[Pranav]

Problem:
(Problem Attached)

Attempt:
I figured out that X should be the one shown in second attachment. Hence, Y must be either A or B. But I have no idea about the correct one.

Any help is appreciated. Thanks!

EDIT: I hope the attachments are not too big to view, please let me know if I should adjust their size.

[AlphaScent]

Why would one hydroxyl group have different stereochemistry than the other two??

[Pranav]

Why would one hydroxyl group have different stereochemistry than the other two??

Why not?

The stereochemistry of hydroxyl groups can be random too but according to the given answer (B), it is not. Is there a reason behind this?

Thanks!

[discodermolide]

Perhaps you need to think about the 3d structure of the intermediate. Are the top and bottom faces equivalent? Could there be a directing factor causing the oxygen to approach from one face or the other?

[Pranav]

Perhaps you need to think about the 3d structure of the intermediate. Are the top and bottom faces equivalent? Could there be a directing factor causing the oxygen to approach from one face or the other?

Hi discodermolide!

I am following the mechanism shown here: http://www.mhhe.com/physsci/chemistry/carey/student/olc/ch15hydroboration_oxidationalkenes.html

Initially, boron is sp2 hybridised, hence, the three C-B bonds lie in a plane. The HOO^- ion can approach boron from either perpendicular sides (top or bottom).  When the migration of C-B bond to form a C-O bond is complete, we have a C-O-B bond. The attack of OH^- can take place from either sides. After the attack of OH^-, we have a C-O^- bond. But I don't see how this helps me to decide the stereochemistry.

Thanks!

[AlphaScent]

The stereochemistry is surely not random.  How does the BH2 and the H add to a double bond?  As disco said, think about the 3D structure.

[discodermolide]

They may lie in a plane but I think that the rest of the molecule is concave so that attack can only happen from one face, probably the top one. I don't have any modelling software here to assist me.

[Pranav]

They may lie in a plane but I think that the rest of the molecule is concave so that attack can only happen from one face, probably the top one. I don't have any modelling software here to assist me.

This is an exam problem so obviously, we don't have access to the computers. The solution to the given problem is presented in a single line that hydroboration oxidation is syn but I couldn't find any reference for it.

Thanks!

[AlphaScent]

Im trying to post this.  Dont now if it works.

[AlphaScent]

Ok, it did.  That is a 3D optimization of that intermediate.  The addition is syn or suprafacial.  The answer is B.

http://www.masterorganicchemistry.com/2013/03/28/hydroboration-of-alkenes-the-mechanism/

[orgopete]

This is an interesting problem that invites some assumptions about the reaction. Since diborane is listed as the reagent, technically, there is an excess of reagent. I might have expected a mixture of products, for example including a 1,5,10-triol in addition to a 1,5,9-triol. However, the choices suggest a single intermediate has formed. If the reaction were heated, that would be plausible, namely as illustrated. The oxidation step preserves the stereochemistry, therefore the product is B. A is also a plausible product that could form from a syn addition, but the intermediate would shift one of the chains into an axial stereochemistry. However, if the reaction were heated, it could revert until the more stable intermediate forms. This suggests to me the suggested intermediate forms and this will give B.