Examples of Alkene Hydrocyanation

Reactions of internal olefins can even generate terminal alkylnitriles by a pathway that involves isomerization of intermediate cyanometal-alkyl complexes. Tlus isomerization is similar to the isomerization that occurs during the hydroformylation of internal olefins discussed in Chapter 17. In fact, the nickel catalyst rapidly isomerizes hexene to the equilibrium ratio of olefins faster than it adds HCN to the C=C bond. Thus, internal hexenes generate the terminal alkane nitrile. 

The toxicity of HCN has limited the use of hydrocyanation in routine synthesis and the development of this reaction as a synthetic method. However, acetone cyanohydrin can be used in some cases as a convenient surrogate for the more noxious HCN. One example of the use of acetone cyanohydrin is shown in Equation 16.4.  

The use of a co-catalyst was crucial to the development of practical hydrocyana-tion. The rate and catalyst lifetime for hydrocyanation of simple alkenes increases dramatically by conducting the reactions in the presence of a Lewis acid. As shown in Table 16.1, the reaction of propene occurs much faster in the presence of aluminum and zinc halides. Lewis acid cocatalysts also promote isomerization and selective additions during some steps of the hydrocyanation of butadiene. This effect is presented later in this section. 

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