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Author Topic: Problem of the Week - 09/07/09  (Read 3107 times)

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azmanam

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Problem of the Week - 09/07/09
« on: September 07, 2009, 04:36:15 AM »

Answer to last week's follow up question has been posted.

http://www.chemicalforums.com/index.php?topic=35398.0

QUESTION: The diastereoselective β-hydroxy ketone reduction has been reported.  By using triethylborane and an external hydride source, a 1,3-syn diol is produced selectively.  Conversely, with an internal boron-based hydride source, the 1,3-anti diol is produced selectively.  Provide a complete arrow-pushing mechanism to account for the differential stereochemistry in the products.
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nj_bartel

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Re: Problem of the Week - 09/07/09
« Reply #1 on: September 08, 2009, 05:16:38 PM »

Any chance the first step of the first mechanism is one triethylborane molecule complexing to both the alcohol and carbonyl oxygen with only a partial set of electrons from each?  Only thing I can think of.  Had something else drawn out but it ended up with opposite stereochem.
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azmanam

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Re: Problem of the Week - 09/07/09
« Reply #2 on: September 09, 2009, 12:10:32 AM »

yes.  the first mechanism involves complexation of both oxygen atoms to boron.  like last week, it's how you draw it that ends up being important. (you can lose one of the ligands on boron so that in the end you don't have more than 4 things on boron)
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ArchStanton

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Re: Problem of the Week - 09/07/09
« Reply #3 on: September 09, 2009, 05:38:08 AM »

Is that top boron reagent a mistake? Should it not be Et2B(OMe). BEt3 can't chelate.

ArchStanton

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Re: Problem of the Week - 09/07/09
« Reply #4 on: September 09, 2009, 05:56:58 AM »

The top example involves external hydride delivery via an axial attack. This transition state goes straight into a Chair. Equatorial attack proceeds through the unfavourable twist boat. The conformation is locked by R2 being equatorial.

Second example is of internal hydride delivery. A chair transition state gives the correct product when you lock R2 in a equatorial position.

azmanam

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Re: Problem of the Week - 09/07/09
« Reply #5 on: September 09, 2009, 07:49:16 AM »

Quote
Is that top boron reagent a mistake? Should it not be Et2B(OMe).

Indeed it should.  My prof wrote it wrong in his lecture notes.  Any alkoxydialkyborane seems to work, but the methoxydiethylborane seems to give the best results.

Quote
The top example involves external hydride delivery via an axial attack. This transition state goes straight into a Chair. Equatorial attack proceeds through the unfavourable twist boat. The conformation is locked by R2 being equatorial.

Second example is of internal hydride delivery. A chair transition state gives the correct product when you lock R2 in a equatorial position.

Right on all accounts.  It's also important to have R1 in the pseudoequatorial position (and thus the carbonyl pseudoaxial).  Otherwise you get the wrong selectivity.

The 'trans-diaxial' attack of the external hydride is Governed by the Furst-Plattner Rules as described above.  Good work.

Prasad 'syn' reduction: http://dx.doi.org/10.1016/S0040-4039%2800%2995673-9
Evans 'anti' reduction: http://dx.doi.org/10.1021/ja00219a035
Furst-Plattner Rules: http://en.wikipedia.org/wiki/F%C3%BCrst-Plattner_Rule
     http://www.chem.wisc.edu/areas/reich/handouts/Nameeffect/named-effects.htm
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