Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: Dan on March 01, 2012, 03:40:11 AM
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I will try to keep up with these, at the moment they are more like problem of the month. Thanks to others who are posting their own problem, keep doing so!
Anyway, predict the products of the following reaction (dba = dibenzylideneacetone):
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That´s a good one. I will give only a small hint to the others: Stanford Chemistry Department
@ Dan: I will try to find a good one too...
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That´s a good one. I will give only a small hint to the others: Stanford Chemistry Department
If that's the case, how many times does the phrase "atom economy" show up in the reference?
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That´s a good one. I will give only a small hint to the others: Stanford Chemistry Department
If that's the case, how many times does the phrase "atom economy" show up in the reference?
;D Back to topic plz...
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Is it a coupling reactions, let's say Heck?
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Well it's not exactly a Heck reaction, but along those lines. Do you want to suggest a product?
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Well I found the answer, so I don't want to spoil anyone's else fun.
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Really unsure but I suspect it is a series of intramolecular cross coupling reactions.
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Really unsure but I suspect it is a series of intramolecular cross coupling reactions.
You're on the right track, but that's not the product. Post your mechanism so we can see the process.
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Sorry for the lazy mechanism, I am inbetween lectures at the moment. I suspect it is an olefin metathesis reaction now. I'm still unsure of the scheme.
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In the first stage, the palladium couples to the alkene substituent, loses a methylene group, and the molecule cyclises to give a five memebered ring. This is repeated 3 times.
The second scheme shows the formation of the 6 membered ring at the end.
I am confused because I do not really know the role of the Ph3Sb? I suspect it is a base, and I think I am missing a transmetallation step.
Edit:
I think the Ph3Sb compound transmetallates with the palladium moiety to add phenyl groups to the alkenes? I am not sure. I say, disregard my posts. I know Im wrong.
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Guitarmaniac86
You're actually not doing too bad, don't get discouraged! Usually unless an olefin is in conjugation with an e- donor or acceptor, or has a leaving group at a vinylic or allylic site, its going to be less reactive than an alkyne. When I see one I usually use that as the initial site of attack. Also, It doesn't look like it from the way the reaction is written, but there's actually something important happening with the Pd and the HOAc too.
The Ph3Sb is in a substoichiometric quantity, so you shouldn't expect it to be incorporated into the major organic product. Its most likely acting as a ligand for Pd in this case. The transmetallation wasn't a bad guess though, if it were in a stoichiometric quantity it could be a pretty good possibility. I've seen Ph3Bi used for that before.
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Guitarmaniac86
You're actually not doing too bad, don't get discouraged! Usually unless an olefin is in conjugation with an e- donor or acceptor, or has a leaving group at a vinylic or allylic site, its going to be less reactive than an alkyne. When I see one I usually use that as the initial site of attack. Also, It doesn't look like it from the way the reaction is written, but there's actually something important happening with the Pd and the HOAc too.
The Ph3Sb is in a substoichiometric quantity, so you shouldn't expect it to be incorporated into the major organic product. Its most likely acting as a ligand for Pd in this case. The transmetallation wasn't a bad guess though, if it were in a stoichiometric quantity it could be a pretty good possibility. I've seen Ph3Bi used for that before.
Is the DBA on Pd imparting chirality to the reaction, such that this is an asymmetric synthesis?
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The dba is an achiral ligand for Pd
http://en.wikipedia.org/wiki/Tris%28dibenzylideneacetone%29dipalladium%280%29
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It's very hard to predict the stereochemical outcome of this reaction. I can't see any steric or stereoelectronic effect that would result in a diastereoselective formation of the poly 5-membered cycle.
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Guitarmanic, there are a number of problems with your mechanism - primarily a) You have Pd(III) intermediates, and b) watch your charges and implicit hydrogen - if you follow your curly arrows, your intermediates would have isolated charges and pentavalent carbons.
Well done to Rory, who is essentially correct. An allene is not formed, the alkyne undergoes hydropalladation, rather than loss of methoxide.
The chemistry is Trost and Shi's "zipper reaction": Link (http://pubs.acs.org/doi/abs/10.1021/ja00002a064)
I have shown the general scheme first. Following hydropalladation, intramolecular carbopalladation of the alkene forms a 5 membered ring, and beta-hydride elimination reforms the alkene in a Heck-like transformation.
In the second scheme, following the intramolecular carbopalladation Trost's alkene does not have a beta-H, but there is another suitable alkene for further intramolecular carbopalladation. So, a series of seven sequential intramolecular carbopalladation reactions constructs seven cyclopentane rings - it is only when the Pd reaches the final alkene that there is a beta-H to eliminate.