What is the stereochemistry resulting from hydroboration of 1-methylcyclopentene?


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> What is the major product of the following reaction sequence?
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> ![](https://i.stack.imgur.com/nzOCP.png)
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I'm supposed to work out the structures of both **2** and **3**, but I already know **2** to be 1-methylcyclopentene (as the most substituted alkene is formed during elimination).


For the hydroboration step, boron should add to the less substituted carbon (in an anti-Markovnikov fashion). However, I'm not sure which of the two choices **B** and **C** is correct. Why should one diastereomer be formed preferentially over the other?




The second step is a *syn* addition of the boron hydride. As depicted below, it will result in an *anti*-Markovnikov product with the boron on the less-substituted carbon. **Both hydrogen and boron are on the same side of the former double bond** and these two are the ones we refer to when we say *syn* addition.


The final step, the oxidation by hydrogen peroxide, is stereospecific with retention. So the newly-formed $\ce{C-O}$ bond is on the same side as the former $\ce{C-B}$ bond. Thus, an *anti* relationship is established between the methyl group and the hydroxy group.


Since 1-methylcyclopentene is planar, a racemic mixture of enantionmers is formed; neither *re* nor *si* side are favoured. The scheme below shows the attack from the *si* side of the $\mathrm{C2}$ atom.


![](https://i.stack.imgur.com/6EkKY.png)


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