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Author: Subject: 4-ethoxy-3-methoxybenzaldehyde from vanillin
Bronstein
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[*] posted on 3-6-2008 at 10:25
4-ethoxy-3-methoxybenzaldehyde from vanillin


I had a need for this compound and it was cheaper to synthesize it than to buy it so I did. I followed more or less hest's procedure.

6.8g (0.063 mol) bromoethane was dissolved in 80 ml DMF, followed by 8.3g (0.0549 mol) vanillin and 13.5g (0.0976 mol) anhydrous potassium carbonate. A big stirbar was dropped in and it was stirred for 2,5 hours. During this time the reaction mixture took on a yellowish colour. The potassium carbonate never fully dissolved.

Next all the mixture was poured into ~800ml ice cold water, it was stirred for a small bit with a glass rod and then put in the fridge for a short while to complete crystallization. The crystals was suction filtered, washed with a small amount of cold water, sucked dry, and allowed to dry on a plate for perhaps 30 minutes.

Yield: 8.4g (83.7%, 0,046 mol)
So it works pretty good.

The crystals smells faintly vanillinish.

Further discussion on the use of this compound will take place elsewhere.

In the future I might follow this same route to make the 4-allyloxy variant.

Edit: Just now I added some HCl to the water filtrate, and it produced some crystals of what I presume is vanillin.

[Edited on 3-6-2008 by Bronstein]

[Edited on 3-6-2008 by Bronstein]
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[*] posted on 3-6-2008 at 14:25


Nice! Thank you for sharing!

My suggestion would be washing the crude product with dilute (1-3%) NaOH to remove phenolic impurities, or dissolve the product and wash it with the same solution, although this implies a little more work.

During my alkylation of salicylaldehyde in DMF with K2CO3, i also noticed the carbonate never dissolved, although from the colour of the recation mixture it was evident at least part of the phenolic aldehdye was converted to the phenolate. Using K2CO3 seems to yield a slightly cleaner product than using KOH, which takes some time to totally dissolved and must be detrimental to the alkylating agent employed.

Looking forward to hear about any further reactions!




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[*] posted on 3-6-2008 at 14:51


Quote:
Originally posted by Klute
My suggestion would be washing the crude product with dilute (1-3%) NaOH to remove phenolic impurities, or dissolve the product and wash it with the same solution, although this implies a little more work.


I had considered washing it with some dilute NaOH, but I figured that all the vanillin should be present as the phenolate as the reaction is finished, and that it should dissolve in the large amount of water you pour the mixture into. So I decided to skip that step.
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[*] posted on 3-6-2008 at 16:09


I would suggest at least one recrystallization of your end product, as usually during these kind of workup the alkylated aldehyde precipitates as very "fluffy" solid, trapping alot of dark reaction mixture. Obvious generous water washing remove most of the color, but you would be surprised how dark a solution of a apparently very clear product can be.
For similar derivatives, I have found DCM/Pet ether or EtOAc/Pet ether to be very effective for that purpose.

I would be delighted to see pictures of your product, if that is possible/suitable for you! :)

What kind of reactions do you plan on doing with the aldehyde? Aldol condensations? Knoevenagel?

I find it very appreciable to see people taking time to share their work.




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[*] posted on 3-6-2008 at 17:33


Is it absolutely imperative to use a relatively expensive solvent like DMF? Are there any alternatives?

I find it interesting that EtBr worked so well! Normally I would just think to use diethyl sulfate or less preferably, EtI. I saw MeBr work for PainKilla, but I never thought EtBr would work. This reaction makes me wonder just how indispensable alkyl iodides or dialkyl sulfates are as alkylating agents.




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[*] posted on 3-6-2008 at 17:53


EtBr is a very efficient alkyl halide! As Nicodem pointed out in Painkilla's thread, the differenc eif reactivity between EtI and EtBr is neglieable... MeI is commonly used becasue more practical to use (not a gas).

