Carbenoid cyclopropanation
Simmons-Smith cyclopropanation
How the Simmons-Smith cyclopropanation works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: CH₂I₂, Zn(Cu) couple (or Et₂Zn, Furukawa). Conditions: Et₂O, reflux or 0–25 °C. Bonds formed: two C–C σ bonds (cyclopropane); bonds broken: alkene π; Zn–C / C–I (carbenoid). Stereochemistry: Stereospecific syn addition; alkene geometry retained. Rate and selectivity: Accelerated by neighbouring OH (directed delivery).
At a glance
Step 1: Zinc inserts into C–I: carbenoid
Zinc inserts into CH₂I₂ to form an organozinc carbenoid (ICH₂ZnI) — it behaves like a free carbene without ever forming one, avoiding side reactions.
Step 2: Butterfly transition state
The alkene's π electrons and the carbenoid's zinc-carbon bond react in a concerted, butterfly-shaped transition state — both new C–C bonds form simultaneously.
Step 3: Cyclopropane, geometry retained
The result is a cyclopropane ring, added with complete retention of the alkene's original stereochemistry — a signature of concerted carbenoid addition.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- Et2O
- Diethyl ether. Low-boiling ether solvent, classic for Grignard reagents.
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
How it compares with related reactions
| Simmons-Smith cyclopropanation | Acetic acid transfers a proton to water | Acid-catalyzed amide hydrolysis | |
|---|---|---|---|
| Reagents | CH₂I₂, Zn(Cu) couple (or Et₂Zn, Furukawa) | Acetic acid (pKa 4.76) + water | Amide, H₃O⁺ (HCl or H₂SO₄), H₂O |
| Conditions | Et₂O, reflux or 0–25 °C | Aqueous solution, 25 °C | 6 M acid, reflux (hours) |
| Stereochemistry | Stereospecific syn addition; alkene geometry retained | None (achiral species) | No stereocentre created; α-stereocentres retained |
| Rate / selectivity | Accelerated by neighbouring OH (directed delivery) | Equilibrium: Ka = 1.8 × 10⁻⁵, lies mostly left | Water attack / C–N cleavage rate-limiting; far slower than ester hydrolysis |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Simmons-Smith cyclopropanation?
CH₂I₂, Zn(Cu) couple (or Et₂Zn, Furukawa). Conditions: Et₂O, reflux or 0–25 °C.
Which bonds are formed and which are broken?
Formed: two C–C σ bonds (cyclopropane). Broken: alkene π; Zn–C / C–I (carbenoid).
What is the stereochemical outcome?
Stereospecific syn addition; alkene geometry retained
What controls the rate or the selectivity?
Accelerated by neighbouring OH (directed delivery)
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go