Adding H and OH, Anti-Markovnikov

In the previous section, we saw how to perform a Markovnikov addition of H and OH across a double bond. In this section, we will learn how to perform an anti-Markovnikov addition of H and OH, for example  

A quick glance at the products indicates that we are adding H and OH across the alkene. Let s take a closer look and carefully analyze the regiochemistry and stereochemistry of this reaction. The OH is ending up on the less substituted carbon, and therefore, the regiochemistry represents an anti-Markovnikov addition. But what about the stereochemistry Are we seeing a syn addition here, or is this anti addition  

Be careful. The example above represents somewhat of an optical illusion. The products seem to suggest an anti addition (the methyl and the OH are trans to each other). But think about what we added in this reaction we did not add OH and a methyl group. Rather, we added OH and H. The H that we added is not shown in the product above (because it does not have to be drawn in a bond-line drawing). If you draw that H on the compound above, you will see that it is on a dash—therefore, this was a syn addition of H and OH. 

To recap, the reaction above is an addition of H and OH, with anti-Markovnikov regiochemistry, and syn stereochemistry. Now we have three important questions to answer  

The answers to all three of these questions are encapsulated in the mechanism. In order to explore the mechanism, we must first acquaint ourselves with the reagents. In the first step, the reagents are BH3 and THF. BH3 is called borane, and it displays very interesting chemistry. The element boron uses its three valence electrons to comfortably form three bonds  

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