Elimination Generation and Reactions of Carbenes

The two unshared electrons in singlet carbenes have opposite spins. Carbenes generated by a-elimination are singlets because the two unshared electrons are derived from the same MO. Triplet carbenes, in which the two unshared electrons have parallel spins, are usually generated by photolysis of diazo compounds. Triplet carbenes are discussed in Chapter 5. 

The most widely used substrates for a-elimination reactions are chloroform and bromoform, but other halides also can undergo a-elimination. For example, allyl chloride undergoes a-elimination when it is treated with a very strong base. 

No matter how they are generated, carbenes and carbenoids undergo four typical reactions. The most widely used reaction is cyclopropanation, or addition to 

Problem 2.13. Draw mechanisms for the following two cyclopropanation reactions. 

The second typical reaction of carbenes is insertion into a C H a bond. This three-centered reaction is similar to cyclopropanation, except that the carbene latches onto a a bond instead of a 77 bond. Again, both carbenes and carbenoids can undergo this reaction. It is most useful when it occurs in intramolecular fashion. If the C of the C-H bond is stereogenic, the reaction proceeds with retention of configuration. 

Carbenes and carbenoids can also be generated from diazo compounds (R2C=N2 — R2C=N=N R2C—N=N) by several nonbasic methods. 

No matter how they are generated, carbenes and carbenoids undergo four typical reactions. The most widely used reaction is cyclopropanation, or addition to a TT bond. The mechanism is a concerted [2 + 1] cycloaddition (see Chapter 4). The carbenes derived from chloroform and bromoform can be used to add CX2 to a 7T bond to give a dihalocyclopropane, while the Simmons-Smith reagent adds CH2. Carbenoids generated from diazoalkanes with catalytic Rh(II) or Cu(II) also undergo cyclopropanations. 

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