Some years ago while learning about the 'Hock process' I had an idea that a similar methodology might be exploited to make simple dialkyl carbonates
and phenol.
The hypothetical methodology would go as follows:
Condense methanol with benzaldehyde under acidic conditions to generate the corresponding ketal.
Sparge in oxygen and stop the reaction before large concentrations of the resulting hydroperoxides accumulates, i.e. ?< 10%? (or else BOOM!).
Hydrolyse the hydroperoxide under mild conditions using acid and cooling to yield dimethyl carbonate and phenol.
Neutralise and isolate the products by distillation. b.p dimethyl carbonate 90°C, phenol 181.7 °C
Condense methanol with benzaldehyde under acidic conditions to generate the corresponding ketal.
Just for information, these compounds are called acetals. "Ketal" is an old term used specifically for acetals derived from ketones, but IUPAC now
discourages its use.
Quote:
Sparge in oxygen and stop the reaction before large concentrations of the resulting hydroperoxides accumulates, i.e. ?< 10%? (or else BOOM!).
No need to dramatize. Such high MW hydroperoxides are generally not prone to detonation, utmost they deflagrate upon heating. They can be safely
isolated.
The hydroperoxidation of acetals is relatively well described in the literature, for example: DOI: 10.1016/S0040-4039(01)97316-2 (with O2
and with AIBN as initiator). I could find no example of hydroperoxidation of PhCH(OMe)2 and PhC(OMe)2OOH is not a known
compound. There is one article describing the preparation of hydroperoxide t-Bu ethers from various acetals including benzylic ones: DOI:
10.1002/adsc.201200410 (excellent yields for cyclic acetals). The plain hydroperoxidation of ethylene acetal of benzaldehyde (1) is
described in DOI: 10.1002/cber.19580910922 (in German, uses O2 with UV initiation).
Quote:
Hydrolyse the hydroperoxide under mild conditions using acid and cooling to yield dimethyl carbonate and phenol.
The hydrolysis of the hydroperoxide of ethylene acetal of benzaldehyde (1) with aq. H2SO4 is described in the
last article cited above and gives glycol monobenzoate (2) and hydrogen peroxide. Treatment with nonaqueous acids such as
BF3.Et2O is not described, but a substructure search of your proposed reaction to give phenols gave zero results.
1:
2:
A hypothesis without references is not truly a hypothesis, it is just an idea. But why don't you do some literature work before posting ideas?
Edit: The formation of PhCH(OMe)2 requires the transacetalization on benzaldehyde with orthoesters such as trimethyl orthoformate. It is
not simply a condensation of methanol with benzaldehyde which is too reversible for preparative use. Such direct condensation is however possible with
1,2- or 1,3-diols (e.g., ethylene glycol), with which the acetals form in a more favorable equilibrium, but the reaction still requires the removal of
water (reflux in toluene or xylene with Dean-Stark trap). This can more or less efficiently work also on less volatile mono-alcohols, but methanol is
too volatile.
[Edited on 6/8/2015 by Nicodem]deltaH - 6-8-2015 at 02:59
A possible application for photoresists?
"A" mixed with a photosensitive initiator could be painted on as a thin film to a substrate. Then exposed to a suitable wavelength of light when open
to air. Then developed with heat to a much tougher polyphenolic-carbonate and undeveloped portions removed by hydrolysis.
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Thank you nicodem for a very nice discussion, I only saw it now. Yes a literature review would have been more prudent on my part and
point taken (apologies)!
I'm happy to hear that they are not prone to detonation, that helps!
I was originally thinking of the traditional acetals that could be prepared from formaldehyde or acetaldehyde, but decided to write with benzaldehyde
as this had the closest relation to the Hock Process scheme.
It's just a rough idea at this stage and the above application also very much juvenile at this point, but just to illustrate a possible direction in
my thinking aside from simple reagent formation application.
In light of what you said i.r.t the benzaldehyde condensation, this doesn't sound like a good substrate to work with, but interesting what you said
i.t.o glycol and published rearrangement to the ester, digesting it for now...
Thanks for the excellent help!
The above schema's are probably junk, rethink/rework then...
[Edited on 6-8-2015 by deltaH]byko3y - 6-8-2015 at 09:26
What you want to do is acutally a https://en.wikipedia.org/wiki/Dakin_oxidation , and it won't work for non-activated aromatics, because there no possibility for any carbocation -
benzylic position is electron defficient, ring is not activated. You need either tertiary carbon on benzylic position for Hock rearrangement, or
activated aromatic for Dakin. The products for Daking are phenol and formate/orthoformate.deltaH - 6-8-2015 at 10:22
I did not know about the Dakin oxidation, thanks for the info byko3y!byko3y - 6-8-2015 at 17:51
If somebody finds this thread and is bothered by the question "what will happen when I try to peroxidize the benzaldehyde derrivative?" - nicodem
already described the reaction, benzoic acid derrivative will be formed. http://onlinelibrary.wiley.com/doi/10.1002/recl.19280470403/...
It will be an ester in case of acetal, and benzaldehyde peroxide will yield a regular benzoic acid (reaction happens when you store benzaldehyde
whithout a stabilizer).
