Addition of Carbenes to Alkenes: Cyclopropane Synthesis

Yet another kind of alkene addition is the reaction with a carbene to yield a cyclopropane. A carbene, R₂C:, is a neutral molecule containing a divalent carbon with only six electrons in its valence shell. It is therefore highly reactive and generated only as a reaction intermediate, rather than as an isolable molecule.

Because they’re electron-deficient, carbenes behave as electrophiles and react with nucleophilic C=C bonds. The reaction occurs in a single step without intermediates.

One of the simplest methods for generating a substituted carbene is by treatment of chloroform, CHCl₃, with a strong base such as KOH. As shown in Figure 1.1, the loss of a proton from CHCl₃ gives trichloromethanide anion, ^-:CCl₃, which spontaneously expels a Cl^- ion to yield dichlorocarbene, :CCl₂.

Figure 1.1 :Mechanism of the formation of dichlorocarbene by reaction of chloroform with strong base. Deprotonation of CHCl₃ gives the trichloromethanide anion, ^-- :CCl₃, which spontaneously expels a Cl^- ion.

 The carbon atom in dichlorocarbene is sp2-hybridized, with a vacant p orbital extending above and below the plane of the three atoms and with an unshared pair of electrons occupying the third sp² lobe. Note that this electronic description of dichlorocarbene is similar to that of a carbocation with respect to both the sp2 hybridization of carbon and the vacant p orbital. Electrostatic potential maps further illustrate this similarity (Figure 1.2).

Figure 1.2 The structure of dichlorocarbene. Electrostatic potential maps show how the positive region coincides with the empty p orbital in both dichlorocarbene and a carbocation (CH₃⁺). The negative region in the dichlorocarbene map coincides with the lone-pair electrons.

If dichlorocarbene is generated in the presence of an alkene, addition to the double bond occurs and a dichlorocyclopropane is formed. As the reaction of dichlorocarbene with cis-2-pentene demonstrates, the addition is stereospecific, meaning that only a single stereoisomer is formed as product. Starting from a cis alkene, for instance, only cis-disubstituted cyclopropane is produced; starting from a trans alkene, only trans-disubstituted cyclopropane is produced.

If dichlorocarbene is generated in the presence of an alkene, addition to the double bond occurs and a dichlorocyclopropane is formed. As the reaction of dichlorocarbene with cis-2-pentene demonstrates, the addition is stereospecific, meaning that only a single stereoisomer is formed as product. Starting from a cis alkene, for instance, only cis-disubstituted cyclopropane is produced; starting from a trans alkene, only trans-disubstituted cyclopropane is produced.

The best method for preparing nonhalogenated cyclopropanes is by a process called the Simmons–Smith reaction. First investigated at the DuPont company, this reaction does not involve a free carbene. Rather, it utilizes a carbenoid—a metal-complexed reagent with carbene-like reactivity.

When diiodomethane is treated with a specially prepared zinc–copper mix, (iodomethyl)zinc iodide, ICH₂ZnI, is formed. In the presence of an alkene, ICH₂ZnI transfers a CH₂ group to the double bond to yield cyclopropane.

For example, cyclohexene reacts cleanly and with good yield to give the corresponding cyclopropane.

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