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Author: Subject: Issues with working up a Beckmann-rearrangement type reaction
Rich_Insane
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[*] posted on 18-9-2015 at 14:31
Issues with working up a Beckmann-rearrangement type reaction


Hey everyone,

So I've posted about amide synthesis a lot. I have run a reaction in which an oxime is rearranged to a primary amide using a Cu(II) catalyst [1]. The reaction itself is very similar to the Beckmann rearrangement. I ran this reaction in xylene (the paper referenced uses toluene and neat conditions regarding the solvent) and recovered a dark green/black solution that tended to "harden" into a granular semi-solid that melts to a liquid when heated. The amides I am working with (see picture below) are soluble in hot water and relatively insoluble in cold water. I have manipulated this property to isolate the amide in three ways: hot filtration, suction filtration of the hot water extract, and passing hot water through the semi-solid recovered upon cooling the reaction mixture. All three of these have yielded some product, but not a lot. The yield was very low.


Cvw0xEI.jpg - 11kB

With hot filtration, the flow rate with the equipment I have was very slow, so slow that the amide crystallized on the filter paper alongside the impurity. A considerable amount was lost when the amide crystallized in the stem of the funnel. Suction filtration of the hot water seemed to work well but the problem is that the xylene/impurities slip through as well. In addition, almost certainly some of the product remains in the impurity. Thus I surmised that the low yields might be caused by incomplete hot-water extraction from the hot water collected after it is boiled with the reaction mixture "semi-solid". I then tried a different method. I took the semi-solid and placed it on a filter paper that had been laid in a typical suction filtration setup. Boiling water was passed through this apparatus and the filtrate was collected. This method yielded the product, but the amount of product relative to the starting material was small.

My question is, how else might I be able to isolate the amide or at least separate it from the rest of the reaction mixture? Possible impurities include the starting material and the nitrile obtained by dehydration of the oxime. Ideally I could just run a column and separate the constituents, but alas, I do not have a column. I am also considering either running the reaction without solvent or boiling away the xylene after the reaction is complete. I'm just looking for suggestions you all might have for me.


(1) -- Tetrahedron Letters 53 (2012) 1413–1416
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AvBaeyer
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[*] posted on 19-9-2015 at 23:25


WHy not just do this in a more time tested manner.
1. convert the oxime to the nitrile with acetic anydride or similar reagent
2. convert the nitrile to the amide with sodium hydroxide/hyrogen peroxide

Your purification woes will be greatly simplified.

Now you are ready to run a Hofmann rearrangement to get your self-medication product.

Safe journey,

AvB

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Rich_Insane
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[*] posted on 20-9-2015 at 11:08


The purpose of this experiment is to try out various divalent metal salts to rearrange the oxime to the amide. Copper acetate is being used at this point simply because it is directly referenced in that paper. My plan is to try metals other than zinc, nickel, or copper to facilitate this rearrangement. I also want to try out different counterions, but one variable at a time is for the best now.

I have tried to do the oxidation of the nitrile using sodium percarbonate. That too creates a mess, but in a different manner.
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