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Author Topic: Problems on Mechanism  (Read 5035 times)

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Heory

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Problems on Mechanism
« on: October 05, 2009, 10:46:19 PM »

What are the mechanisms of the following several reactions? Could you please give me some clues or tell me the reaction's name, if you know any of these reactions? Thank you very much.
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Heory

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Re: Problems on Mechanism
« Reply #1 on: November 13, 2009, 06:24:16 PM »

*Ignore me, I am impatient*
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Heory

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Re: Problems on Mechanism
« Reply #2 on: November 13, 2009, 11:28:10 PM »

 :)
Though 1,2,3 have been solved, help is still needed.
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Dan

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Re: Problems on Mechanism
« Reply #3 on: November 14, 2009, 04:00:28 AM »

Maybe something like this for 7?
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Heory

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Re: Problems on Mechanism
« Reply #4 on: November 14, 2009, 04:09:41 AM »

Thank you Dan!
It's the most reasonable mechanism I've seen for this reaction. But the defect is that if CN- can kick away O-,  O- is unlikely to kick away CN- initially, and CN- would attack from the less hindered side of the epoxide.

correction:
Q6, 1) PCC
Q9, the last step, H2O2 in basic condition.(the last step also has been solved.)
« Last Edit: November 14, 2009, 04:57:25 AM by Heory »
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Dan

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Re: Problems on Mechanism
« Reply #5 on: November 14, 2009, 04:42:36 AM »

Yes, the epoxide opening step bothers me too. It's the only mechanism I can come up with right now though.

The cyanide exchanges I think are ok. We see reversible cyanide addition to carbonyls in the Benzoin condensation, so maybe it's no so unreasonable.

I should have used equilibrium arrows here, because then I think it makes more sense - the epoxide might open at the other end most often, but this just regenerates the previous intermediate and the epoxide can re-form. Even if opening of the epoxide as I've proposed is rare, the protonation of the final carbanion is going to lie heavily to the product side, since this is the most basic intermediate in the mechanism by several orders of magnitude - perhaps this step is "essentially irreversible" like the thermodynamic sink of a deprotonated Claisen condensation product?

I don't know, I see the holes too and I'm not entirely convinced either, but I think it's plausible at least. It's a neat transformation in any case.
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Heory

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Re: Problems on Mechanism
« Reply #6 on: November 14, 2009, 05:03:07 AM »

Yes! I got it when you mentioned equilibrium arrows. I much agree with you now. Good job!
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orgopete

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Re: Problems on Mechanism
« Reply #7 on: November 14, 2009, 09:12:32 AM »

Are we talking TOSMIC here? It looks like it might be, see http://pubs.acs.org/doi/abs/10.1021/jo00439a002

I just glanced through a bunch of the others. I could tell you the mechanism or type of mechanism for several others, but it also looks like there are some errors in this list as well. For example, it looks as though the Br3 is missing the 3 from the pyridinium hydrobromide in ???. The last one has a conjugate epoxidation reaction without an oxidant.

If the list were split up, it would be easier or more sensible (for me) to give individual answers. Putting them in separate list may exhaust the poster though.
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Author of a multi-tiered example based workbook for learning organic chemistry mechanisms.

Heory

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Re: Problems on Mechanism
« Reply #8 on: November 14, 2009, 04:25:06 PM »

I extracted them from a very old book on total synthesis, which puzzled me for a long time. I also thought there were some mistakes and tried to make modification, for example, I guessed CrO3 was missing and for the last step H2O2 was missing. I've no access to chemical database so cannot search them to find the answer.
Thank you Dan and Thank you orgopete!
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blind

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Re: Problems on Mechanism
« Reply #9 on: November 14, 2009, 08:42:39 PM »

For the last question:

1) NMP is a high boiling solvent, perhaps they reflux the reaction?
2) LiCl is used in Stille reactions to accelerate the reaction, it stabilizes the intermediate formed by oxidative addition to the catalyst.
3) You have two double bonds, one is in an enone, the other is not.  The one that is with the enone is electron deficient, therefore is selectively epoxidized over the (relatively) electron rich double bond.
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tmartin

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Re: Problems on Mechanism
« Reply #10 on: November 18, 2009, 03:29:39 AM »

I've been looking thee over for a bit, and trying to figure out a few things:
#4) Is there something missing?  I can draw a mechanism for the reaction...but I wouldn't put a whole lot of stock in it, seems a little too off.
#5) Seems like there should be some sort of rearrangement, but I just can't move the arrows to get a product yet.  What is the order of addition?  Is the oxidant in there at the same time?
#6) Maybe the first step is formation of the ketone and a N-O bond, and the second step forms the C-N bond at the alpha carbon after enolization?
#8) Is a Tsuji-Trost reaction, I believe

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Heory

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Re: Problems on Mechanism
« Reply #11 on: November 18, 2009, 04:03:28 AM »

Thank you! blind and Mr. cock! ;D
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tmartin

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Re: Problems on Mechanism
« Reply #12 on: November 18, 2009, 04:52:48 AM »

Haha, it's technically a blue hen chicken  (a tribute to my undergraduate alma mater)... :)
Edit: I picked a pretty building instead, so I wouldn't cause any commotion  :P

For #5, it is really going to bug me.  I'm not 100% convinced any of this is on the right track, but I figure I'll throw out my ideas for peer review.  I'm guessing the amine base can grab either the blue protons, or the red proton.  Because there is copper (II) chloride in the reaction, I'm think of a conjugate addition... so I choose a blue proton.  Adding into the conjugated system gives me the desired 6 membered framework...but from there, I dunno.
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stewie griffin

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Re: Problems on Mechanism
« Reply #13 on: November 18, 2009, 04:58:58 AM »

Yes #8 is definitely a Tsuji-Trost... it goes through a pi-allyl palladium intermediate.
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Heory

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Re: Problems on Mechanism
« Reply #14 on: November 19, 2009, 07:26:58 PM »

 :) I think the authors of the old book drew some structures incorrectly. However, thank everyone who gave me a hand.
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