenes but are often more stable or easier to generate, frequently involving a metal atom bonded to a carbon.

The most common method for propination involves the Simmons-Smith reaction, which uses diiodomethane and a zinc-copper couple. This reaction generates an organozinc carbenoid intermediate that efficiently transfers a methylene (CH2) group to the alkene. Other methods include the use of diazomethane with a catalyst, or transition metal-catalyzed reactions employing diazo compounds. The stereochemistry of the alkene is generally preserved during the reaction, meaning a cis-alkene will yield a cis-cyclopropane and a trans-alkene will yield a trans-cyclopropane. Propination is a valuable synthetic tool in organic chemistry for the construction of cyclopropane rings, which are found in various natural products and pharmaceuticals. The high ring strain of cyclopropanes also imparts unique reactivity, making them useful building blocks for further chemical transformations.