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Offline 190303458

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question about diastereoselective reaction
« on: March 20, 2016, 06:02:09 AM »
Hi all, I have a question about diastereoselective reaction. From my understanding, a diastereoselective reaction is a stereoselective reaction producing diastereomers in unequal amount.
The following reaction is from my lecture material. It says the reaction is diastereoselective and the product is a diastereomer of 2-methylcyclohexanol.
However, hydroboration of alkenes is a syn addition of H-BH2 to the alkene. If H-BH2 is added from the top face of the alkene, A will be formed. If H-BH2 is added from the bottom face of the alkene, B will be formed. A and B are enantiomers. Since a pair of enantiomers are formed, why is the reaction diastereoselective?
Please help, I am very confused about this. Thank you very much in advance.

Offline AWK

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Re: question about diastereoselective reaction
« Reply #1 on: March 20, 2016, 06:25:52 AM »
Consider optical active compound with substituent in position 3 or 6 of ring.
AWK

Offline 190303458

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Re: question about diastereoselective reaction
« Reply #2 on: March 20, 2016, 06:45:14 AM »
Consider optical active compound with substituent in position 3 or 6 of ring.
Thanks for your reply first, but what's the relationship between your suggestion and my question? I have no idea about that.

Offline Guitarmaniac86

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Re: question about diastereoselective reaction
« Reply #3 on: March 20, 2016, 07:26:11 AM »
Consider optical active compound with substituent in position 3 or 6 of ring.
Thanks for your reply first, but what's the relationship between your suggestion and my question? I have no idea about that.

I think I know what AWK is getting at.

Think about the conformation of the ring in the starting material. Now, what difference would there be between a substituent at the 3 position and the substituent at the 6 position? How will this affect the reaction? Why does this give you more of one diastereoisomer?
« Last Edit: March 20, 2016, 07:37:31 AM by Guitarmaniac86 »
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Offline AWK

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Re: question about diastereoselective reaction
« Reply #4 on: March 20, 2016, 07:46:45 AM »
I striked the right chord in Guitarmaniac86 guitar
AWK

Offline Alwin Kristen

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Re: question about diastereoselective reaction
« Reply #5 on: March 20, 2016, 12:40:09 PM »
I see no reason why borane can't attack from both sides. I have done fisher projection, conformation analysis, done research from books and googled it without a clue. Maybe there is just an error at your lecture material?

Can you clarify your idea AKW because I got nothing from your hint?
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Offline sjb

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Re: question about diastereoselective reaction
« Reply #6 on: March 20, 2016, 12:50:59 PM »
What does diasteroselective, as opposed to diastereospecific, mean?

Offline Dan

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Re: question about diastereoselective reaction
« Reply #7 on: March 20, 2016, 01:28:57 PM »
. Since a pair of enantiomers are formed, why is the reaction diastereoselective?
Please help, I am very confused about this. Thank you very much in advance.

I would explain this as follows:

The product is 2-methylcyclohexan-1-ol.

There are two pairs of enantiomeric steroisomers of this compound: cis-2-methylcyclohexan-1-ol (2 enantiomers) and trans-2-methylcyclohexan-1-ol (2 enantiomers). The cis and trans forms are related as diastereomers, and the hydroboration/oxidation sequence only produces one of these forms - so it is diastereoselective. But it is not enantioselective because trans-2-methylcyclohexan-1-ol is formed as a racemate.
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