4.6.7 Addition of Carbenes
The simplest carbene is methylene (:CH2), which is uncharged and very reactive intermediate. The reactions of carbenes are especially interesting because, in many instances, the reactions show a remarkable degree of stereospecificity. Methylene is a very poisonous yellow gas that can be prepared by the decomposition of diazomethane (CH2N2).
The structure of diazomethane is a resonance hybrid of three structures:
Methylene reacts with alkenes by adding to the double bond to form cyclopropanes.
The Simmons-Smith reaction is one of the best ways of making cyclopropanes. The Simmons-Smith reagent is made by adding methylene iodide to the "zinc-copper couple" (zinc dust that has been activated with an impurity of copper). The reagent probably resembles iodomethyl zinc iodide, ICH2Znl. This kind of reagent is called a carbenoid because it reacts much like a carbene (Scheme 23)
Scheme 23
Dibromocarbene formed from CHBr3 can add to a double bond to form a dibromocyclopropane (Scheme 24).
Scheme 24
4.6.8 Epoxidation
An epoxide is a three-membered cyclic ether, also called an oxirane. An alkene is converted to an epoxide by a peroxyacid. The epoxidation takes place in one step as shown below (Scheme 25).
Scheme 25
Alkene molecule cannot rotate and change its cis or trans geometry during the reaction. So the epoxide retains whatever stereochemistry is present in the alkene. For example, epoxidation of cis -2-butene and trans -2-butene retains its stereochemistry when reacts with meta -chloroperbenzoic acid (Scheme 26).
Scheme 26