Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: kinghumanity on November 15, 2013, 11:19:15 AM
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I can't figure out this reaction mechanism. I'm trying to think of ways to deprotonate the aromatic ring and beta hydride elimination doesn't seem to be it. Using the base to deprotonate doesn't seem to make sense either. Would really appreciate some help :(
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I can't figure out this reaction mechanism. I'm trying to think of ways to deprotonate the aromatic ring and beta hydride elimination doesn't seem to be it. Using the base to deprotonate doesn't seem to make sense either. Would really appreciate some help :(
What the Heck? ;)
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I can't figure out this reaction mechanism. I'm trying to think of ways to deprotonate the aromatic ring and beta hydride elimination doesn't seem to be it. Using the base to deprotonate doesn't seem to make sense either. Would really appreciate some help :(
What the Heck? ;)
Yes, I've thought about the Heck coupling. But one key part of that is beta-hydride elimination, and that doesn't seem possible here. And also wouldn't the Heck coupling leave the alkene intact instead of breaking the double bond?
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You would want the double bond to be intact for the subsequent cyclysation to occur.
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You would want the double bond to be intact for the subsequent cyclysation to occur.
How? :(
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I can't see any reason why the Heck reaction wouldn't work, draw it out and you will see.
I am not familiar enough with Palladium catalysed reactions so I would leave this up to my colleagues to comment on whether Palladium is involved in the cyclysation.
To give you a guide on how the mechanism might proceed if the palladium is not involved look up aromatic electrophillic substitution reactions with alkenes.
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sjb...
I am sitting in the computer lab and read the post. Said Heck to myself and then saw what you wrote and laughed out loud and proceeded to be stared at...any way, this may be a variation of cross-coupling. Will look over and more of a coherent response in a bit.
lol what the heck... every time.
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lol what the heck... every time.
This really brightened my day, glad I am not the only one who finds chemisry humour funny ;D
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Chemistry humor is the best. Read my tag line lol.
So I have reasoned through this reaction and believe it is a type of Heck cross-coupling. I am not at my work computer so I cannot draw at this time. I will later this evening though. Being a type of Heck reaction, it does follow the classical reaction mechanism. The oxidative addition and addition of the olefin are classic in my opinion but not the beta-hydride shift/ abstraction (what have you). The transference of the hydride comes from the phenyl ring. Where the bond is made to make the final cyclohexyl ring is where the hydride is abstracted from. The treatment with triethyl amine regenerating the catalyst finishes the reaction.
Again I will have a picture soon.
Cheers
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Bare with the drawing. My program isnt the best.
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Bare with the drawing. My program isnt the best.
Thanks! So it would be a typical electrophilic aromatic substitution using palladium that's bonded to the carbon after migratory insertion?
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Bare with the drawing. My program isnt the best.
Forgive my ignorance in palladium chemistry, this is an area of mechanistic chemistry in which I have very little experience, how does that intermediate palladium species form?
The carbon in the beta position to the palladium bearing carbon has become protonated, is that by way of hydride attack with the π electrons being donated to the palladium or is the palladium attacking the double bond and the π electons picking up an H+?
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I was thinking about either of these two, I apologise for the quality of the arrows
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@ King,
There is migratory insertion forming the first bond between the two species, but no electrophilic aromatic substitution as I have it written. I have surmised, BASED ON THE CONDITIONS GIVEN, that there is no electrophilic aromatic substitution but a hydride elimination from the phenyl ring and a new bond forming to close the ring. A colleague of mine and I discussed if it may be possible and we came to the conclusion that due to the ability of a phenyl ring to coordinate with the palladium center it isnt a reach to say the aforementioned mechanism is possible.
Again I came up with this based on the conditions. You would need to generate a electrophile after the initial Heck reaction and regeneration of the pi system.
@Archer
The way I have it written it is not the most correct representation. The bond to the bicyclic ring is actually more of a pi complex with the palladium after the migratory insertion of the biphenyl sysyem.
I have attached a picture of my thoughts with some explanations.
