Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: Heory on December 14, 2009, 10:02:08 PM
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I think the first problem has been solved so let's turn to a new one. And again, I suggest that everyone who has taken part in the previous problems share good reactions.
Have fun, guys. ;D
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Well, this is certainly interesting, I don't think I have ever seen DBU activated at those methylene positions before, and I think that is what is going on here? I can't really find a good way to get there just yet though. I'm assuming a few things, the first compound is in excess, DBU will not deprotonate itself (it is a commercial base, right), there will be 2 eq N2 given off. So since DBU is not going to deprotonate itself under my assumption, I'm assuming there is a reactive intermediate that is formed that will act as a base?
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???
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Well, I was working on this while you posted (without internet sources :P)
I agree the last part is a Diels-Alder/retro Diels Alder to kick out N2.
So what was the real reaction they wanted to have occur? I can't imagine someone mixed these together in the hope of getting this product? Perhaps this was one of those side reactions that led to something cool?
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I don't have the original literature and copied it from my teacher's courseware. I don't know whether "O" in the square bracket refers to the tetrazine or O2 in the atmosphere. I believed the dehydrogenating reagent was O2 until Oxy mentioned that the tetrazine could also work that way. Well, let's put it aside temporarily.
The Diels-Alder/retro Diels-Alder mechansim is right. Believe it or not, more resonable mechanism was proposed by the authors for steps before D-A/retro D-A.
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I'll have to ponder more then...I don't think the above mechanism is unreasonable though ???.
I was thinking along that pathway mainly because I did not see the DBU deprotonating itself to form an enamine.
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Well, I guess Ive locked myself into thinking about this one pathway, and really am not seeing a second that I like. Anyone else have any thoughts, or perhaps a subtle hint (that won't spoil it) so as to get thought back on track?
And maybe this is my cynicism, but does anyone else wonder what reaction they were really trying to do or what they were trying to make? The reagents seem odd to throw together under the given conditions and expect a certain product... but I may not be thinking outside the box enough.
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I can't imagine that they planned this. ??? Probably noticed that this was a background reaction when trying to do something else, so they decided to just run it and see what happens.
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I'm also confused, but I feel they discovered this reaction just by accident. When this problem is finished, you can help me find out the answer. And I also want to know whether Oxy's dehydrogenating reagent is reasonable.
The way proposed by tmartin in which the ring opened and released N2, which was followed by formation of an unusual vinyl anion, is less perfect. As for the ring closure step, the enamine with 2 strongly electron donating N atoms which was nuleophlically attacked by the imine should actually act as a nucleophilic reagent. The first 2 steps are right. With this hint look for another pathway to release N2. :)
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To be specific, in the step that immediately follows tmartin's step 2, the enamine acted as a nuleophilic group. After that you will certainly follow the logic train of thought to the product.
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When I follow that mechanistic pathway I have a hard time arriving at the desired product. Namely, I cannot form the desired C-N double bond in the new ring, nor can I find a way to push electrons to form the correct N-C bond to the enamine carbon in question and the correct nitrogen. ???
Anyone else see what I'm missing?
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I don't want to diminish your sense of fulfillment too much. I hope this would help.
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Still not 100% sure where this is going... starting to look like the original pathway, however other ways of pushing electrons seem to lead to a dead end.
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Still not 100% sure where this is going... starting to look like the original pathway, however other ways of pushing electrons seem to lead to a dead end.
You are right until the last step. Look back to the intermediate cited by Oxy. After you obtain it, make full use of (retro-)electrocyclic reactions.
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I'm not sure if another equivalent of "A" (named by Oxy) is required or not. I really can't get there without it so I will assume it is needed. The -RCN step is a bit of voodoo the way I've drawn it. I think that movement of electrons will make the desired bonds, but not sure if it is the best way to get there. Thinking of it in a stepwise fashion, the imine nitrogen pushes electron density into the carbon, breaking the C-C bond, which pushes its electron density into the second nitrogen to break the N-N bond and liberate RCN.
The second pathway I would think would require heat or light, but who knows. Looking at it, that way may not be so crazy really if there is added energy in the system.
I've kind gone a little off my rocker thinking about this, so hopefully one of these is close to the solution, as I'm kind of at an impasse. :P
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That's close.I think it is the O2 in the atomosphere that dehydrogenated the intermediate. As for the electrocyclic reaction, you'd better leave the seperated charges as it were. Don't draw the 3-memered cycle. But essentially you are right. Would mind drawing the whole mechanism for this problem? :)
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I actually did not save any of my schemes as ChemDraws this time around... but if I get bored later in the day I may find time to redrawn them all...if not I'll leave it to you :P
Is there a reference to this problem? It would be interesting to see what led to this discovery, and also what type of yields are obtained.
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I don't have the original document. But just now I found its tittle frow which we can see the reaction was just a surprise and the oxidant was the tetrazine.