I'm sure this reaction works perfect in acetone, which is th e"usual" replacement for DMF in more sensible alkylations, but surel ethanol or methanol would work well here. DMF being a good aprotic polar solvent, it is very usefull for such SN2 alkylation, but certainly not mandatory! It has the other advantage of having a high bp, so it can be sued to conduct reactions at high temp, and dissolves quite some inorganic salts.
But indeed it is expensive and toxic, and it's worht using other suitabel solvant when possible. plain phenolic substartes perform great in methanol or ethanol, haven't tried with phenolic aldehdyes though as I have a supply of DMF.




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[*] posted on 4-6-2008 at 00:18


Bronstein, what makes you believe what you got is 4-ethoxy-3-methoxybenzaldehyde? Have you measured the mp?
I hope you understand that the way you did this reaction (at room temperature for only 2.5 hours) it seems particularly strange you would obtain the alkylated product and in such good yield. Also your belief that the vanillin would stay dissolved as a phenolate after dilution with water is not particularly correct since the carbonates have the pKa comparable to the phenolates and thus only little phenolic compound gets dissolved unless you use concentrated K2CO3(aq) solutions. Also, it seems particularly strange that a product with a mp reported to be anything from 58-65°C would crystallize so immediately and nicely by dilluting its DMF solution. Such low melting products most commonly precipitate as oils after dilution.
Procedures of ethylation with EtBr described in the literature using potassium bases and vanillin in DMF generally call for a prolonged heating, none at room temperature.

Klute, K2CO3/acetone reflux sucks for these reactions. Nevertheless, here is one example:
Quote:
(i) A mixture of 3.0 g (19.7 mmol) of vanillin and 4.08 g (29.5 mmol) of dry K2CO3 in 30 mL of dry acetone was stirred and heated to reflux for 30 min, and 2.35 g (21.6 mmol) of ethyl bromide were added dropwise. The mixture was refluxed for 8 h, then filtered, and the solvent was removed under vacuum. The residue was dissolved in EtOAc (30 mL), successively washed with saturated solutions of NaHCO3 and NaCl (3×5 mL), and dried (Na2SO4). The solvent was removed under vacuum, and the residue was purified by column chromatography over silica gel (80 g, hexane/EtOAc 95:5) to give 3.19 g (90%) of 4-ethoxy-3-methoxybenzaldehyde as a white solid: Rf 0.38 (hexane/EtOAc 8:2); mp 59–60 °C (hexane/EtOAc 9:1) [lit.20 58 °C].

Tetrahedron, 61 (2005) 10061-10072 (DOI: 10.1016/j.tet.2005.08.015)


Using KOH/EtOH or KOH/iPrOH reflux works great in most cases when using primary alkyl bromides and is not so unreliable as K2CO3/acetone (and you don't need to bother with drying K2CO3 prior every use). Unfortunately, if using secondary alkyl bromides, using alcohols as solvents generally does not work and aprotic polar solvents like DMF, NMP or DMSO are required. Primary alkyl chlorides (except allylic and benzylic) on phenols also give a low yield with KOH/iPrOH reflux (~30% or less, even after several days of reflux), though using an autoclave at ~100-120°C could make it work.

[Edited on 4/6/2008 by Nicodem]




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[*] posted on 4-6-2008 at 04:45


Ok, I did a melting point test. I put a small sample in a tiny test tube and put a termometer in it. The test tube was placed in a heating bath. At about 57-58 C it had all melted. Chemfinder has a mp 59-60 C for it, so perhaps it's not totally pure. I think I might do a recrystallization, but then again, I might not need the extra purity for the reaction I'm going to do with it. (Knoevenagel)
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[*] posted on 4-6-2008 at 05:42


Nice! A mp of 57-58°C would make it extraordinarily pure.
Now, just to be on the sure side, you can put a few mg of your product in a test tube containing ~1ml of ~2M aqueous NaOH and stir a bit.
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[*] posted on 4-6-2008 at 05:59


Nicodem, in my experience some low-melting aldehdyes canc risatillze fine out of DMF solution by adding aqueous solution, all depends on how clean the reactuion was. Look at the different almkylation I did on salicylaldehyde to o-methoxybenzaldehyde (mp 39). When the reaction was done suing TMP, the product crystallized in seconds after adding water, but with MeI it satyed as a semi-solid low melting substance (even though the colour was much more dark with TMP).
So I wouldn't be surprised if his product, wich melts 20°C over methoxybenzaldehyde would precipitate immediatly.