I have to also admit that I am not very experienced with mechanistic chemistry of transition metals. I have been told by some very experienced and smart chemists that no one really knows what goes on for sure.
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My errors in the picture.
Pd should start at Pd(0) then go to Pd(2-).
Archer,
Where does the hydride come from that is bonded to palladium. The hydride is generated in the last step of the cycle. The H would have to be an aryl, benzyl or vinyl moiety. Not a hydride.
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Ignore me, I was thinking this was a two-step process :-[. Looking at the Pd Species from the Heck reaction as Pd(L2)HBr and thought that this maybe reacted with the Heck product (as per my mechanism). Looking back at the Heck mechanism I can see that what you had drawn was one of the Heck Intermediates.
Sometimes it is best not to try and over engineer things....
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Well, you are not completely out of bounds thinking it was a two-step process. That could be true if the compound indeed has to then be treated with an acid to form an electophile to go through EAS.
But the Pd(L2 HBr) is then going to react with the triethylamine present to regenerate the catalyst.
You need not be ignored, you post good stuff. We are conversating, bouncing ideas. To paraphrase Borek, I am occasionally wrong. This is how problems are worked through.
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It's good to talk these things through, I miss being an academic when you can talk about these things over coffee or a cold beer on a Friday afternoon, happy days :)
This is my only place where I can discuss things like this, it's been really helpful to me in these kinds of applications.
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I am also no longer an academic. I work for a small company that my professor in college consults for, so it is a happy medium of talking chemistry but also having to produce compounds for commercial use.
I have only gone as far as my bachelors at the moment but working here with him as a mentor and also all the people on this sight has allowed me to grow as a chemist by leaps and bounds.
I do not have any friends any more where I can have that beer and talk chemistry. If I do that now while I am out I get the look like I have 8 heads.
As you stated, this is a great place where learning can take place.
I like the rules about trying your own problems. In college I tutored friends. They wanted to me basically to do the home work and the copy. I do not do that. If I am going to tutor, be ready to work and learn. Thats what I like about this site.
Cheers,
Archer
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I had a different approach. I assumed the Heck would occur as normal. At that point the Pd dissociates, and the resultant olefin can do a 6π-electrocyclization (http://en.wikipedia.org/wiki/Electrocyclic_reaction) followed by a [1,5]-hydride shift (http://en.wikipedia.org/wiki/Sigmatropic_reaction).
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I had a different approach. I assumed the Heck would occur as normal. At that point the Pd dissociates, and the resultant olefin can do a 6π-electrocyclization (http://en.wikipedia.org/wiki/Electrocyclic_reaction) followed by a [1,5]-hydride shift (http://en.wikipedia.org/wiki/Sigmatropic_reaction).
I'm not saying that isn't the mechanism, but the electrocyclic reaction must de-aromatize two rings. If ortho vinyl biphenyl undergoes this reaction, then I'd agree. Does it?
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Good point orgopete. Photo conditions can break aromaticity to do reactions very similar to what I proposed (http://en.wikipedia.org/wiki/Stilbene_photocyclization, http://en.wikipedia.org/wiki/Diarylethene), but we aren't under the proper conditions.
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What about a tandem heck? Migratory insertions into aromatic systems followed by beta elimination to regenerate the aromatic ring are known.
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What about a tandem heck? Migratory insertions into aromatic systems followed by beta elimination to regenerate the aromatic ring are known.
I think that is the point being debated. How does that reaction take place? Undoubtedly, it is the electrons of the aromatic carbon that must participate. The examples look like the Heck intermediates are receiving electrons from the neighboring ring with an electrophilic-aromatic substitution driving force. Certainly, these look more likely than a direct participation of an aromatic C-H bond.
By the way, while I was skeptical of the earlier proposed electrocyclic reaction, I would have been more likely to agree given the conditions of these examples (though I didn't work through con v disrotary and it's implications). I anticipated mild conditions for the Heck, but heating to 160oC I think invites consideration of an electrocyclic reaction.
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Thanks guys :)