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Author: Subject: Metal/acid reduction of benzoin
Newton2.0
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[*] posted on 19-2-2021 at 14:41


karlos³! I am enamored with this! I tried both methods and I have found that Zn in AcOH is the most efficient method, at least for me.

Tin/HCl Reduction of Benzoin:

-53g benzoin
-53g Sn powder which was obtained by using a bandsaw to make the powder and using a sieve to try and obtain the finest particles (These were on average likely larger than 100 mesh, however, there were some fine particulates which were in the 100-200 mesh range)
65mL EtOH (absolute)
65mL 32% HCl

Refluxed this mixture for approximately 35 hours and additional portions of Sn powder totalling 15g were added to compensate for the large particle size of the Sn.

A slightly oily, yellowish material is present and it appears the reduction is taking place.



In a separate run, I have tried the GAA/Zn mix and have found it far superior in both reaction time and simplicity. I ran the reaction similarly to SC, refluxing for approximately an hour.

Zinc/GAA Reduction of Benzoin
-29g benzoin
-40g GAA
-6.6g Sn (325 mesh) activated by immersion in 3-5% HCl and swirled around and then decanted and washed with dH2O.

An oily, yellowish precipitate was noted and upon pouring the mixture into a beaker of warm dH2O, a mass of chunky, off-white solid was formed. This will be collected and recrystallized in boiling EtOH at which point the solids will be dried and weighed and a melt test will be performed.



I'm excited to follow up with this method! I have seen others discussing additional reactions involving tosylates, etc. in an attempt to improve amination yields. I think this is a really promising avenue of discovery for the amateur chemist!


I wonder if piperidine can be used in place of DMA ala Chemplayer's piperidine ephedrone synthesis. And I will look into the effects of various N-, and N, N-substitutions.
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mr_bovinejony
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[*] posted on 19-2-2021 at 15:53


I've found that adding the metal in increments helps a lot during reduction. I'm excited that someone else is trying this! I have deoxybenzoin oxime sitting around still but I think I'm going to try again with nabh4 and molecular seives first when I get back to this project
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Newton2.0
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[*] posted on 20-2-2021 at 09:21


I haven't gotten to the point of the reductive amination yet.

I do somewhat wonder about the presence of Sn and Zn in the deoxybenzoin. Hopefully, recrystallization will separate this out. I have to do a few more of these because it seems pretty dirty still.

I have found a very interesting bit of information for the reductive amination step. Probably common knowledge to most of you but here it is:

https://www.organic-chemistry.org/abstracts/literature/363.s...
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karlos³
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[*] posted on 20-2-2021 at 11:42


As for the enamine/imine formation, you want to do that separately before over molsieves or under dean-stark conditions(for higher boiling amines like piperidine).
Lower boiling amines like ethylamine and dimethylamine require to be left in, for example DCM, over molsieves for a week or the like, then the organic solvent is removed and the enamine/imine used as it is in alcohol.
This is the key to achieve a moderate yield, it is a bit discussed over at HL with references.

We haven't done this though, none of us, at least yet.
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Newton2.0
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[*] posted on 13-4-2021 at 10:13


So, I decided to run the Zn/GAA reduction again but I did not properly activate the Zn grains. As a result, the amount of Zn at the bottom of the flask seems to be unchanged after a few hours.

As this is the last of my benzoin, I must salvage the reaction. I added a small amount (approximately 2mL) of 32% HCl dropwise in hopes of etching the Zn enough to activate it. Hopefully, this plan works and doesn't screw the whole thing up.

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