Chemical Forums

Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: 190303458 on March 20, 2016, 06:02:09 AM

Title: question about diastereoselective reaction
Post by: 190303458 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.

Title: Re: question about diastereoselective reaction
Post by: AWK on March 20, 2016, 06:25:52 AM

Consider optical active compound with substituent in position 3 or 6 of ring.

Title: Re: question about diastereoselective reaction
Post by: 190303458 on March 20, 2016, 06:45:14 AM

Quote from: AWK on March 20, 2016, 06:25:52 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.

Title: Re: question about diastereoselective reaction
Post by: Guitarmaniac86 on March 20, 2016, 07:26:11 AM

Quote from: 190303458 on March 20, 2016, 06:45:14 AM

Quote from: AWK on March 20, 2016, 06:25:52 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?

Title: Re: question about diastereoselective reaction
Post by: AWK on March 20, 2016, 07:46:45 AM

I striked the right chord in Guitarmaniac86 guitar

Title: Re: question about diastereoselective reaction
Post by: Alwin Kristen 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?

Title: Re: question about diastereoselective reaction
Post by: sjb on March 20, 2016, 12:50:59 PM

What does diasteroselective, as opposed to diastereospecific, mean?

Title: Re: question about diastereoselective reaction
Post by: Dan on March 20, 2016, 01:28:57 PM

Quote from: 190303458 on March 20, 2016, 06:02:09 AM

. 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.