Frontier orbital effects

The second key to success in making sure that the alkyl radical behaves well is to use a reactive radical trap. In fact, this is a major limitation of intermolecular radical carbon–carbon bond-forming reactions: for the trapping of alkyl radicals only electrophilic alkenes (attached to electron-withdrawing groups such as –CN, –CO2Me, and –COMe) will do. This is a limitation, but nonetheless cyclohexyl iodide adds to all these alkenes with the yields shown and the rate of addition to most of these alkenes is 103 to 104 times that of addition to 1-hexene.

To explain why, we have to go back to our analysis of the electronic structure of radicals and the energy of SOMOs. We said there that, while both electron-withdrawing groups and electron-donating groups will stabilize radicals, electron-withdrawing groups tend to lower the energy of the SOMO, while electron-donating groups tend to raise the energy of the SOMO.

●Electrophilic and nucleophilic radicals

• Low-energy SOMOs are more willing to accept an electron than to give one up; radicals adjacent to electron-withdrawing groups are therefore electrophilic.

• High-energy SOMOs are more willing to give up an electron than to accept an electron; radicals adjacent to electron-donating groups are therefore nucleophilic.

Hence the preferred reactivity of these alkyl radicals: they are relatively nucleophilic and therefore prefer to react with electrophilic alkenes. Reaction between a nucleophilic alkyl radical and an unfunctionalized (and therefore nucleophilic) alkene is much slower. Similarly, radicals adjacent to electron-withdrawing groups do not react well with electrophilic alkenes. We can represent all this on an energy level diagram.

We can now consider the third type of radical in the reaction mixture highlighted—the nitrile-stabilized alkyl radicals. The diagram above explains the third aspect of radical chemoselectivity in the reaction: why both the product radical and the radicals produced by AIBN choose to react with Bu3SnH and not with acrylonitrile. These radicals are electrophilic—they have an electron-withdrawing nitrile group attached to the radical centre, so reaction with an electron-poor alkene is slow.

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مواضيع ذات صلة

Attack of carbenes on lone pairs

Nitrenes are the nitrogen analogues of carbenes

Rearrangement reactions

Insertion into C–H bondsInsertion into C–H bonds

Making cyclopropanes

Carbenes react with alkenes to give cyclopropanes

The structure of carbenes depends on how they are made

Carbenes can be divided into two types

اكتشاف العناصر

العناصر

Diazomethane makes methyl esters from carboxylic acids

Intramolecular radical reactions are more efficient than intermolecular ones