Simmons-Smith Reaction Mechanism

What is Simmons–Smith Reaction?

Simmon Smith reaction is most powerful organic cheletropic (reaction) method for cyclopropane preparation from diiodomethane (CH2I2) in the presence of zinc-copper couple (Zn-Cu) from unfunctionalized alkenes (e.g., cyclohexene, styrene) stereospecifically.

This method is highly stereocontrolled, and it involves a C=C bond-breaking reaction and the addition of the CH2 group. For example, if the alkyl groups of the alkene are trans-alkenes, give trans-cyclopropanes, and cis- alkene gives cis- in the cyclopropane.

simmons-smith reaction
Simmons-Smith cyclopropanation, Simmons-Smith reaction

Simmon Smith Reaction Mechanism –

The number of reactions, theoretical studies, and the stereochemical outcome show that the Simmon Smith reaction mechanism is a concerted process. It proceeds via a three-centered “butterfly-type” type transition state.

simmons smith reaction intermediate is methylene free radical intermediate. simmons smith reaction  is stereospecific to the alkene.

Simmon Smith Reaction Mechanism
simmons-smith reaction mechanism, Simmons-Smith cyclopropanation mechanism , mechanism ofSimmons-Smith reaction

Additional Information –

  • Simple alkenes, α,β-unsaturates ketones and aldehydes, electron-rich alkenes (enol ethers, enamines).
  • More electron-rich alkenes will react faster bcz the electrophilic nature of the reagent.
  • Highly substituted alkenes can react slowly bcz of steric hindrance.
  • The cyclopropanation is diastereoselective, and it occurs less hindered face of the double bond.
  • When alkene substrate has OH, OAc, OMe, OBn, and NHR functional groups, they act as directing groups for delivering stereochemical outcomes.

Related Reactions:


  1. E. Lévesque, S. R. Goudreau, A. B. Charette, Org. Lett.201416, 1490-1493.
  2. A. Voituriez, L. E. Zimmer, A. B. Charette, J. Org. Chem.201075, 1244-1250.

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