Sciencemadness Discussion Board - Improving yields of TMP methylation?

Improving yields of TMP methylation?

horribilis - 1-4-2019 at 16:05

Not too long ago I attempted o-methylation with trimethyl phosphate (TMP.) US patent US4453017A was used as an inspiration with 4-hydroxy-3,5-dimethoxybenzaldehyde as the substrate.

The setup was a 3-neck rbf, pressure equalizing addition funnel, reflux condenser with CaCl2 drying tube, and magnetic stirring over a heating mantle. To the addition funnel was added 17 ml (90 mmol) TMP. The funnel was stoppered and then 10.4 g (57 mmol) 4,-hydroxy-3,5-dimethoxybenzaldehyde and 15.0 g (108 mmol) freshly fused K2CO3 (heated in 250C oven for 1 hour) were added to the flask. The flask was stoppered and heated to ~80C with the goal of carrying out melt phase methylation. Once melted, TMP was quickly added to the flask but stirring/mixing was still quite poor. About 35 ml of anhydrous DMF was charged into the addition funnel and this was immediately added to the mixture. The reaction was allowed to reflux for ~3 hours with constant stirring. The solution was then cooled for several minutes, dumped into a beaker, and then crashed out with 300ml of RT water. The beaker was cooled to room temperature then placed into a freezer until 0C. Crystals were separated with vacuum filtration and washed with several portions of cold water. As they looked visibly impure, the wet crystals (11.6 g) were dissolved in minimal hot methanol (~35 ml) and then RT water was added until cloudiness persisted (~35 ml.) The beaker was once again cooled to RT and placed into a freezer to 0C. The crystals were vacuum filtered, washed with small portions of water, and dried overnight to yield 6.7 g of 3,4,5-trimethoxybenzaldehyde (53% yield) as off-white or slightly yellow plates, MP 65-67C (lit. 73C.)

Obviously this is a poor yield. The patent reports a 92% yield. Although an inert atmosphere is used in the patent, I found record of someone else carrying out this reaction without inert atmosphere and with a higher yield than the patent, 95% reported: https://www.sciencemadness.org/whisper/viewthread.php?tid=19...

I am wonder what I can do different (or should do differently) to improve yields? My thoughts are:

1. I don't have access to a true inert atmosphere, although I can do an initial purging with relatively inert gases (CO2/argon blend) and then seal it with stoppers and a balloon over the reflux condenser, assuming it isn't immediately destroyed by DMF vapors.

2. The SM post isn't clear about the reflux temperature, but my reaction did reflux quite heavily for an hour before I lowered the temperature. The patent recommends 105C for 3 hours. Would lowering the reaction temperature to 105C improve yields? I'm not sure.

3. I wonder if conducting this experiment in solvent rather than melt phase, as the patent suggests, is critical? I am sure my solvent is anhydrous or close to it (dried over sieves for several days.)

Any thoughts or input would be appreciated!

[Edited on 2-4-2019 by horribilis]

[Edited on 2-4-2019 by horribilis]

bipolar - 3-4-2019 at 02:53

1. IMO, inert atmosphere is unnecessary here.

2. It is a bad idea to reflux DMF as it will decompose quite significantly (presense of K2CO3 doesn't help with that either). You will get more impurities in your final product at the very least.

3. As I see it, methylation without solvent leads to higher concentration of reactants -> faster reaction rate...

Quote:

wet crystals (11.6 g) were dissolved in minimal hot methanol (~35 ml) and then RT water was added until cloudiness persisted (~35 ml.) The beaker was once again cooled to RT and placed into a freezer to 0C. The crystals were vacuum filtered

Are you sure there was no product left in the filtrate?
This aldehyde was quite soluble in 40% aqueous IPA for me.

Also your aldehyde is very crude, judging by mp...

horribilis - 3-4-2019 at 03:21

Quote: Originally posted by bipolar

1. IMO, inert atmosphere is unnecessary here.

2. It is a bad idea to reflux DMF as it will decompose quite significantly (presense of K2CO3 doesn't help with that either). You will get more impurities in your final product at the very least.

3. As I see it, methylation without solvent leads to higher concentration of reactants -> faster reaction rate...

Quote:

Are you sure there was no product left in the filtrate?
This aldehyde was quite soluble in 40% aqueous IPA for me.

