Chemistry of Alkenyl Halides

We have encountered haloalkenes—alkenyl halides—as intermediates in both the preparation of alkynes by dehydrohalogenation and also the addition to alkynes of hydrogen halides. Alkenyl halides have become increasingly important as synthetic intermediates in recent years as a result of developments in organometalUc chemistry. These systems do not, however, follow the mechanisms familiar to us from our survey of the haloalkanes (Chapters 6 and 7). This section discusses their reactivity. 

Unlike haloalkanes, alkenyl halides are relatively unreactive toward nucleophiles. Although we have seen that, with strong bases, alkenyl halides undergo elimination reactions to give alkynes, they do not react with weak bases and relatively nonbasic nucleophiles, such as iodide. Similarly, SnI reactions do not normally take place, because the intermediate alkenyl cations are species of high energy. 

Alkenyl halides, however, can react through the intermediate formation of alkenyl organometallics (see Exercise 11-6). These species allow access to a variety of spedficaUy substituted alkenes. 

Grignard additim to ketone gives tertiary alcoh d 

Metal catalysts couple alkenyl halides to alkenes in the Heck reaction 

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