Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: beheada on February 17, 2007, 01:23:35 PM
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So here's a question about the Diels-Alder rxn...
In my notes from class I have noted that an addition cannot occur to a cyclic conjugated diene because it cannot rotate it's bond?!
Yet, I'm under the impression Diels-Alder CAN in fact happen with a cyclic system, because my final problem for homework is as follows:
Come up with the diene and dienophile of the molecule, the molecule being:
A cyclohexene with trans nitro groups on C4,C5 and a bridged (not sure what to call it) carbon connecting C3, C6.
So the question: How the hell does that happen unless you CAN add to a conjugated diene ring?
If you CAN then the answer for the question ought to be IMHO
A cyclopentadiene combining with a dienophile which has trans nitro groups.
What say you?
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Cyclopentadiene is a great diene for D-A chemistry. The ones that don't work are like the attached structure where you can't get a so-called s-cis conformation of the diene. Cyclopentadiene is so good because it is always in the s-cis conformation.
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HAR! I just found an excerpt that says cyclopentadiene is SOOOO good for D-A rxn that it will actually react with itself to form a polycyclic molecule. How about that?
Ohhhh, one other small question about conjugated dienes. I know that if the molecule is sterically crowded by 2 methyl groups, then the addition can't take place. But if there is only one, does that mean the addition may take place?
For example, instead of 1,3-butadiene you tack on a methyl group to C1 or C4 which is facing toward the reacting side. I guess we'll call it (Z)-4-methyl-1,3-butadiene . will it go or not?
Thanks
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Cyclic dienes cannot roatate into a trans conformation with fewer than 8 carbons!
And this is why cyclopentadiene and cyclohexadiene work!
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Ohhhh, one other small question about conjugated dienes. I know that if the molecule is sterically crowded by 2 methyl groups, then the addition can't take place. But if there is only one, does that mean the addition may take place?
For example, instead of 1,3-butadiene you tack on a methyl group to C1 or C4 which is facing toward the reacting side. I guess we'll call it (Z)-4-methyl-1,3-butadiene . will it go or not?
In that example, you would expect the D-A to be significantly more difficult because you introduce a steric clashing in the reactive conformation, so that conformation is less accessible.
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So that means it will happen, but it has less likelihood of happening?
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I hate to say it will never happen, because I am sure there are examples, but it will be a whole lot harder to do (may require extreme temp, pressure). It's the kind of reaction that you wouldn't want to propose in a synthesis, but it might work if you really put your mind to it.
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I know why this is so odd.
Here's the question straight from the homework:
"Dimethyl butynedioate undergoes a Diels-Alder reaction with (2E,4Z)-5-methyl-2,4-heptadiene. (a) Draw the structure of the diene with the correct stereochemistry and in its reactive s-cis conformation. (b) Draw the structure of the product."
I found the same question in my textbook except the diene is 2,4-hexadiene. I think he may have just jacked up by putting an extra group at the 5 position, perhaps maybe not realizing it?
It's my understanding that this rxn is plausible but not probable due to the steric hindrance. So maybe it's a slip on the professor's part from altering book questions.
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Maybe. It's not always clear why one D-A goes but another will not. My guess is that his question is meant to demonstrate the stereochemical outcome of the D-A depending on what olefin isomers you take into the reaction.