Also your aldehyde is very crude, judging by mp...

The solubility of 3,4,5-trimethoxybenzaldehyde in water is reported as something like 1490mg/ml. I don't think there would have been much in the filtrate but I could be wrong. Yes, MP was crude. I tried recrysallizing a few more times from hot methanol (no crashing out with water) and it did not improve. Perhaps recrystallization from hexanes/heptanes would be more appropriate, or short path vacuum distillation as a last resort.

[Edited on 3-4-2019 by horribilis]

bipolar - 3-4-2019 at 03:38

Quote: Originally posted by horribilis

Perhaps recrystallization from hexanes/heptanes would be more appropriate, or short path vacuum distillation as a last resort.

Just do purification via bisulfite adduct, it works well for this aldehyde (only use EtOAc as solvent).

horribilis - 3-4-2019 at 03:55

Quote: Originally posted by bipolar

Quote: Originally posted by horribilis

Perhaps recrystallization from hexanes/heptanes would be more appropriate, or short path vacuum distillation as a last resort.

Just do purification via bisulfite adduct, it works well for this aldehyde (only use EtOAc as solvent).

Does bisulfite adduct work to purify? I understand it's used to prepare aldehydes for storage but I've never heard of using it for purification. What sort of conversion % do you get from aldehyde to bisulfite adduct and then back to aldehyde?

bipolar - 3-4-2019 at 04:17

Quote: Originally posted by horribilis

Does bisulfite adduct work to purify? I understand it's used to prepare aldehydes for storage but I've never heard of using it for purification.

Why wouldn't it?

Quote:

What sort of conversion % do you get from aldehyde to bisulfite adduct and then back to aldehyde?

I'm not sure exactly, conversion should be very high when procedure done correctly.

Dissolve your crude aldehyde in small amount of EtOAc, to this add concentrated sodium bisulfite (min. 3 eq) in water, stir overnight.
1) If there's no adduct precipitate (like it was in my case with this aldehyde), then just separate water layer, wash 2 times with EtOAc and basify with a little excess of aqueous alkali, you'll get your pure aldehyde.

2) If there's precipitate, it is probably best to filter it, wash it several times with EtOAc, dry it and then suspend in water layer (separated from the filtrate) and basify this suspension.
Instead of filtering, you can also just add more water until adduct dissolves completely, separate water layer etc...

Also you can prepare sodium bisulfite solution by mixing aqueous sodium sulfite and acetic acid.

This all information above is from my personal experience.

[Edited on 3-4-2019 by bipolar]

horribilis - 6-4-2019 at 17:47

Same reaction was attempted under slightly different conditions.

The reflux condenser was topped with a balloon. Into the 500ml 3-neck round bottom flask went 13.6 g syringaldehyde (MP 107-110.5C uncorrected -- somewhat impure compared to lit. 110-113C), 13.4 g fresh anhydrous K2CO3, and 29 ml anhydrous DMF. Via addition funnel, 12 ml TMP added over a 5-10 minute period. During addition and stirring (3 hours), reaction temperature was always between 100-110C, much lower than the previous attempt. Flask contents dumped into 350ml of room temperature water and the flask rinsed with an additional 75ml of water. Refrigerated for about 45 minutes then vacuum filtered. Wet crystal mass was taken at 9.02 g. After drying under vacuum, crystals are now pretty dry at 7.00 g but will continue to dry overnight. A MP will be taken in the morning. The crystals look much, much purer than the last run; hoping they don't have to be purified as this will result in even more loss. If necessary, recrystallization will be from pet. ether (65-135C fraction) as there's no ethyl acetate on hand for a bisulfite adduct (yet.)

It was also considered that the filtrate still held some product, so this mixture was distilled until all water driven off, then vacuum distilled to drive off approximately 50% of the DMF at 75C. Reaction mixture turned red (no clue why), which was crashed out with a small amount of water, and placed into a freezer. A few needles precipitated but nothing else -- what a waste of time.

Next run will be in a much smaller flask so melt phase methylation can be attempted. No access to a smaller 3 neck flask, so this will be attempted with a 50ml or 100ml single neck round bottom flask, by swapping out addition funnel and reflux condenser quickly. Hoping the melt phase reaction shows a better yield as this is kind of a pain in the ass to carry out. Methylation with dimethyl carbonate on this same substrate shows better yields, although this reaction has yet to be carried out personally. https://www.sciencemadness.org/whisper/viewthread.php?tid=22...

