[jinclean]

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

[a student]

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

[KritikalMass]

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.

[orgopete]

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, 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.

[jinclean]

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

[KritikalMass]

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.

[jinclean]

Yes but i am confusing about the C-P and the C-O  forming simultaneously,the accurate mechanism about it

[Heory]

yes, phosphore likes to bond with oxygen very much.

[Ulfsaar]

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.

[KritikalMass]

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.

[orgopete]

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.
         


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.

[KritikalMass]

I would like to offer another possibility(s).

      

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.

[orgopete]

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.

[Heory]

 

[jinclean]

Hi Heory!
Do you have some document about the coupling in your pocture? it's so amazing!