I think I have my terms mixed up but I'm confused about the concept of stereoselectivity as they relate to reactions with alkenes.

For example, hydroboration is a stereoselective reaction. It uses syn addition. However, you can form a racemic mixture from it because you can add from different sites. enter image description here

I think this is a good depiction of what I mean. So it seems to me that hydroboration is called stereoselective because it does syn (rather than anti AND syn), but it still forms two stereoisomers. So how does that work?


You haven't mixed up your terms: a stereoselective reaction is defined as one which preferentially forms one stereoisomer over another.

However, you have to remember that the term stereoisomer encapsulates both enantiomers and diastereomers. The hydroboration reaction is stereoselective because it selects for a particular diastereomer, not a particular enantiomer. Basically, hydroboration leads to the formation of a racemic mixture of a particular diastereomer. The alkene you have chosen, 2-methylbut-2-ene, is not a good example substrate that can actually show how the reaction is stereospecific (since one of the alkene carbons is symmetrically substituted, there's no diastereomerism present in the products).

If I use (­E)-3-methylpent-2-ene as the substrate, then it becomes clearer:


Note how neither of the (2​R,3​R) or the (2​S,3​S) stereoisomers are formed. However, the two products that are formed are enantiomers of each other.

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