[Edited on 7-4-2019 by horribilis]

bipolar - 7-4-2019 at 07:32

IMO, when doing this procedure with a solvent (DMF) you don't have to add TMP via addition funnel (just mix everything at once at RT and proceed with the procedure).

And you are using finely grinded K2CO3, right?

Quote: Originally posted by bipolar

Are you sure there was no product left in the filtrate?

I hope it was clear that I was talking about the filtrate after recrystallization from aq. MeOH.

Quote: Originally posted by bipolar

This aldehyde was quite soluble in 40% aqueous IPA for me.

I checked my lab records and did some calculations for the exact info:
~67 mL of 37% aq. IPA contained at least 2.67 g of 3,4,5-TMBA dissolved in it.

Quote: Originally posted by horribilis

[...]so this will be attempted with a 50ml or 100ml single neck round bottom flask, by swapping out addition funnel and reflux condenser quickly.

Can't you just add TMP through the top of the condenser tho? (via syringe or something, so there's no excessive pressure build-up in the system of course)

[Edited on 7-4-2019 by bipolar]

horribilis - 7-4-2019 at 08:04

MP of the 6.92 g "much, much purer" crystals was taken, something like 65-67 C. The impure crystals were then recrystallized from ~175 ml of hot water, as recrystallization from water was observed in a journal article. Small brown oily droplets were observed on the bottom of the beaker, upon addition of boiling hot water. (In hindsight, less than 175 ml of water could have been used.) The hot water was carefully decanted and the remaining small amount of water and oily droplets were passed through a filter paper, which collected the droplets quite well. Not sure if these droplets were impurities or just the dissolved 3,4,5-trimethoxybenzaldehyde behaving weirdly -- the crystallized oil droplets were set aside and saved for this reason. Will report back with % recovery and another melting point tomorrow.

The K2CO3 is anhydrous microprilled, photography grade. It was noted that the K2CO3 didn't fully dissolve in the reaction mixture until 1 hour into the 3 hours heating/stirring. No access to TLC or the reaction progress would have been monitored. Your comment and my observation now makes me think that perhaps it wasn't allowed to react to completeness? This may also explain why the yield of impure crystals in the first trial (at the ~150 C DMF reflux temperature) was much greater than the impure yield of the second trial (at ~100 C).

Yes, TMP could be added through the condenser -- good idea. Syringde will have to be used, as you suggested, as the patent recommends maintaining a constant reaction temperature.

Edit: It seems that water was a poor choice of solvent for recrystallization, as small prills of something (K2CO3?) were locked into the fluffy crystal matrix. Recrystallization from petroleum ether will be tried next, and then bisulfite adduct on a separate trial.

[Edited on 7-4-2019 by horribilis]

bipolar - 7-4-2019 at 20:43

Those 'oil droplets' are probably undissolved [crude] melted 3,4,5-TMBA. This compound should have pretty low solubility even in boiling water.
edit: Or maybe not

, if your reference says that this amount of aldehyde could be recrystallized from 175 mL of water. But I still think that these droplets contain a lot of the product.

Quote: Originally posted by horribilis

It seems that water was a poor choice of solvent for recrystallization, as small prills of something (K2CO3?) were locked into the fluffy crystal matrix.

It's definitely not K2CO3 as it's highly soluble in water.

It's very unlikely, but maybe your impurity is unreacted syringaldehyde? Unlikely - because it should have formed phenolate with K2CO3.
Maybe, just to be sure, you can try to dissolve your crude aldehyde in suitable solvent (DCM, Et2O, etc.), wash it 2 times with 3-5% aq NaOH, dry organic solution, evaporate solvent and see if melting point of the aldehyde raised.
Recovery will bee near 100% if there's no phenolic (any acidic) impurities.

Recrystallization from petroleum ether is probably a good idea, but be aware that it will be more like hot extraction-recrystallization as colored impurities won't dissolve in it. Recovery should be very high too as 3,4,5-TMBA should have very low solubility in pet. ether at RT°.
It will probably take a lot of solvent tho, but I'm not sure.

