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I need to run multiple aza-Michael additions of methyl acrylate with various amines (dimethylamine, benzylamine, phenylethylamine, aniline, etc.) in order to get the bis-ester. I was wondering if the inhibitor (MEHQ,hydroquinone, others) that is in the acrylate as-purchased will cause me any problems/prevent the intended reaction? I asked my coworkers and no one can agree on if I should remove it or not.

Oh, and I’m using methanol as my solvent.

Thank you!!

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2 Answers 2

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It is common for the hydroquinone inhibitor to be added to aclates to prevent polymerization during storage. It is possible that MEHQ could react with the amine and hinder the aza-Michael addition. To minimize the potential interference, try removing the inhibitor first to prevent side reactions.

You mentioned that you are planning to use methanol as your solvent. In aza-Michael reactions, the amine acts as a nucleophile and attacks the electrophilic carbon of the Michael acceptor. Methanol can act as a proton donor and protonate the amine, making it less nucleophilic. Have you considered using THF or DMSO instead?

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    $\begingroup$ Methanol protonate the amine? Fat chance of that. Even MeHQ would do that only slightly. $\endgroup$
    – Mithoron
    Dec 18, 2023 at 1:50
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I would not worry that the typical polymerization inhibitors added to acrylic esters will interfere with the aza-Michael reaction. I guess you will add the amine in excess, and the relatively small amount of the inhibitors, if they react at all (which I doubt), will do no harm. Depending on your reaction conditions, you may even want inhibition of the radical polymerization, for example at elevated temperatures.

I also assume you will have effective workup procedures to isolate your products in pure form, so you may want to pay attention to remove the inhibitors then so that they will not disturb whatever you plan later on. After the double bond is lost due to the reaction, the inhibitors are not needed any more.

Something else to keep in mind: Your reaction products are $\beta$-aminoesters which are much more prone to hydrolysis or alcoholysis than their counterparts without the amino group.

I have used various acrylic esters successfully in the past in Michael-type addition reactions and never bothered with removing inhibitors first.

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