Concerning the use of acetone, I have never tried it I admit. It just that I haev seen it used in some many insatnce, I always considered it to be routine in similar alkylations. I do particulairly remember this article about asymetric methoxybenzaldheydes were they alkylate in acetone with various alkyl halide in good yields IIRC, but then again it's hard to know if the results are trustworthy until you've tried it. If your experience with alkylations in acetone leads you to beleive it's crap, I take your word to it and will stick to using alcohols in less delicate alkylations. Sorry for the bad advice Fleaker.




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[*] posted on 4-6-2008 at 06:39


Actually, the method using K2CO3/acetone often works very nice, especially for p-hydroxy substituted benzaldehydes, acetophenones and other such phenols more acidic than phenol itself (like vanillin). The problem with it is in that with some combinations of alkyl bromide and substrate it either does not work at all or it gives low or unreproducible yields. In comparison K2CO3/DMF or KOH/DMF are more or less completely general and reliable to all substrates, so the choice is obvious unless DMF unavailable. That's why I say K2CO3/acetone sucks, because it often leads to disappointment.

About this vanillin ethylation... I'm not claiming that the product is not the expected one, I just want Bronstein to confirm it beyond doubts because it is simply hard to imagine such good results without heating like it is normally done. What he did is not standard procedure, so what I expect is some non standard confirmation of the product's identity. Of course, if he would have used extraction in the work up and washed the extract with some 2M NaOH and obtained so much product, then it would be more or less obvious that what remains is O-ethylvanillin. Yet, this way some additional confirmation is advisable.




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[*] posted on 5-6-2008 at 09:38


Ok, so a tiny sample was placed in ~1ml 5% NaOH and swirled about for a bit. It was very hard to judge wheter or not anything dissolved at all, but it seemed to me that at least the majority of the crystals did not dissolve.
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[*] posted on 5-6-2008 at 17:47


All nice work- two comments: @Nicodem- I noticed a prep for Calebertine in the article you cited. Are you able to d/l and post it in ref? I'd appreciate getting it, thanks. Whatever happened to Thiele and capillary tubes?



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[*] posted on 1-7-2008 at 12:25


Well, I tried this one more time, this time after I filtered the crystals I put them into 40ml 5% NaOH and swirled them around for some minutes, then filtered again followed by another water wash.

The yield this time was 4.9g (49.3%). So I guess Nicodem was right (as always), there was lots of unreacted vanillin in the previous batch.

I took some pictures of the crystals still suspended in water, I could attach them later when I have copied them to the computer.
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[*] posted on 1-7-2008 at 12:44


You should try refluxing the recation mixture then. Also, dissolving the crude product in a solvent such as DCm before the dilute base wash might be more adviseable. Hoping to hear more on the subject :)



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[*] posted on 3-7-2008 at 04:21


Heating is certainly a good idea, but not to the reflux since the bp of DMF is considerably higher than what the reaction needs (not to mention that at reflux most of EtBr will simply remain in the condenser due to its high volatility). A few hours at 60-80°C should do, but of course the reaction should be followed with TLC to make sure all the starting material reacted.

Washing crystalline matter is not particularly efficient in removing soluble impurities since a lot of the impurities can remain unaccessible to the solvent due to inclusion in/among the crystals. Washing a solution (for example the product dissolved in CH2Cl2, toluene, etc.) is way more efficient. To efficiently remove small phenolic compounds you need to use ~2M NaOH and wash 3 or more times.




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[*] posted on 3-7-2008 at 06:17


I was referring to acetone, as this is what he used. I usually heat DMF to ~50°C when doing such alkylations (with TMP though, much higher bp). Even just 40°C would speed things up with this solvent, but room temp for 24H is often all it takes for reactive substartes.



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[*] posted on 9-10-2014 at 14:28


After a few attempts with varying success, I was able to make some 4-ethoxy-3-methoxy benzaldehyde, in a fairly convenient manner.

