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Topic: mechanism of the reduction of the double bond by LiAlH4  (Read 16226 times)

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Azazel

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mechanism of the reduction of the double bond by LiAlH4
« on: February 24, 2005, 12:29:51 PM »
Hello, I am new to this forum. :)

I am an undergraduate student in my third year, and a step in my synthesis project has been the reduction of the C=C double bond with LiAlH4. Now, as many of us know, this usually doesn't occur. However, in this case, it does occur. This happens due to the fact that the reactant has a Hydroxy group in the allylic position. I've tried to find the proper mechanism online, but failed. If anyone knows the mechanism for this ( esp. with a reference to the proper literature. ;) ), I would be very glad.

I am also not a selfish bastard, and will try to assist others in question they have.


Offline Mitch

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Re:mechanism of the reduction of the double bond by LiAlH4
« Reply #1 on: February 24, 2005, 12:57:54 PM »
Have you tried scifinder scholar?
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Azazel

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Re:mechanism of the reduction of the double bond by LiAlH4
« Reply #2 on: February 24, 2005, 01:04:31 PM »
The paper that I got in the scifinder scholar was exactly what I needed wrt diastereoselectivity of the reaction ( the main issue about that part of the project). No mechanism in that paper though. ( Organic letters ) .

Maybe a more general search is needed.

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Re:mechanism of the reduction of the double bond by LiAlH4
« Reply #3 on: February 24, 2005, 08:17:05 PM »

It's an alkene, not an alkyne, right?

Offline AWK

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Re:mechanism of the reduction of the double bond by LiAlH4
« Reply #4 on: February 25, 2005, 01:36:24 AM »
LiAlH4 does not reduce unpolarized double or triple bonds
AWK

Azazel

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Re:mechanism of the reduction of the double bond by LiAlH4
« Reply #5 on: February 25, 2005, 02:34:24 AM »
Quote
It's an alkene, not an alkyne, right?

yes.

Quote
LiAlH4 does not reduce unpolarized double or triple bonds

I know. I've wrote about it in my opening post. See? :p
Quote
This happens due to the fact that the reactant has a Hydroxy group in the allylic position.

I need the mechanism for that, and I couldn't find it...

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Re:mechanism of the reduction of the double bond by LiAlH4
« Reply #6 on: February 25, 2005, 02:22:45 PM »
So I'm not familiar with the reaction with alkenes, but with alkynes containing a propargyl hydroxy group, the key is that you can form a five membered ring chelate between the alkyl aluminum and the alkoxy group.  I would suspect that the same thing would be key to the alkene reaction.

The mechanism would probably proceed as follows:

1) deprotonation of the alcohol by LAH (LAH is quite basic)
2) hydroalumination of the alkene via a 4-member transition state
3) chelation of the resultant alkyl aluminum to the pendant alkoxide
4) the alkoxide chelate is then broken down on workup

Azazel

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Re:mechanism of the reduction of the double bond by LiAlH4
« Reply #7 on: February 25, 2005, 03:41:46 PM »
Hey, I think I sort of worked it out on paper. I'll recheck on scifinder tomorrow.

Thanks a bundle!

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