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LAH reduction of Amide mechanism
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Topic: LAH reduction of Amide mechanism (Read 12595 times)
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azmanam
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LAH reduction of Amide mechanism
«
on:
March 09, 2011, 09:11:55 AM »
I don't know how I feel about the mechanism of LAH reduction of an amide generally proposed in undergrad textbooks. I've always lived by the mantra 'never draw a positive charge in a basic mechanism/negative charge in acidic mechanism' (except for the counter ion... but we don't care about that part). We don't kick out RO- under acidic conditions, nor have an alcohol directly attack an unactivated carbonyl because we'd draw O- in the intermediate.
The step I have problems with is the expulsion of the aluminum hydroxide in the first reduction step. That generates a carbocation/iminium ion intermediate. I'm real uncomfortable drawing a carbocation in a basic mechanism. I'm also uncomfortable with
-
O-Al
-
H
3
as a leaving group with a 2
-
charge.
Am I being too meticulous or do others share my concerns? I'm teaching this mechanism in class in a few days and don't want to sound like a doofus. Please advise!
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azmanam
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Re: LAH reduction of Amide mechanism
«
Reply #1 on:
March 09, 2011, 09:16:27 AM »
I like this mechanism much better, which completely avoids positive charges... but I've never seen it drawn this way. Not in my textbook, not in another textbook, not on the MSU e-text[1], not on the UCalgary site[2]...
[1]
http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/crbacid2.htm
[2]
http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch20/ch20-3-4-2.html
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Doc Oc
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Re: LAH reduction of Amide mechanism
«
Reply #2 on:
March 09, 2011, 09:55:51 AM »
I like the 2nd one better as well, but I doubt that many of the undergrads will care one way or the other. Most of them will groan about having to learn another mechanism.
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stewie griffin
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Re: LAH reduction of Amide mechanism
«
Reply #3 on:
March 09, 2011, 11:48:27 AM »
I don't have the correct answer for you but I too was always bothered by the mechanism typically taught here. I like the second mech better.
Perhaps if it were a more advanced class you could actually explain the merits/drawbacks of each mechanism, but for a typical 2nd yr orgo class I assume most won't ever notice the positive charge in basic media problem.
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stewie griffin
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Re: LAH reduction of Amide mechanism
«
Reply #4 on:
March 09, 2011, 06:46:14 PM »
Azmanam, what happens though when you have two alkyl groups on the amide nitrogen? Your second mechanism wouldn't be applicable then.
Is there a reason why can't we just postulate a direct S
N
2 by hydride on the intermediate aminal.
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azmanam
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Re: LAH reduction of Amide mechanism
«
Reply #5 on:
March 10, 2011, 08:03:02 AM »
Yeah, I thought about that after my op, too. Secondary amides couldn't be deprotonated. But I agree, I like an S
N
2-like attack of hydride displacing the
-
OAlH
2
.
Thanks for the thoughts, all.
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Re: LAH reduction of Amide mechanism
«
Reply #6 on:
March 10, 2011, 10:53:02 AM »
Don't forget about the Li in your mechanism. The amide is almost certainly coordinated to Li before the first hydride addition (not the reactivity difference between LiAlH
4
and NaAlH
4
). You could then consider that the leaving group might be the aggregate of Li–O–AlH
3
.
I am skeptical of the direct S
N
2 displacement. It's pretty sterically demanding.
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LAH reduction of Amide mechanism