ziqquratu
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Methylations with Methyl Hydrogen Sulfate
Just been looking around, filling in time, and I stumbled across some things on Organic Synthesis that really interests me.
The following three links are what interested me most:
http://www.orgsyn.org/orgsyn/prep.asp?prep=cv3p0564
http://www.orgsyn.org/orgsyn/prep.asp?prep=cv2p0619
Now, in each of those preparations, they use "methyl sulfate" as the methylating agent. It's used to methylate two different aromatic
aldehydes - m-hydroxybenzaldehyde and vanillin. On first reading, it just seemed that they were talking about DMS, but something sounded wrong with
that - I have no idea why. So, I looked it up by the CAS number, and guess what? These references are using Methyl Hydrogen Sulfate (or Methyl
Sulfuric Acid, or Monomethyl sulfate, whichever you prefer) to perform these reactions.
Methyl hydrogen sulfate is easily prepared by mixing sulfuric acid and methanol, and adding anhydrous sodium sulfate. There was a prep on the Hive by
Antoncho, from memory, but I dont actually have time to look it up right now. His preparation gives the acid at a density of around 1.2-1.3g/ml,
which is the "technical" or "practical" grade that they use in those references.
Now, looking at the substrates they use, I'm wondering exactly what this agent - previously only useful for making 1,4-dimethoxybenzene or as a
precursor to DMS itself - is actually capable of. If it can methylate vanillin, this may imply it is capable of doing syringaldehyde to
3,4,5-trimethoxybenzaldehyde; likewise, perhaps it would be efficient in methylating 2-hydroxy-5-methoxybenzaldehyde after a Reimerr-Tiemann or
magnesium methoxide formylation of p-methoxyphenol.
Forgive me if this has been covered before - I wasnt able to find it, so I hope that it's new territory. At worst, it's probably something
that should be revisited!
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S.C. Wack
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Unfortunately, this was some screwup made when they computerized. Some nomenclature nazi, likely computer generated, decided that when someone wrote
methyl sulfate, they meant the acid sulfate.
The authors in these cases meant neutral methyl sulfate, the nasty stuff.
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Chris The Great
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Well, assuming you don't mind working with dimethyl sulfate, you can easily turn your methyl hydrogen sulfate into dimethyl sulfate by distilling
at 130 to 140*C under vacuum. The methyl hydrogen sulfate decomposes almost quantitively into dimethyl sulfate and sulfuric acid at this temperature.
According to The War Gases (giving a good indication of the safety of this chemical) the procedure recommends the flask be heated to 140*C
with a 20mm Hg vacuum. The Handbook of Preperative Inorganic Chemistry also has a procedure for distilling methyl hydrogen sulfate into
dimethyl sulfate, IIRC.
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ziqquratu
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Should have known better than to get so optomistic about these things!!
I had another look today with my computer (which has the appropriate plug-in installed) and saw DMS in the reaction schemes. Ahh well, would have
been a good find if it was true!
In any case, synthesis of DMS is simple enough, you're right, assuming proper precautions are taken. Ahh well, back to the vacuum distilling...
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