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Author Topic: Problem of the Week - 11/23/09  (Read 7141 times)

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Dan

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Re: Problem of the Week - 11/23/09
« Reply #15 on: November 24, 2009, 03:19:23 AM »

(and, not to nit pick, but your 'more BuLi' in your scheme is incorrect.  You've already accounted for the 2 eq of base, and already have the negative charge you need to extrude nitrogen.

Yeah, sorry the "more BuLi" refers to the Bu- in the mechanism. You need to deprotonate the N and the carbon alpha to the hydrazone - 2 eq (or are you saying -SO2 is basic enough to do the second deprotonation?).

ie: RCH2C(NNHSO2Ph)R' + 2BuLi ---> RCH=CLiR' + N2 + LiSO2Ph + 2BuH
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stewie griffin

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Re: Problem of the Week - 11/23/09
« Reply #16 on: November 24, 2009, 03:46:52 AM »

How about this?
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Heory

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Re: Problem of the Week - 11/23/09
« Reply #17 on: November 24, 2009, 04:01:41 AM »

I wanted to go that way at first but thought it was not very resonable, for the 4-membered ring shoud be destroyed. your structure doesn't conform to azmanam's NMR data. 6.22, 6.51, 6.66 are shifts of prontons on the Ar ring, am I right?
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stewie griffin

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Re: Problem of the Week - 11/23/09
« Reply #18 on: November 24, 2009, 04:07:13 AM »

Well I wasn't sure about that. In my mechanism I remove one aromatic proton and replace with one alkene proton. So the three protons in that region are still accounted for.
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tmartin

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Re: Problem of the Week - 11/23/09
« Reply #19 on: November 24, 2009, 04:23:13 AM »

After looking at this NMR data I am becoming a bit skeptical...

I like stewie's mechanism, it makes sense to me that this type of reaction could proceed.  I think the four membered ring would be just fine, there are natural products with this type of scaffold (pinene, for example).  I can also see the protons on stewie's final product mapping up pretty well with the peak list provided.  The singlet around 3.4 could be the allylic/benzylic proton, perhaps (I think stewie is missing a methyl group there, assuming chemdraw mistake).

The only think I cannot make sense of it the two peaks at 0.93 that are singlets for 3 protons...I cannot make them fit any product I could think of currently. Could this be just a chemdraw error, azmanam?
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Dan

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Re: Problem of the Week - 11/23/09
« Reply #20 on: November 24, 2009, 05:10:17 AM »

The only think I cannot make sense of it the two peaks at 0.93 that are singlets for 3 protons...

Yeah, this is bothering me too - from the list it looks like there are 18H in the 0.9-0.95 region:

0.90, s, 6H
0.90, s, 6H
0.93, s, 3H
0.93, s, 3H

Is it possible that one of the "0.90, s, 6H" is a duplicate that shouldn't be there? 18H in this region implies 6 Me groups, if this "duplicate is removed we have 12H - 4 Me groups...

If this is the case, then Stewie's structure (with the missing Me added in) fits well.

« Last Edit: November 24, 2009, 05:29:17 AM by Dan »
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Heory

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Re: Problem of the Week - 11/23/09
« Reply #21 on: November 24, 2009, 11:07:02 AM »

Stewie griffin's structure will go through furthur rearrangement under acidic condition to destroy the 4-membered ring, I think. However, Stewie griffin's way seems much more reasonable than I thought, for I didn't know it's a name reaction.
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azmanam

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Re: Problem of the Week - 11/23/09
« Reply #22 on: November 25, 2009, 04:50:45 AM »

Ok, sorry it took me so long to get back here.  I was fusing some ZnCl2 yesterday by melting it under hi vac... and started to melt the flask which started to curl in on itself under the vacuum.  Fortunately nothing got out of hand, but I decided to call it a day after that :)

Anyway... Stewie wins the second round with a Nazarov-type cyclization (and also a F.C. alkylation for the double bonus).  It is formally a 4pi conrotatory electrocyclization, but who's counting?  Given that it's a pericyclic reaction, you can get to the tetracyclic product without a carbocation intermediate following ring closure.  Alcohol is protonated, water leaves to give a resonance stabilized allylic/benzyllic carbocation.  Electrocyclization of the resonance structure (faciliated by a pair of electrons from the OMe group) can put the double bond in place directly and the positive charge on oxygen, followed by rearomatization to the final product.  4-membered ring remains intact throughout.