[Edited on 8-4-2019 by bipolar]

clearly_not_atara - 7-4-2019 at 22:14

I'm gonna disagree with bipolar about the inert atmosphere -- aldehydes are oxidized by air, and the carboxylic acid is more hydrophilic than the aldehyde, causing it to be retained in water if it's formed. It's probably not necessary, but I figure it helps.

https://www.fpl.fs.fed.us/documnts/pdf1998/batte98a.pdf

Try forming the bisulfite adduct to selectively precipitate aldehydes in the presence of oligomers and carboxylates.

[Edited on 8-4-2019 by clearly_not_atara]

horribilis - 8-4-2019 at 03:45

The recovery of crystals (from the filtrate which was separated from the oily droplets) was something like 2-3 g, which was a terrible recovery of the 6.92 g of impure crystals. The now-crystallized oily droplets and the 2-3 g of crystals were dumped back into the filtrate and water was boiled off to ~100 ml and left it to cool. The "crystals" were absolutely fucked when they precipitated upon cooling. Tired of dealing with this bullshit for the day, the entire beaker was dumped. Ugh...

bipolar, the NaOH wash is a great idea -- just need to find some low BP nonpolar solvent. (DCM is pretty much impossible to find OTC these days.) atara, yes, it now seems like bisulfite adduct is the best method for purification. Interesting paper, too.

For sure, the next run will be solvent-less, in an appropriately sized flask, with an initial purging of an inert atmosphere, and for 3 hours after all K2CO3 is dissolved. Hopefully the ethyl acetate and sodium bisulfite will arrive soon so the bisulfite adduct can be used.

bipolar - 8-4-2019 at 12:10

Quote: Originally posted by horribilis

and for 3 hours after all K2CO3 is dissolved

K2CO3 won't dissolve and nor it should; just grind it (e.g. manually in a mortar) and it will be fine for the reaction.

[Edited on 8-4-2019 by bipolar]

horribilis - 8-4-2019 at 17:08

Quote: Originally posted by bipolar

Quote: Originally posted by horribilis

and for 3 hours after all K2CO3 is dissolved

K2CO3 won't dissolve and nor it should; just grind it (e.g. manually in a mortar) and it will be fine for the reaction.

[Edited on 8-4-2019 by bipolar]

Isn't it dissolved as it's consumed? It's understood that it has a catalytic role in the reaction (deprotonation) but it isn't recovered in the end, at least not in the amount it was added at the beginning.

[Edited on 9-4-2019 by horribilis]

bipolar - 9-4-2019 at 02:36

I meant that you will always observe solids in the reaction mixture. So you won't be able to see K2CO3 'dissolving'.
K2CO3 reacts with -OH group of syringaldehyde to form corresponding potassium phenoxide and KHCO3 (solid).
Phenoxide reacts with TMP forming... uhh, I'm not sure... Potassium dimethyl phoshate, I guess? It should be solid too.
And I think that solid phenoxide can be observed too when performing this reaction without solvent.

(also formed KHCO3 will decompose to H2O (+ K2CO3 & CO2) at the reaction temperatures, so there will be propably some hydrolysis of TMP occuring forming solid potassium phosphates.)

[Edited on 10-4-2019 by bipolar]

horribilis - 20-4-2019 at 14:55

Ethyl acetate is on back-order so no attempts at the bisulfite adduct.

The procedure was repeated with some modifications. A 100ml 3-neck round bottom flask was set up with magnetic stirring over an oil bath. A pressure-equalizing addition funnel, thermometer, and a reflux condenser capped with a balloon were affixed to the flask. To the flask was added 12.0g K2CO3 (crushed by mortar and pestle and dried in a 250C oven for 2 hours) and 12.0g syringaldehyde (recrystallized several times from water as long needles with a light straw-yellow color.) The addition funnel was charged with 12 ml of TMP. The reaction flask was purged with a CO2/argon mix and then stoppered. The solids were brought to melting, approximately 110C, and TMP was added over the course of 10 minutes. No solvent was used. Reaction temperature was maintained at 95-110C for 4 hours with magnetic stirring at its maximum. The reaction was allowed to cool to 50C and then flooded with 50ml of room temperature water. Crystal formation immediately followed. The crude product was shaken, scraped, and washed out of the round bottom flask with 200ml of water onto a Buchner funnel. The solids were vacuum filtered for several minutes, noting an orange/yellow filtrate. The solids were then recrystallized from hot water, which caused the impure 3,4,5-TMBA to oil out as a brown oil. This was allowed to cool, yielding 3,4,5-TMBA as a large, singular mass. An MP was not taken as the product was visibly impure with brown/yellow/orange discoloration. Recrystallization from (50C) C6-C8 aliphatic hydrocarbons was then attempted. Despite the solvent being below the melting point of the intended product (73C), it oiled out once again. As it cooled, precipitate formed as before, as a singular mass with visible brown/yellow/orange impurities.

