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Author Topic: Problem of the Week - 11/23/09  (Read 6738 times)

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azmanam

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

QUESTION: Mechanism, X, and Product.  Bonus for naming the reactions.  Double bonus for the alternative name for the last step.
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Dan

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Re: Problem of the Week - 11/23/09
« Reply #1 on: November 23, 2009, 07:06:01 AM »

The first two steps look like a Shapiro reaction. Is there an aqueous workup after the BuLi, or will the Shapiro carbanion be treated directly with the aldehyde?
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azmanam

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Re: Problem of the Week - 11/23/09
« Reply #2 on: November 23, 2009, 07:17:19 AM »

you can assume no workup until after the addition of the aldehyde.
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Dan

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Re: Problem of the Week - 11/23/09
« Reply #3 on: November 23, 2009, 07:25:36 AM »

How about
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tmartin

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Re: Problem of the Week - 11/23/09
« Reply #4 on: November 23, 2009, 08:25:40 AM »

I like your mechanism, Dan.  I do have a question though.  Why did you choose that carbon-carbon bond to migrate?  I was thinking the other carbon-carbon bond would migrate... not a huge difference in product though, I know.
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Dan

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Re: Problem of the Week - 11/23/09
« Reply #5 on: November 23, 2009, 09:35:05 AM »

Why did you choose that carbon-carbon bond to migrate?  I was thinking the other carbon-carbon bond would migrate...

Good point! That was a lapse in concentration, I agree with you.
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Dan

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Re: Problem of the Week - 11/23/09
« Reply #6 on: November 24, 2009, 12:25:45 AM »

So like this?
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jinclean

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Re: Problem of the Week - 11/23/09
« Reply #7 on: November 24, 2009, 12:50:40 AM »

Wow!that is very interesting!but i think the Bamford-Steven reaction should occur in advance with the 2.2 eq BuLi  instead of the addition to the aldehyde,and the addition should be made by the diazonium like the wolff rearrangement?
« Last Edit: November 24, 2009, 01:20:26 AM by jinclean »
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Dan

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Re: Problem of the Week - 11/23/09
« Reply #8 on: November 24, 2009, 01:36:31 AM »

Hmmm, I'm not sure I agree. The Bamford-Stevens requires only 1 eq of base, and the Shapiro requires 2 eq - both reactions proceed via the same intermediate, so given the 2.2 eq of BuLi, I still think it's a Shapiro. Basically, I think the diazo - generated by 1 eq BuLi - would collapse to the vinyl anion (with loss of N2) in the presence of a second equivalent of BuLi.
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Heory

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Re: Problem of the Week - 11/23/09
« Reply #9 on: November 24, 2009, 01:57:17 AM »

 :)
Dan's answer is at least very very close to the answer. (The last step is Wagner-Meerwein rearrangement?)
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jinclean

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Re: Problem of the Week - 11/23/09
« Reply #10 on: November 24, 2009, 02:14:20 AM »

Thank you,Dan!
 i have a little knowdge about the Shapiro reaction ,i only know that this reactrion has the  similar mechanism with the Bamford-Steven reaction.that's very interesting that the difference between them is the amount of the base used in the reaction
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Heory

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Re: Problem of the Week - 11/23/09
« Reply #11 on: November 24, 2009, 02:29:14 AM »

 BTW, the intact OH made me uneasy.;D I tried to kick it away but then found nowhere to go, so I have to agree with DAN.
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azmanam

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Re: Problem of the Week - 11/23/09
« Reply #12 on: November 24, 2009, 02:29:27 AM »

Dan is correct for steps i. through iii. and has successfully drawn product X (the secondary benzyllic alcohol that is the first compound on the second line of the first scheme Dan drew).  Dan also successfully noted the Shapiro reaction as the name for the first transformation: conversion of a ketone to a vinyl lithium.  Bamford Stevens is usually run in protic or aprotic solvents without strong alkyl lithium bases (and usually only 1 equivalent).  In protic solvents, a saturated carbocation is actually formed at the former carbonyl carbon - and will react like normal carbocations do.  In aprotic solvents, carbenes can be formed to do carbene chemistry.[1]

Trapping of the vinly lithium with an electrophile is the Shapiro reaction.[2]  Dan's mechanism goes astray, however, in the acidification step.  The alkene is not the most Lewis basic site on the molecule.

(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.  No more BuLi is needed after explusion of the sulfonate)

I had this thought yesterday, and thought I'd try it out.  I've attached the predicted NMR spectrum of the product - as predicted by NMRdb.org[3], my new favorite NMR prediction website.  The delta1 column is chemical shift, pattern is multiplicity, and the integ. column is integration.  I don't know if this will help or hinder, so let me know how it is received :)

[1] http://www.organic-chemistry.org/namedreactions/bamford-stevens-reaction.shtm
[2] http://www.organic-chemistry.org/namedreactions/shapiro-reaction.shtm
[3] http://www.nmrdb.org/predictor
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Heory

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Re: Problem of the Week - 11/23/09
« Reply #13 on: November 24, 2009, 02:54:15 AM »

F-C reaction didn't happen. But how are there so many protons? Is something wrong with the prediction?
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tmartin

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Re: Problem of the Week - 11/23/09
« Reply #14 on: November 24, 2009, 03:09:37 AM »

I like this idea as a hint, azmanam...however I'm still trying to wrap my head around your NMR data  :P
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