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Topic: Need help with reaction mechanism  (Read 7966 times)

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Offline Archer

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Re: Need help with reaction mechanism
« Reply #15 on: November 18, 2013, 11:28:20 AM »
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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Offline AlphaScent

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Re: Need help with reaction mechanism
« Reply #16 on: November 18, 2013, 12:24:53 PM »
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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Offline Archer

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Re: Need help with reaction mechanism
« Reply #17 on: November 18, 2013, 12:34:14 PM »
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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Offline AlphaScent

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Re: Need help with reaction mechanism
« Reply #18 on: November 18, 2013, 01:05:37 PM »
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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Offline stewie griffin

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Re: Need help with reaction mechanism
« Reply #19 on: November 20, 2013, 03:17:08 PM »
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).

Offline orgopete

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Re: Need help with reaction mechanism
« Reply #20 on: November 20, 2013, 11:18:43 PM »
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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Offline stewie griffin

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Re: Need help with reaction mechanism
« Reply #21 on: November 21, 2013, 08:29:04 AM »
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.

Offline stewie griffin

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Re: Need help with reaction mechanism
« Reply #22 on: November 21, 2013, 09:07:26 AM »
What about a tandem heck? Migratory insertions into aromatic systems followed by beta elimination to regenerate the aromatic ring are known.


Offline orgopete

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Re: Need help with reaction mechanism
« Reply #23 on: November 21, 2013, 09:53:25 AM »
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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Offline kinghumanity

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Re: Need help with reaction mechanism
« Reply #24 on: November 22, 2013, 11:30:48 AM »
Thanks guys :)

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