Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: jinclean on October 31, 2009, 11:35:06 AM
-
the reaction named by perkow is a wildly used reaction in organophosphorus ,but the mechanism is very strange and vague .i need some help to explain the mechanism(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fwww.chemdrug.com%2Fdatabases%2FOrganicReactions%2F20051126203231109.gif&hash=ed77e8671c78803ff3dec09afcd7c996157e8b4c)
-
the only thing that I know about phosphore is: it likes to bond with oxygen very much.
here is some information about it's properties, I hope that helps a little.
http://en.wikipedia.org/wiki/Phosphorus#Chemical_bonding (http://en.wikipedia.org/wiki/Phosphorus#Chemical_bonding)
-
There is an error in the way you wrote out your reaction. The reaction is between a triakyl phosphite ester and a haloketone. As you have your reaction written now it is lacking the trialkyl phosphite ester. I do agree though that it has a strange reaction mechanism. Do you have an example of the mechanism that you can upload? The example on Wikipedia is very confusing because they are leaving out the keto-enol tautomerization steps.
-
This is an interesting question and one in which I spent a lot of time in thinking about a related reaction of phosphorous tribromide and an alcohol. The problem I wrestled with was whether attacks on phosphorous should occur from a tetravalent form rather than a trivalent form (as PBr3 itself). The Arbusov reaction was one I was familiar with in which phosphorous reacts first as a nucleophile (also in PBr3 with Br2, or PPh3 with CCl4, and others).
As indicated in the Perkow Wikipedia reference, (http://en.wikipedia.org/wiki/Perkow_reaction) it is the initial step for the Perkow reaction as well. The mechanism does not show how the cleavage of a P-C bond occurs, but it does indicate from examples some groups that undergo the Perkow (rather than the Arbusov) reaction. The examples have stabilized enolates. Presumably then, addition by oxygen of an enolate to the phosphonium salt could give a pentavalent phosphorous and loss of the enolate with cleavage of the P-C bond would revert back to a phosphonium salt. Dealkylation would give the phosphoryl enolate.
The driving force of the reaction could then be the preference of phosphorous to be bonded to oxygen rather than carbon would drive this equilibrium toward the enol phosphate.
-
Thanks for your reply!
yes,i;m confusing is the tranform tabout the ketone and the enol by the phosphate shift,this is so strange,but only do i have an example is the wikipedia documents,but i know another article in chem rev :The Chemistry and Properties of Enol Phosphates.
the link is :http://pubs.acs.org/doi/abs/10.1021/cr60214a004?prevSearch=perkow&searchHistoryKey=
i can't download it,so i don't know the details in it
-
I'll just post the mechanism from Wikipedia here. Currently I am baffled by how the C-P+ bond is broken and how the new O-P bond is formed.
I think what is going on is that one of the double bonds on the ketone attacks the P+ intramolecularly, and the bond between the C-P+ cleaves simultaneously forming the new C=C. However, that is just a hypothesis.
-
Yes but i am confusing about the C-P and the C-O forming simultaneously,the accurate mechanism about it
-
yes, phosphore likes to bond with oxygen very much.
-
Hello Heory!
your mechanism is possible,but i think the phosphorus cation,is steadier than the four members oxacyclonion.while the chlorin acion attack the alkoxy group is slower than the cyclo-opening,so the reaction can't occur at the same time.
-
While Heory's mechanism is interesting, think that it is wrong in some respects, mainly in that think the phosphorous does not attack the oxygen in the initial step.
-
I would like to offer another possibility(s).
I don't know that I could favor one or the other exclusively. In the hexachloroacetone example posted in Wikipedia, I would favor direct attack on the chlorine. On others, I might expect attack on carbon and if some enolate could be generated, it could be used to equilibrate to the enolphosphate. The dealkylation step would stop the equilibration process.
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fwww.curvedarrow.com%2Fchem%2FZZ5487C216.jpg&hash=69ad89b919d056955959d7e89c05c33885b8a9d0)
As I noted in my earlier post, I was persuaded by the dealkylation of phosphonium salts that the PBr3 mechanism seemed like it ought to be like this Perkow reaction.
On one of the proposed mechanisms, I am not aware of any nucleophiles delivering electrons to an oxygen of a carbonyl group. If anyone can give a reference to a process of this type, please reference it.
-
I would like to offer another possibility(s).
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fwww.curvedarrow.com%2Fchem%2FZZ5487C216.jpg&hash=69ad89b919d056955959d7e89c05c33885b8a9d0)
The problem with this is that you are using a halodiketone. But I like the way it is looking other than the diketone aspect of it.
-
The Perkow reaction is not one that I am familiar with. I simply had noted "The phosphite ester salt is subject to keto-enol tautomerism and if the enol isomer is predominant the Perkow adduct is formed otherwise the keto form results in the Michaelis-Arbuzov adduct." in Wikipedia. The examples they had were from hexachloroacetone and another beta-carbonyl compound.
If the Perkow reaction occurs with ethyl bromoacetate, then I wouldn't have any problem with using it there also. I just thought that it gives the Arbusov product. I believe bromoacetone does also. Somewhere there must be mixtures and a swing to the Perkow. If someone could advise me, I'll remove the second carbonyl.
-
:)
-
Hi Heory!
Do you have some document about the coupling in your pocture? it's so amazing!
