Introduction

Carbenes were once only thought of as short lived intermediates. The reactions of this section only deal with these short lived carbenes which are mostly prepared in situ, at the time of the main reaction. However, there do exist so called persistent carbenes, which are stabilized by a variety of methods often including aromatic rings or transition metals. In general a carbene is neutral and has six valence electrons, two of which are non bonding. These electrons can either occupy the same sp² hybridized orbital to form a singlet carbene (with paired electrons), or two different sp² orbitals to form a triplet carbene (with unpaired electrons), but we will focus exclusively on the more common singlet carbenes.

The reactivity of a singlet carbene is concerted and similar to that of electrophilic or nucleophilic addition. The highly reactive nature of carbenes leads to very fast reactions in which the rate determining step is generally carbene formation.

Preparation of methylene (:CH₂)

The preparation of methylene starts with the yellow gas diazomethane, CH₂N₂. Diazomethane can be exposed to light, heat or copper to facilitate the loss of nitrogen gas and the formation of the simplest carbene methylene. The process is driven by the formation of the nitrogen gas which is a very stable molecule.

Carbene reaction with alkenes

A carbene such as methylene will react with an alkene which will break the double bond and result with a cyclopropane. The reaction will usually leave stereochemistry of the double bond unchanged. As stated before, carbenes are generally formed during the reaction; hence the starting material is diazomethane not methylene.

In the above case cis-2-butene is converted to cis-1,2-dimethylcyclopropane. Likewise, below the trans configuration is maintained.

Other carbenes

In addition to the general carbene with formula R₂C: there exist a number of other compounds that behave in much the same way as carbenes in the synthesis of cyclopropane derivatives. Halogenated carbenes such as dichlorocarbene, Cl₂C:, are more stable than simple alkyl carbenes. Dichlorocarbene can be conveniently prepared from chloroform (CHCl₃) with base in the presence of a phase transfer catalyst:

These halogenated carbenes form cyclopropanes in the same manner as methylene: