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Offline kinghumanity

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Need help with reaction mechanism
« on: November 15, 2013, 11:19:15 AM »



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 :(

Offline sjb

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Re: Need help with reaction mechanism
« Reply #1 on: November 15, 2013, 11:37:43 AM »



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? ;)

Offline kinghumanity

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Re: Need help with reaction mechanism
« Reply #2 on: November 15, 2013, 12:04:45 PM »



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?

Offline Archer

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Re: Need help with reaction mechanism
« Reply #3 on: November 15, 2013, 12:21:50 PM »
You would want the double bond to be intact for the subsequent cyclysation to occur.
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Offline kinghumanity

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Re: Need help with reaction mechanism
« Reply #4 on: November 15, 2013, 12:43:02 PM »
You would want the double bond to be intact for the subsequent cyclysation to occur.

How?  :(

Offline Archer

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Re: Need help with reaction mechanism
« Reply #5 on: November 15, 2013, 12:56:24 PM »
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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Offline AlphaScent

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Re: Need help with reaction mechanism
« Reply #6 on: November 15, 2013, 01:45:08 PM »
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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Offline Archer

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Re: Need help with reaction mechanism
« Reply #7 on: November 15, 2013, 02:25:47 PM »

lol what the heck... every time.

This really brightened my day, glad I am not the only one who finds chemisry humour funny  ;D
“ I love him. He's hops. He's barley. He's protein. He's a meal. ”

Denis Leary.

Offline AlphaScent

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Re: Need help with reaction mechanism
« Reply #8 on: November 15, 2013, 04:03:26 PM »
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
If you're not part of the solution, then you're part of the precipitate

Offline AlphaScent

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Re: Need help with reaction mechanism
« Reply #9 on: November 15, 2013, 05:12:04 PM »
Bare with the drawing.  My program isnt the best.

If you're not part of the solution, then you're part of the precipitate

Offline kinghumanity

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Re: Need help with reaction mechanism
« Reply #10 on: November 15, 2013, 06:09:22 PM »
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?

Offline Archer

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Re: Need help with reaction mechanism
« Reply #11 on: November 16, 2013, 03:47:58 AM »
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+?

« Last Edit: November 16, 2013, 04:06:48 AM by Archer »
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Offline Archer

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Re: Need help with reaction mechanism
« Reply #12 on: November 18, 2013, 10:25:53 AM »
I was thinking about either of these two, I apologise for the quality of the arrows
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Offline AlphaScent

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Re: Need help with reaction mechanism
« Reply #13 on: November 18, 2013, 10:37:38 AM »
@ 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. 
If you're not part of the solution, then you're part of the precipitate

Offline AlphaScent

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Re: Need help with reaction mechanism
« Reply #14 on: November 18, 2013, 10:41:23 AM »
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.
If you're not part of the solution, then you're part of the precipitate

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