-
hello! KritikalMass
i think the Diketone might have the same mechanism as any other ketone, but i have never seen the circumstance which the five phosphor can reactive as a reductive reagent.is it possible in mechanism?
-
# Heory
the phosphite ester is a nucleophilic reagent in the case of perkow reaction,why does it attact the carbonly group and just attact the O in C=O? and the zinc might attack the α-carbon substituted by the bromine ,just as the reformasky reaction ,and in the acetate acid ,the organozinc is destroied by the H+,so we can get an enolate substance,instead of the zinc attact the carbonly to this product
-
:)
-
Re: Phosphorous as a reductant
That is a fairly common reaction. PCl3 + Cl2 :rarrow: PCl5
or
Diethyl diazodicarboxylate + PPh3 :rarrow: hydrazide + triphenylphosphine oxide
Re: Electron donation to oxygen of carbonyl
I generally think of carbonyl groups having a resonance structure with a negative charge on the oxygen and positive on carbon. Hence, I did not expect to add electrons to an electron rich atom. I had not thought of the hexafluoroacetone example cited, kudos. Given that reaction, I too would be inclined to donate electrons to the oxygen to generate a negative charge on the carbonyl carbon for the condensation step. That example seems quite rational since there are two electron stabilizing trifluoromethyl groups.
While the mechanism accomplishes the goal of generating an enolate with zinc, I do not find it a compelling argument. Zinc will also reduce benzyl bromide to toluene. Thus, it would seem reasonable for zinc to insert in the carbon-bromine bond (if not adding directly to bromine). Iodide is a good reductant for dehalogenation, presumably by attack of iodide on an a bonded iodine.
-
I agree with orgopete- the mechanism Heory is proposing is at least an attempt at solving the problem, but still think it is fundamentally wrong. I am all for an intramolecular attack of the carbonyl oxygen on the P+. I might just have to take a pic to show what I am talking about.
-
:)
-
:)
-
Re: KriticalMass intramolecular rxn
I agree with Heory about the missed charge on the 4-membered ring and his doubt of the mechanism. I am doubtful more from a bond angle and electrophilicity point of view. The carbonyl carbon should have a bond angle of about 120°. An intramolecular attack on the oxygen seems unlikely (unless you show some precedent). Will an alpha-halo amide should be more conducive of this reaction?
Re: other mechanisms
I concede to being at a disadvantage in that the only examples I am aware of for the Perkow reaction are those cited in Wikipedia. In the proposed mechanism with bromoacetone, how much Perkow product results in this reaction? Does the Arbusov reaction compete with it?
I am familiar with a bisulfite addition to aldehydes, but I am unaware of a phosphite equivalent. If phosphite does add to the carbonyl, then the reaction of triethylphosphite with bromo or chloroacetaldehyde should be an even better Perkow reaction. Is it?
Re: addition of electrons to the oxygen of a carbonyl
I would argue with Woodward on this one. While that may have been written, I would argue that it goes more to writing a short explanation of what must have happened, but not necessarily how. I am more inclined to think along the lines of a dissolving metal reduction or an acyloin condensation in which electrons are added to the carbon and resulting in breaking the pi-bond. If a second electron were added, it would add to the radical.
This example points to some very real quandaries in proposing mechanisms. The carbonyl group is not particularly basic in this example, so protonation by acetic acid seems an unlikely initial step. The mechanism does not involve acetic acid. Does the reaction succeed without it? Is acetic acid present to "clean" the zinc? There are other similar reductions, like a Clemmensen reduction. For it, I thought it important to protonate a carbonyl group first and then the electrons are added to a increasingly electrophilic carbon.
I do concede that upon searching, it is not difficult to find the proposed mechanism for a Reformatsky reaction. I had been taught that it inserted into the C-Br bond, but that should not be important. Since it now appears that a Zimmerman-Traxler transition state exists for addition of a Reformatsky reagent to an aldehyde, therefore the zinc is complexed with the oxygen and not the carbon. I can understand that a mechanism of addition of electrons to the carbonyl oxygen will result directly in that enolate. Is that compelling to the mechanism? How do electrons add to carbonyl groups? The reason I was skeptical was I can find many examples in which electrons are added to the carbonyl carbon, but I was not aware of a single example of an addition to the oxygen. For example, if it adds to the oxygen, then addition of a methyl Grignard to a bromoketone ought to give the same enol ether product. I don't know that none of that forms, but I don't think it is an important reaction.
Even so, I am not persuaded to believe any of the proposed mechanisms for the quinoline dione example in Wikipedia. An SN2 displacement on a tertiary trifluoroaceate does not seem likely. Direct displacement by attack on the oxygen of the trifluoroacetate does not seem likely. Perhaps addition to the oxygen of the carbonyl group would work, but I don't like that it is an aryl ketone with a vinylogous nitrogen donating electrons as well. That does not mean that one of those mechanisms may not present. It just means I am uncomfortable with matching their electron movements with other chemistry that I have knowledge of.
-
Well, either no one in this thread is correct, or Wikipedia is wrong (which would not be surprising).
-
Touché!
-
Heory- I noticed you spend alot of time over in the graduate level ochem board. Can you ask one of the mods to come over and clear this situation up? And yes, I disagreed with some of your mechanisms but hope my disagreements didn't come across as ad hominem. If they came across that way than please accept my apologies.
This has been quite a thread and looks to me as if the Wikipedia article needs some revisioning.