Alkenes, addition reactions Halogenation Hydroboration

Both ( )- and (Z)-l-halo-l-alkenes can be prepared by hydroboration of 1-alkynes or 1-halo-l-alkynes followed by halogenation of the intermediate boronic esters (244,245). Differences in the addition—elimination mechanisms operating in these reactions lead to the opposite configurations of iodides as compared to bromides and chlorides. 

The first pathway represents hydroboration-oxidation of the alkene to achieve irwri-Markovnikov addition of water. The second pathway represents a radical ((r ft-Markovnikov) addition of HBr followed by an Si,j2 reaction to replace the halogen with a hydroxyl group. Each of these pathways represents a valid synthesis. As we learn more reactions, it will become more common to encounter synthesis problems with several perfectly correct answers. The goal should always be efficiency. A 3-step synthesis will generally be more efficient than a 10-step synthesis. 

We begin with a discussion of hydrogenation, focusing on the details of catalytic activation. Then we turn to the largest class of addition processes, those in which electrophiles such as protons, halogens, and metal ions are added to the alkene. Other additions that will contribute further to our synthetic repertoire include hydroboration, several oxidations (which can lead to complete mpture of the double bond if desired), and radical reactions. Each of these transformations takes us in a different direction the Reaction Summary Road Map at the end of the chapter provides an overview of the interconversions leading to and from this versatile compound class. 

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