Formation of cyclopropanes from alkenes using methylene carbene (:CH2)

by James

Description: Alkenes treated with methylene carbene (:CH2) will form cyclopropanes.

Notes: In practice there are several ways to write this. Two common examples are with diazomethane (CH2N2) in the presence of heat (Δ) or light (hν), with diiodomethane (CH2I2) in the presence of zinc-copper couple (Zn-Cu). For our purposes the end result is exactly the same. The example with CH2I2 and Zn-Cu is called the Simmons-Smith reaction.

Examples:


Notes: Note that the syn product is always formed. See how the hydrogens on the double bond have become dashes?

Mechanism: Treatment of diazomethane with light (or heat) results in fragmentation of the C–N2 bond (Step 1, arrow A), liberating nitrogen gas. The resulting carbene then does the cyclopropanation of the alkene (Step 2, arrows B and C). Note that the reaction proceeds through a concerted transition state, so B and C happen simultaneously.

For the reaction with Zn–Cu the mechanism is essentially the same with a slight twist. First, Zn inserts itself into the C–I bond in a mechanism similar to that for Grignard formation but is not depicted here (Step 1). Then, breakage of the C–Zn bond (Step 2, arrow A) leads to a species that performs the cyclopropanation (Step 3, arrows B and C) to give the cyclopropane. It also proceeds through a concerted transition state.

3-B-mech

Notes: This example with Zn–Cu is called the “Simmons Simith cyclopropanation”

{ 6 comments… read them below or add one }

vijaykumar

wow it was a very nice information!!!!
i just gone through all the reactions of alkenes it is very helpfull for me!!!
thank you for uploading them in website!!!!

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james

Glad you find it useful.

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Jeffery

Why do carbenes not react with alkynes? Has it got to do with the angle strain in the product which makes it unstable?

Also, how do ZnCu react with CH2I2? Is ZnCu a catalyst? Where can I find the mechanism?

Reply

James Ashenhurst

Carbenes can react with alkynes to give cyclopropenes, but it’s a slower reaction due to the fact that the products (cyclopropenes) have more ring strain. Generally cyclopropanation of alkynes is not covered in these introductory courses, so it isn’t mentioned here.

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Andrew Geller

why do you have bromine addition images here?

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Isaac Traynis

I think he linked it to the wrong photo on the site…just ignore it until he fixes it.

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