KOMe from K2CO3 and possibly MeOCO2iPr?
The solubility of potassium bicarbonate in alcohols is extremely low. So, when K2CO3 is added to anhydrous methanol, the precipitation of KHCO3 drives
the formation of KOMe. The attached paper from Platonov et al 2002 describes how to prepare potassium methoxide from K2CO3 and dry methanol. The
mixture is filtered, concentrated, filtered (to remove K2CO3), and distilled to dryness to leave KOMe.
Unfortunately, the insolubility of KHCO3 prevents the simple preparation of unsymmetrical alkyl carbonates from MeOCO2-. The equilibrium constant
(Sauers 1975)
[H2O][MeOCO2-]/[MeOH][HCO3-] = 3.57
implies that most potassium bicarbonate in methanol solution will convert to MeOCO2-, because [H2O] is approximately equal to [MeOCO2-] and both are
much smaller than [MeOH]. However, the extremely low solubility of potassium bicarbonate in methanol (0.01M at reflux) prevents a direct reaction from
being used to obtain potassium methyl carbonate. The half-life of methoxocarbonate in water is approximately ten minutes; that means that (since the
reverse reaction should have a similar rate) about 14 hours of refluxing (1000 minutes) over a very strong dehydrating agent (ensuring <0.01M H2O)
would be required to convert a mole of KHCO3 to KMeCO3. Sauers et al show the conversion proceeds exclusively through the intermediacy of CO2, so such
a procedure may result in significant loss of CO2.
But the reaction of CO2 with KOMe is nearly instantaneous (same paper), so gassing the solution of KOMe obtained via K2CO3 with (dry) CO2 should
rapidly produce KMeCO3, which ought to decompose quite slowly as long as the methanol is reasonably dry. This opens up the possibility of alkylating
KMeCO3 with alkyl halides. Similar reactions have been done with the KHCO3/N(Hexyl)4Cl/dimethylacetamide solvent system (Cella & Bacon 1983),
though, as we're avoiding KHCO3, we should be able to omit the PTC.
The reaction of isopropyl bromide with a typical carboxylate (mesitoate) in HMPA takes all day at room temperature; with isopropyl iodide, however,
the reaction is complete in less than an hour (Shaw et al 1973).
Methyl isopropyl carbonate boils at about 117 C. This is useful because dimethyl carbonate typically requires a temperature of about 120 C to act as a
methylating agent; refluxing DMC is not sufficiently hot at standard pressure. Refluxing MiPC, however, probably methylates. Higher alkanes may be
used to obtain even higher-boiling carbonates. Methyl benzyl carbonate may also be interesting; BnCl is very reactive. Potassium iodide catalysis may
be applicable.
No paper discusses the solubility of KMeCO3 in methanol, but I assume it is similar to potassium acetate, which is very soluble in methanol. And if
the esterification doesn't work, at least there's a new way to make potassium methoxide :p
EDIT: Tundo and Selva 2001 tell us that unsymmetrical methyl alkyl carbonates tend to act as methyl-transfer reagents, which is nice.
Attachment: sauers1975.pdf (929kB)
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Attachment: platonov2002.pdf (27kB)
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Attachment: cella1984.pdf (575kB)
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Attachment: shaw1973.pdf (191kB)
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Attachment: tundo2001.pdf (229kB)
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[Edited on 12-9-2017 by clearly_not_atara]
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