Simmons–Smith cyclopropanation: practice and mechanism
CH₂I₂ and Zn(Cu) form a carbenoid that transfers a CH₂ group onto the double bond. A cyclopropane forms and the alkene geometry is retained (stereospecific).
Example
The zinc carbenoid ICH₂ZnI transfers a CH₂ group to the double bond in one step. A cyclopropane forms and the arrangement of the alkene substituents is kept (stereospecific).
Mechanism step by step
- 1
Zinc (activated with copper) inserts into the C–I bond of CH₂I₂: ICH₂ZnI forms, a carbenoid.
- 2
The carbenoid transfers the CH₂ group to both alkene carbons in one step ("butterfly" transition state).
- 3
ZnI₂ leaves and a cyclopropane forms. The substituents keep their positions: a cis alkene gives a cis cyclopropane.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
Often confused with: Addition of dichlorocarbene · Epoxidation
Frequently asked questions
Why not free carbene from diazomethane?
Diazomethane is toxic and explosive, and free CH₂ carbene is so reactive that it also inserts into C–H bonds. The carbenoid is gentler and selective.
Is the reaction stereospecific?
Yes. Because both new bonds form at once, the cis/trans relationship of the alkene is preserved in the cyclopropane.