Cyclopropanation of Alkenes
Cyclopropanation refers to any chemical process which generates cyclopropane rings. The triangular structure of cyclopropane ring leads to a high ring strain between the carbon. Therefore, cyclopropane itself is challenging to produce and generally requires the use of highly reactive species such as carbene intermediate. A carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons.
The Simmons-Smith Reaction
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Cyclopropane structure
Of all the reactions used to produce cyclopropane rings, the Simmons-Smith reaction is preferred, because of its ability to be used in the presence of many functional groups. The basis of the
Simmons-Smith Reaction is the formation of two C-C bonds simultaneously. The catalyst used for this reaction is iodomethyl zinc iodide.
Formation of iodomethyl zinc iodide
. diiodomethane iodomethyl zinc iodide
Iodomethyl zinc iodide is a organozinc carbenoid, which means it shares reaction characteristics with a carbene. The electronic structure of the compound allows the formation of two C-C bonds at the same time.
+
ZnI
2
The nucleophilic double C=C bond causes the loss of the iodide and the electrons from the nucleophilic C-Zn bond being used to form the other C-C bond. Overall, this results in a cyclopropane ring where the double bond on the alkene previously was.

Cyclopropanation of Alkenes
Use of Cyclopropanes
Cyclopropne itself is a colourless gas that is highly flammable. Cyclopropane has been used as a general anesthetic in medicine since 1934, however, it has since then been replaced due to it’s stability and reactivity. Cyclopropane rings are also present in many medications such as ciprofloxacin.
Isomerism
The use of cyclopropane rings in drugs means it’s important to consider it’s enantiomers. Although cyclopropane itself is achiral by itself, a substituted cyclopropane may display chirality. Therefore, enantiomer analysis is important when producing molecules with cyclopropane rings.
A cyclopropanation reaction is stereospecific with respect to the alkene, meaning the stereochemistry is retained. For example, if the alkyl groups of the alkene are cis - then they are also cis - in the cyclopropane and trans -alkenes give trans -cyclopropanes
Problem Set
1.
What would be the result of a Simmons-Smith Reaction that used trans-3-pentene as a reagent.
2.
What starting material could be used to form trans-1,2-diethylcyclopropane
3.
Draw the products of this reaction. What type of reaction is this?

Cyclopropanation of Alkenes
Answers
1.
Trans-1-ethyl-2-methylcyclopropane
With a Simmons-Smith Reaction we know that the cyclopropane ring is formed where the double bond is on the reactant. We can see on trans-3-pentene that the double bond is on
C3 and C4 or C2 and C3 from the right, so a cyclopropane ring would be formed on C3 and
C4. The branches would be trans to each other since the reaction is stereospecific.
2.
Trans-3-hexene
Once the molecule is drawn out we can see that the cyclopropane ring is located on C3, therefore the double bond must also be on C3. The red lines on the diagram show that the longest chain excluding the cyclopropane ring is 6 carbons long, therefore the starting molecule must be 6 carbons. Since stereochemistry is retained the molecule must be trans-3-hexene
3.
Trans-1-ethyl-2-methylcyclopropane
This is a Simmons-Smith reaction because of the use of
Stereochemistry not shown iodomethyl zinc iodide. The double bond is located on C2 on the reactant, so the cyclopropane ring would be located on C2 on the product. The branches on the cyclopropane would be trans to each other.