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

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.


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.


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