It's unclear how the patent can claim a workup of "dump the reaction mixture into water and wash the crystals with water" because it seems like complete bullshit. The same applies to the ScienceMadness post. A molar excess of TMP is used and large amounts of K2CO3 remain in the reaction mixture -- how can dumping the reaction mixture into water result in any semblance of purity?

With any luck the ethyl acetate will be here soon. If not, short-path vacuum distillation seems like the only option. The biggest challenge will be vacuum distilling a product that is a solid at 73C. Perhaps an attempt at vacuum distillation will be made this weekend.

[Edited on 21-4-2019 by horribilis]

[Edited on 21-4-2019 by horribilis]

bipolar - 20-4-2019 at 19:36

So did you took melting point of the recrystallized product? It will melt much below it's mp in the solvent, in case you're wondering. Just take recrystallized product, powderize it, put at the tip of thermometer and slowly heat approximately 2-5 mm above electric hotplate.

Also I didn't quite understand, did the product dissolved completely in hexanes (by "hexanes" I mean aliphatic hydrocarbons) at boiling?
If not, I would change process: do extraction of the crude product (it should be dry) with boiling hexanes, decanting it from oil / semi-solids and repeating this process with fresh portions of boiling hexanes until only insoluble colored resins left.
Although it's probably unnecessary. Take melting point first, please.

[Edited on 21-4-2019 by bipolar]

horribilis - 20-4-2019 at 20:13

Quote: Originally posted by bipolar

So did you took melting point of the recrystallized product? It will melt much below it's mp in the solvent, in case you're wondering. Just take recrystallized product, powderize it, put at the tip of thermometer and slowly heat approximately 2-5 mm above electric hotplate.

Also I didn't quite understand, did the product dissolved completely in hexanes (by "hexanes" I mean aliphatic hydrocarbons) at boiling?
If not, I would change process: do extraction of the crude product (it should be dry) with boiling hexanes, decanting it from oil / semi-solids and repeating this process with fresh portions of boiling hexanes until only insoluble colored resins left.
Although it's probably unnecessary. Take melting point first, please.

[Edited on 21-4-2019 by bipolar]

The MP of the impure product was not taken because it was visibly impure: the white plates had many obvious streaks of yellow and orange.

I made a mistake in my last post and left out an important part. Please see the revision in the previous post.

A short path vacuum distillation was attempted -- quite the pain in the ass. The product was collected somewhere around 120C. The culprit of the impurity, a small amount of an oily brown/yellow liquid, remained in the reaction flask. The distilled product nearly plugged the short-path condenser despite circulation of scalding hot water, and so the entire short-path apparatus was heated in an oven for 10 minutes. The hot, crystal-clear liquid is now being recrystallized from water (to form crystals rather than a singular large mass.) An MP will be taken sometime soon -- at least this will prove that the reaction works, to some extent, although the losses from multiple recrystallizations and distillation are heartbreaking.

Honestly, an alternative to hexanes would be ideal. Can you think of a solvent that could be used to extract the crude mixture (after crashing it out with water) that would leave behind the K2CO3, unreacted TMP, and other bullshit? Maybe DCM or ethyl acetate?

