Simmons Reaction

Alkene Reactions: Simmons Reaction - Cyclopropane Formation

Simmons–Smith reaction – cyclopropanation of alkenes

Diiodomethane and an activated zinc–copper couple assemble the zinc carbenoid iodomethylzinc iodide (ICH₂ZnI). That closed-shell reagent adds syn across an alkene, preserving the relative stereochemistry (E → trans, Z → cis). The result is a cyclopropane that inherits the alkene’s substitution pattern without rearrangement or radical side chemistry.

Quick Summary

Reagents/conditions: CH₂I₂ with activated Zn(Cu) (or Et₂Zn/CH₂I₂, Furukawa modification) in ethereal solvents at 0–25 °C.
Outcome: Concerted syn transfer of CH₂; alkene configuration is retained (E → trans cyclopropane, Z → cis).
Mechanistic spine: Zinc inserts into CH₂I₂ → zinc carbenoid aligns with C=C → syn delivery of CH₂ as ZnI₂ departs.
Selectivity: Allylic alcohols coordinate the zinc, steering addition to the OH face.
Safety: CH₂I₂ is toxic and dense; handle behind a shield with efficient ventilation.

Mechanism (4 Steps)

Class: Closed-shell carbenoid addition (concerted syn cycloaddition).

Step 1: zinc inserts into diiodomethane to make the zinc carbenoid — Step 1 — Zn(Cu) inserts into CH₂I₂, polarising both C–I bonds to build the ICH₂ZnI carbenoid.

The zinc–copper couple converts CH₂I₂ to the zinc carbenoid iodomethylzinc iodide (ICH₂ZnI); no free-radical methylene is involved.

Step 2: the carbenoid coordinates the alkene in a syn approach — Step 2 — The carbenoid approaches the alkene from one face, maintaining the intact C=C before bond formation.

Zinc remains bound to iodide while the methylene carbon positions above the alkene π cloud. Both alkene carbons see the same face of the reagent, pre-setting the syn relationship.

Step 3: concerted cyclopropanation — Step 3 — The C=C engages the carbenoid carbon in a single concerted step, forming two C–C bonds on the same face while zinc is oxidized to ZnI₂.

The π bond supplies both electrons needed to forge the new C–C bonds as ZnI₂ departs, completing the syn cyclopropanation.

Step 4: cyclopropane product and ZnI2 are released — Step 4 — Syn delivery completes the cyclopropane and ZnI₂ departs from the face opposite the new ring.

The alkene carbons now share a single bond to the methylene carbon, forming a cyclopropane. Zinc leaves as ZnI₂, closing the concerted syn addition.

Mechanistic Checklist

Zn(Cu) inserts into CH₂I₂ to make the ICH₂ZnI carbenoid — no free radicals are produced.
The carbenoid approaches the alkene from a single face; both new C–C bonds form syn.
Electron flow is polar; ZnI₂ is expelled during bond formation.
Alkene configuration is preserved (E → trans, Z → cis cyclopropane).
Allylic alcohols coordinate to zinc, steering addition to the OH-bearing face.

Worked Examples

Reagents: CH2I2, Zn(Cu) couple — CH₂I₂, Zn(Cu)

Product: ethylcyclopropane — Product — Ethylcyclopropane (syn addition, no new stereocenters)

Substrate: substituted alkene — Substrate

Product: trans-disubstituted cyclopropane — Product — Trans-disubstituted cyclopropane (alkene E-geometry retained)

Multiple Alkenes & Selectivity

Simmons–Smith is highly chemoselective for C=C bonds; isolated alkenes cyclopropanate sequentially if present.
Allylic alcohols coordinate zinc and direct delivery to the OH face; the Et₂Zn/CH₂I₂ (Furukawa) variant enhances this effect.
Functional-group tolerance is generally good, but strongly coordinating heteroatoms can sequester zinc and slow the reaction.

Practical Tips & Pitfalls

Activate zinc dust with dilute acid rinse, then copper(II) sulfate to create the Zn(Cu) couple immediately before use.
Keep CH₂I₂ cold and capped — it is dense, volatile, and toxic (moderate skin absorption risk).
Et₂Zn/CH₂I₂ (Furukawa reagent) is hotter and more reactive; add slowly and watch for exotherms.
Moisture quenches reactivity; run under inert atmosphere with dry etheral solvents.
Workups typically include saturated ammonium chloride to dissolve Zn salts and trap residual carbenoid.

Exam-Style Summary

Reagents: CH₂I₂ + Zn(Cu) (or Et₂Zn/CH₂I₂).
Mechanism: carbenoid formation → syn cyclopropanation → ZnI₂ departure.
Stereochemistry: syn addition; alkene configuration retained (E → trans, Z → cis).
Products: Cyclopropanes bearing the original substituent pattern; no rearrangements.
Control: Allylic alcohols direct delivery to the OH face.

Interactive Toolbox

Explore variations and stereochemical outcomes in the Reaction Solver.
Generate additional practice problems with the Mechanism Explorer.

FAQ / Exam Notes

Why is the addition stereospecific? The ICH₂ZnI carbenoid engages the alkene in a concerted, closed-shell transition state; both C–C bonds form on the same face, locking in syn delivery.

How does the Furukawa modification differ? Et₂Zn/CH₂I₂ forms a more reactive carbenoid that is especially effective for allylic alcohols, but the outcome (syn cyclopropanation, ZnI₂ by-product) is the same.

Can the reaction create new stereocenters? Yes — newly formed cyclopropane stereocenters are set syn relative to one another, matching the starting alkene geometry and any directing groups (e.g., allylic OH).

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