2,5-Dibromo-1,4-benzoquinone Reactivity?

I have myself reacted chloranal with sodium methoxide in methanol (refluxed). I will try to recall some details for you from memory (it was many years ago):

Although the chloranil didn't dissolve well in the concentrated sodium methoxide/methanol solution I was using, it netherless reacted nicely in time forming a yellow mixture of precipitate and solution under stirring with a bar magnet that also kinda milled the solids on the bottom as it stirred to form a slurry.

After running for some time (can't remember how long) I filtered and washed with an organic solvent (can remember which but probably methanol) to [ultimately] recover a red crystalline flake product highly soluble in organic solvents and a bright white amorphous powder after washing heavily at the filter with water (it was completely insoluble in everything I could try).

IR suggested that the red soluble flakes was indeed tetramethoxyquinone. No surprise, it looked very much like chloranil shiny plates, simply a beautiful deep red colour.

The question was what was the second super bright white amorphous looking insoluble in everything product I had made a lot of that too.

IR of this material suggested it was also heavily methoxylated, but it had clear hydroxyl note on the IR, so I concluded I had over methoxylation here attacking the carbonyl as well and upon washing it released the sodium and formed a hemiacetal that proved to be very insoluble and very pretty white and non-crystalline.

This should be reversible then and indeed, heating the white product in a test tube under flame caused vigorous boiling upon melting yielding the same red crystalline product in time as previously obtained (the tetramethoxyquinone). I am thus fairly sure that methanol was eliminated from the hemiacetal (seen as a vigorous boiling) and so this was no problem and I could convert the white product into what I wanted easily.

Long story short, don't be surprised if you possibly obtain a hellishly insoluble white product, in fact, it may be worthwhile to use excess methoxide and target it specifically as it's unavoidable. It is easier to isolate this thing by making use of its extreme insolubility, just wash with copious water, dry and then do the methanolysis by heating and then recrystallise your quinone product.

Hope my suggestions prove useful.

The structures of the two products that I believe were made are:



[Edited on 14-11-2013 by deltaH]

No, these MIGHT work, you won't know till you try The best is to follow a known procedure that has been shown to already work if you want to maximise your chances of success. Failing that, DABCO or TEA are probably half reasonable trial choices but are needed in stoichiometric amounts to affect dehydrohalogenation.

The worry with chloromethanes is the possibility of forming reactive carbenes by the special dehydrohalogenation of an alpha proton... HOWEVER, this is not an easy task at all and requires super bases which DABCO and TEA aren't, so on second thought, DCM might in fact be fine.... again experimentation will tell all!

As you said, I would prefer you don't use DCM, but if you must for solubility reasons, it may well work just fine. So ether is first choice, failing that try DCM.

[Edited on 16-11-2013 by deltaH]

Amines tend to readily attach to the ring position of quinones under basic conditions, accompanied by the reduction of the quinone to it's hydroquinone form. So for DBU, the fast reaction you have read about is probably the formation of:



While for some reason the DABCO version is not favoured, probably because of steric hindrance. The DABCO equivalent that would have formed would be:



In this animation of the 3d structure, the extreme steric hindrance is clear (click on structure to see animation):



[Edited on 18-11-2013 by deltaH]

Thank you DeltaH, that's very helpful.

Below is a rough draft of the proposed reaction scheme:



The question mark specifies where the suitable reagents from this threads' conclusions will come into play.

Quote: Originally posted by vulture

1) Do we have any references for this attack of amines on quinones? If yes, please supply. The proposed reaction involves a proton transfer which I would suspect to be hard under aprotic conditions. 2) I don't see how DABCO would cause more steric hindrance than DBU. 3) You obviously got this reaction from the literature, again, a reference would be nice, plus, which conditions do they use?

No it does not have anything on the reactivity of non-nucleophilic bases per say, just simply supplying the reference as requested, that certain amines are known to add to the 2 and 5 positions of quinones coupled with a reduction of the quinone and then reoxidation by unsubstituted quinones. It's not so easy to find... I had no luck when googling it, for example. I promised before that I would give it a better go after my hectic week was over and so now I could I had to pull it from an old presentation of mine and then find some online source for it.

[Edited on 22-11-2013 by deltaH]

Oh, I see.

Also I feel it is important to note that specifically in this case, from what I've read, ketal derivatives do not work, and neither do 2,2,5 bromine groupings.

From what I know, the only possible brominated derivative of Cyclopentanone that will work for this reaction is the ketone form with 2,3,4 bromine groupings, so using 2,2,5-Tribromocyclopentanone or it's Ethylene Ketal isn't possible.

So, even though there are other derivatives that seem like they would work, it apears that only one will do the job.

Additionally, on the reaction scheme I posted the last step is actually two, hv, and then NaOH, not at the same time.

Below is the revised, more comprehensive version, including steps for 2-Cyclopenten-1-one, as I will have to make it.



So, any suggestions on what will work for the DABCO step if this doesn't seem likely to work?

DeltaH thinks it has good probability, but Vulture, not so much.


[Edited on 23-11-2013 by APO]

...and that Vulture... is why you ask "Are you following published steps up to the cyclopentenone or proposing? Do you know the conditional details and yields?" first

Thanks APO, the reason I asked is because the same concerns occured to me as did to Vulture, except the 1,3 product made sense to me, but I was also worried about the selectivity of the partial oxidation and acid catalysed dehydration.