How does asymmetric hydroboration achieve stereoselective additions to alkenes?

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In summary, alpha-pinene and borane react to form a dimer, which is a commonly used asymmetric reagent in asymmetric synthesis. The reaction does not produce a trimer due to steric hindrance. This compound is used for stereoselective additions to alkenes, where the alkene reacts with the boron and forms a trimer, replacing the hydrogen. This method may only work on small alkenes due to steric hindrance. The bulky pinocampheol groups make the hydroboration reaction stereoselective by only allowing the boron to approach from the less hindered side of the alkene. This process is stereoselective, not regioselective.
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mycotheology
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So I'm reading about asymmetric synthesis and read that a commonly used "asymmetric reagent" is this:
http://img513.imageshack.us/img513/6447/ipcb.png
thats the dimer you get when you react alpha-pinene with borane. If I'm not mistaken, the reason this reaction doesn't go all the way and produce a trimer is because there is too much steric hindrance for a third a-pinene molecule to fit in there.

I'm trying to understand how this compound is used for stereoselective additions to alkenes. I'm guessing that the alkene has to react with the boron and form a trimer (by replacing the hydrogen). Firstly does that mean that this will only work on small alkenes because steric hindrance will block bigger alkenes from reacting with the boron?

My main question is how exactly does this make the hydroboration reaction stereoselective? Is it purely sterics? In other words, do the big bulky pinocampheol groups make it so the boron can only approach from the less hindered side of the alkene?
 
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Stereoselective or regioselective?
 

What is asymmetric hydroboration?

Asymmetric hydroboration is a chemical reaction that involves the addition of a boron atom and a hydrogen atom to an unsaturated compound, resulting in the formation of a chiral molecule. This reaction is often used in organic synthesis to create enantiomerically pure compounds.

What is the difference between asymmetric hydroboration and regular hydroboration?

The main difference between asymmetric hydroboration and regular hydroboration is that asymmetric hydroboration produces chiral molecules, while regular hydroboration produces achiral molecules. This means that asymmetric hydroboration can create compounds with distinct left- and right-handed versions, while regular hydroboration cannot.

What are the benefits of using asymmetric hydroboration?

Asymmetric hydroboration has several benefits, including the ability to create enantiomerically pure compounds, which are important in industries such as pharmaceuticals, materials science, and agriculture. It also allows for the creation of compounds with specific stereochemistry, which can have a significant impact on their properties and reactivity.

What types of compounds can undergo asymmetric hydroboration?

Asymmetric hydroboration can be used on a wide range of unsaturated compounds, including alkenes, alkynes, and heterocycles. These compounds must have a carbon-hydrogen bond adjacent to the double or triple bond in order for the reaction to occur.

What is the mechanism behind asymmetric hydroboration?

The mechanism of asymmetric hydroboration involves the coordination of the boron atom to the unsaturated compound, followed by the addition of the hydrogen atom. This is followed by rearrangement and protonation steps, resulting in the formation of the chiral product. The use of chiral ligands is crucial in controlling the stereochemistry of the reaction.

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