Cheletropic Reaction - Carbene Additions To Alkenes

Carbene Additions To Alkenes

One of the most synthetically important cheletropic reactions is the addition of a singlet carbene to an alkene to make a cyclopropane (see figure at left). A carbene is a neutral molecule containing a divalent carbon with six electrons in its valence shell. Due to this, carbenes are highly reactive electrophiles and generated as reaction intermediates. A singlet carbene contains an empty p orbital and a roughly sp2 hybrid orbital that has two electrons. Singlet carbenes add stereospecifically to alkenes, and alkene stereochemistry is retained in the cyclopropane product. The mechanism for addition of a carbene to an alkene is a concerted cycloaddition (see figure). Carbenes derived from chloroform or bromoform can be used to add CX₂ to an alkene to give a dihalocyclopropane, while the Simmons-Smith reagent adds CH₂.

Interaction of the filled carbene orbital with the alkene π system creates a four-electron system and favors a non-linear approach. It is also favorable to mix the carbene empty p orbital with the filled alkene π orbital. Favorable mixing occurs through a non-linear approach (see figure at right). However, while theory clearly favors a non-linear approach, there are no obvious experimental implications for a linear vs. non-linear approach.