Stereospecificity of Carbene Addition Reaction

Carbenes are highly reactive intermediates that cannot be isolated, so they are generated in the presence of a selected reactant. When dichlorocarbene forms in the presence of an alkene, the electrophilic carbene reacts with the double bond of the alkene to form a cyclopropane. 

We won t discuss the mechanistic details of the addition of a carbene to an alkene since it would require a detailed consideration of molecular theory. However, we note that a pair of electrons of the 71 bond and the unshared pair of electrons of the carbene provide the four electrons required to form two C-C bonds of the cyclopropane ring. The reaction shown for cyclohexene and dichlorocarbene is stereospecific. Even with acyclic aUtenes, cis alkenes yield cw-substituted cyclopropanes and trans alkenes yield tnzwr-substituted cyclopropanes. 

Methylene ( CH2), the simplest carbene, can be prepared by decomposition of the highly toxic and explosive reagent diazomethane (CH Nj). However, more easily used reagents have been developed that, while they do not produce methylene directly, function as methylene transfer agents. They are called carbenoid species because they react like carbenes. lodomethylzinc iodide, known as the Simmons-Smith reagent, is a carbenoid. In the Simmons-Smith method, diiodomethane reacts with a zinc-copper alloy to produce an intermediate I—CH Zn—I compound. 

The Simmons-Smith reagent transfers a CH unit to the alkene in a stereospecific reaction. For example, trans-2-h xtene reacts with the Simmons-Smith reagent to give only tra ws-1,2-dimethy-l-cyclopropane. 

Write the expected product of the reaction of rri r-2-butene with CHBr and potassium r t-butoxide. Write the mechanism for the formation of the expected intermediate. 

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