I drew the electron flow in more of a FC type of mechanism, but there's nothing wrong with the way stewie drew it.  He could have alternatively move the electrons in a circle in the other direction to put the positive charge on one of the aryl carbons - also keeping the positive charge away from the 4-membered ring.

As for the NMR... I don't know what happened.  I redrew the product 3 different ways in case overlapping bonds in the bridged bicycle was throwing off the predictor.  Every time the same line list was generated.  After blowing up the alkyl region, it is clear the spectrum is correct... but the line list is somehow incorrectly populated.  I have let the admin of the site know... Sorry for the confusion :)
« Last Edit: November 25, 2009, 05:09:25 AM by azmanam »
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azmanam

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Re: Problem of the Week - 11/23/09
« Reply #23 on: November 25, 2009, 04:58:48 AM »

btw, you are right Dan, I was misreading your scheme re: more BuLi.  I saw the arrow from Bu- to expel the sulfonate as the 2nd eq... then saw 'more BuLi' over the arrow and took it to mean a third eq.  I think you meant the Bu- and the 'more BuLi' to refer to the same 2nd eq.  You definitely need 2 eq, and sulfonate won't deprotonate anything, imo.  Sorry for the confusion.
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azmanam

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Re: Problem of the Week - 11/23/09
« Reply #24 on: November 25, 2009, 04:59:43 AM »

barring the line list error... did the NMR help/hurt/no difference?  would it be helpful to include it in predict the product questions in the future?
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Heory

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Re: Problem of the Week - 11/23/09
« Reply #25 on: November 25, 2009, 05:10:58 AM »

Well done stewie griffin!
Could you explain how the 4-membered ring kept intact?
Why didn't the double bond gain a proton to form a tertiary carbocaton and then lead to rearrangement?
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stewie griffin

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Re: Problem of the Week - 11/23/09
« Reply #26 on: November 25, 2009, 05:16:57 AM »

re: azmanan, I like having some sort of spectroscopic data when the question is "draw a mechanism and predict the product." Sometimes there are so many things that could happen it's tough to really know where to start. Knowing NMR or even IR data can help guide the thought process.
re: Heory, I guess the nazarov cyclization is just faster than wagner-meerwein shift ?? To be honest I don't know precisely why the 4 membered ring stays intact, but my intuition here just didn't make me feel like the 4-membered ring had to break.
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Dan

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Re: Problem of the Week - 11/23/09
« Reply #27 on: November 25, 2009, 07:21:46 AM »

I like having some sort of spectroscopic data when the question is "draw a mechanism and predict the product."

Agreed, I like the NMR too, it lets me rule out some of my more outlandish ideas before they hit the forums...
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Heory

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Re: Problem of the Week - 11/23/09
« Reply #28 on: November 25, 2009, 01:34:13 PM »

I propose other guys such as Dan, stewie griffin and Tmartin share instereting reactions like azamanam so we can learn more in a week. I'd like to share some from my text book, but maybe most are too simple for you. :)
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azmanam

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Re: Problem of the Week - 11/23/09
« Reply #29 on: November 25, 2009, 02:53:40 PM »

From Dr. Luc Patiny, the contact person of the nmrdb.org website:

Quote
Thanks for your comments.

Indeed there is some problems.

1. Today the program is not completey aware on diastereotopic atoms. We are working on this and it will be solved beginning of next year. It is a top priority for us because we have a lot of projects depending on this. However the predictions of the 4 different methyl groups are correct in principle.

2. In order to show the integrals we simply + / - 0.05 ppm and therefore for each of the 2 methyl groups that are around 0.93 there is the integral that partially integrates the other methyl leading to a confusing results. Because we first simulate the spectrum and then define the zones to integrate the integration is really done on the real spectrum and it is difficult to change.
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