[Edited on 21-4-2019 by horribilis]

horribilis - 21-4-2019 at 09:05

MP of the 3,4,5-TMBA crystals, brilliantly white in appearance, observed as 70-73 C. Judging by MP, they're still slightly wet. Yield ~4.4 g (~34% on starting material.)

horribilis - 1-6-2019 at 13:00

After receiving some much appreciated direction, here is the latest iteration of this procedure:

1.25 molar equivalents of K2CO3 was ground with a mortar and pestle for several minutes and oven dried at 250 C for 2 hours. To a 3-neck round bottom flask was added 1 molar equivalent of syringaldehyde. The flask was set up for magnetic stirring over an oil bath. To the flask was affixed a reflux condenser, pressure equalizing addition funnel, and a thermometer. A hose adapter was affixed to the reflux condenser, which was lead to a smaller round bottom flask with an anti-suck back adapter, which was then lead to a 50 ml graduated cylinder filled with mineral oil. The dried K2CO3 was allowed to cool for a few minutes and then added to the reaction vessel. The addition funnel was charged with 2.5 molar equivalents of TMP and a balloon was placed over the neck of the additional funnel to act as a septum. The apparatus was then gently flushed with CO2/argon wine preserver for perhaps 15 seconds (purging evident by bubbles rising through the mineral oil in the test tube) and the balloon was quickly swapped for a glass stopper.

The flask was heated to 110 C, at which point the syringaldehyde melted. Stirring was initiated and the TMP was added over the course of about 5 minutes. Gas evolution was noted. The reaction was allowed to proceed for a total of 14-16 hours with stirring at a reaction temperature of 110 C. In some cases, suck back of mineral oil was observed despite a constant reaction temperature. In these cases of total suck back of the mineral oil, it was unclear how long the reaction proceeded without atmospheric protection. (It appeared to have no significant impact on yield.) Upon discovery of total suck back, more mineral oil was added to the test tube and the apparatus was purged with CO2/argon as before. In some cases the reaction mixture turned mostly solid toward the end of the reaction, despite a constant 110 C reaction temperature. It is unclear why this happened, but the yield was unaffected.

At the 14-16 hour mark the reaction mixture was quickly dumped (as a liquid, or as broken-up solids) into a beaker containing an equal volume of room temperature water. The beaker was allowed to sit for several minutes. Some liquid from the beaker was poured back into the round bottom reaction flask and swirled, to transfer over all solids. The contents of the beaker were vacuum filtered and set aside in another beaker. The filtrate was extracted with three portions of ethyl acetate. The organic extracts were added to the beaker containing the solids, and the mixture was stirred until all solids dissolved, with additional ethyl acetate being added as necessary. The ethyl acetate was then washed with two portions of ~5% w/w NaOH (to remove unreacted syringaldehye), two portions of water (to remove any remaining TMP or inorganics), and two portions of brine (to pre-dry.) All aqueous layers were discarded.

The organic extracts were dried over MgSO4 and filtered. The ethyl acetate was stripped under gentle vacuum over a water bath to yield 85-92% of a crude 3,4,5-trimethoxybenzaldehyde as an amorphous solid with a color that ranged from champagne to light brown/yellow. Further purification with vacuum distillation is advised, although this was not yet attempted. Bisulfite adduct may also work but I don't have enough experience to feel comfortable with this procedure -- vacuum distillation is comparatively simple. It's believed that the impurities were carried over from the starting material, as the syringaldehyde displayed a similar color.

[Edited on 1-6-2019 by horribilis]

Tkuze - 5-6-2019 at 11:25

All I can advise is that this reaction DOES work with Trimethoxyphosphate without an ineet atmosphere, BUT an inert atmosphere SHOULD ALWAYS be used in non aqueous reactions, even if it is well known to work without is. This is standard operating procedure in graduate level air-free organic synthesis. One can easily obtain equiptment for an inert atmosphere. Buy a size 20 Nitrogen tank with standard female threading (580C or something). Get a $35 dual guage regulator ((low flow rate) from Amazon. Heres a bulleted list with prices and sources
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horribilis - 5-6-2019 at 17:52

A vacuum distillation of the combined crude products of several trials was performed. Recovery was ~90%. A MP was not taken but the product is snow-white in appearance and was collected in a narrow temperature range -- it should be pure. The remaining ~10% is dark yellow/brown/orange in appearance, but not a tar. Strange.

Tkuze, thank you for the input. It appears my ghetto-rigged CO2/argon atmosphere and bubbler setup worked reasonably well. A real inert atmosphere setup like you suggest would be very nice, though.

[Edited on 6-6-2019 by horribilis]