A Tutorial on the Simmons-Smith Reaction

In the Simmons-Smith reaction, an alkene reacts with an organozinc carbenoid to form a cyclopropane.¹ To form the organozinc carbenoid, a zinc-copper couple (Zn/Cu) reacts with a dihalomethane, often diiodomethane (CH₂I₂). This reaction produces iodomethylzinc iodide, an organozinc carbenoid with the structure I-Zn-CH₂-I. Conventionally, excess of the dihalomethane is added to a solution containing the alkene and excess Zn via syringe to form the carbenoid. Then, a concerted reaction occurs between the organozinc carbenoid and the alkene; the electrons from the double bond form a bond with the carbon atom in the organozinc compound, the electrons in the carbon-zinc bond form a bond with the alkene, and electrons from iodine will form a bond with zinc. This reaction is stereospecific, preserving the positions of the groups on the alkene with respect to each other.

Formation of the Organozinc Carbenoid

Zn + CH₂I₂ → I-Zn-CH₂-I

The zinc is coupled with another metal; in the original Simmons-Smith reaction, a Zn/Cu couple was used. In the total synthesis of maoecrystal V, a Zn/Ag couple was used in diethyl ether for a higher yield and faster reaction rates.²

Reaction of the Carbenoid with the Alkene

 

 

 

 

In this example, cyclohexene reacts with the organozinc carbenoid to form the cyclopropane norcarane.

[1] Simmons, H. E.; Smith, R. D.  J. Am. Chem. Soc. 1959, 81, 4256.
[2] Rousseau, G.; Conia, J. M. Tetrahedron Lett. 1981, 22, 649−652.