Reactivity in Chemistry

Electrophilic Addition to Alkenes

 

EA9. Cyclopropanation

Earlier, we saw that alkenes can donate their pi electrons to oxygen electrophiles in peroxides. The result is transfer of an oxygen atom from the peroxide to the alkene.  An epoxide or oxirane ring is formed.

Another, related example is cyclopropanation of an alkene.  In alkene cyclopropanation, an alkene is converted into a cyclopropane.  A carbon atom is donated to the alkene.

A classic example is the addition of dichlorocarbene to an alkene.

 

Carbenes are electrophiles because the carbon does not have an octet.  The carbon has only two bonds and one lone pair.  That's just three electrons, not eight.

On the other hand, there is a lone pair.  The carbene can be nucleophilic, too.

The reaction involves addition of the alkene to an electrophilic carbene.  At the same time, that lone pair can donate back, so that a carbocation does not actually form.

The carbene is sometimes formed through an unusual "alpha"-elimination in the presence of strong base.  Strong bases are often alkyllithium reagents, such as CH3Li, but KOH will work with some compounds.

The reaction is unusual because a proton is abstracted from one carbon atom, and a leaving group departs from the same atom.  It is much more common to see the leaving group depart from the next atom over, in a beta-elimination. 

Carbenes are frequently formed from diazo compounds.  These include diazomethane, CH2N2.

In diazo compounds, an N2 group is hanging on by a thread.  It can easily leave, resulting in a carbene.  This reaction is often promoted by metal catalysts, such as copper (II) salts.

The carbene, CH2, is even less stable than CCl2.  However, in the presence of metal salts, it can be stabilised as a metal carbene complex.

Problem EA9.1.

Why is a carbene more stable with chlorine atoms attached?

Problem EA9.2.

Some metal carbene complexes, such as the one shown below, are particularly stable.  Explain why.

Problem EA9.3.

Free carbenes, those not attached to a metal ion, are typically so unstable that they can only be generated briefly in solution before they react with a nucleophile, such as an alkene.  Arduengo carbenes, such as the one below, are stable enough to be put in a bottle and stored in the refrigerator.  Explain why.

Problem EA9.4.

Predict the products of the following reactions.

 

 

 

 

 

This site is written and maintained by Chris P. Schaller, Ph.D., College of Saint Benedict / Saint John's University (with contributions from other authors as noted).  It is freely available for educational use.

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Structure & Reactivity in Organic, Biological and Inorganic Chemistry by Chris Schaller is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License

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This material is based upon work supported by the National Science Foundation under Grant No. 1043566.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

 

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