20.0 g vanillin and 150 mL ethanol were placed in a 1000 mL beaker, heating and stirring to dissolve. 9.0 g ~85% KOH in 60 mL warm ethanol was added with swirling. The beaker was cooled in a water bath and then in the freezer until ~5 degrees C, and the resulting precipitate was filtered, squeezed of excess alcohol, and dissolved in 65 mL dimethylsulfoxide. The resulting solution was added to a 350 mL swing-top beer bottle, followed by 15 g ethyl bromide (only a slight molar excess). A bit of plastic wrap (I think it was PVC) was placed over the mouth of the bottle and the lid was clamped shut.

The bottle was heated on a boiling water bath for about 15 minutes. As the reaction proceeded, a precipitate could be seen accumulating (potassium bromide). After 10 minutes, the amount of precipitate did not appear to increase and the reaction was interpreted to be complete.

The reaction vessel was allowed to stand at ambient temperature for two days, then poured into a 1000 mL beaker, followed by 300 mL distilled water. The solution had a pH of 3.5 at this point (Cannizaro reaction?). A slightly yellow oil began to crash out, followed by some white solids. KOH in water was added to bring he pH to 14 with vigorous stirring. The oil solidified and some more solid precipitated. The beaker was cooled to ~5 degrees C, the yellow-white crystalline solids filtered (only smells vaguely of vanilla) washed with cold water and dried over CaCl2.

Yield: 12.05 g, 51%
MP: 57-59 degrees C (lit 59-63, Alfa Aesar)

I cannot think of any reason for the acidic post-reaction mixture, except for base-catalyzed disproportionation of vanillin into the corresponding benzoic acid and benzyl alcohol. Maybe this happened when forming the phenolate in hot ethanol?

[Edited on 10-10-2014 by Crowfjord]

I forgot to mention, the method used here was somewhat inspired by Sandmeyer's post here, in regards to alkylation of syringaldehyde under autogeneous pressure.

[Edited on 10-10-2014 by Crowfjord]
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[*] posted on 10-10-2014 at 11:59


Quote: Originally posted by Crowfjord  
I cannot think of any reason for the acidic post-reaction mixture, except for base-catalyzed disproportionation of vanillin into the corresponding benzoic acid and benzyl alcohol. Maybe this happened when forming the phenolate in hot ethanol?

It's probably because you used a solvent unsuitable for the reaction. DMSO reacts with ethyl bromide, it O- and S-ethylates it. O-Alkylation of DMSO results in an unstable intermediate that can decompose by loss of a proton, thus neutralizing the phenolate and acidifying the mixture. This is a typical side reaction of alkylations in DMSO, that's why this solvent is preferably avoided when reactive alkylating reagents are used (methyl, ethyl, benzyl, allyl halides and sulfonates...).
You would probably get better yields by using ethanol or 2-propanol as solvents at longer reaction times (given that the volatility of EtBr reduces the reflux temperature). Though DMF or NMP would probably be better solvents than alcohols.
You probably also wasted a substantial amount of the starting material by that separate potassium salt formation in ethanol. What was the purpose of that? If you wanted to ethylate vanillin, why didn't you just use KOH as the base?




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[*] posted on 10-10-2014 at 12:48


Thanks for weighing in, Nicodem. Yes, I am aware of the side reaction; that probably did consume at least some of my ethyl bromide. I wanted to see for myself exactly how it would pan out. I would expect the ethyldimethylsulfonyl bromide (that is one that forms, yes?) to react with the phenolate to add a methyl rather than ethyl group. The melting point suggests mostly 4-ethyoxy-3-methoxybenzaldehyde as the isolated product, but I should do a TLC. Having already tried the same method with syringaldehyde (with both methyl and ethyl bromide - methyl gave nearly quantitative yields), I thought that another shot with vanillin would be interesting.

I found, with syringaldehyde at least, that forming the phenolate in a separate step gave better results when using DMSO as the reaction solvent. I can speak of that in another thread, though. I had tried previously using ethanol and KOH; I had a vinyl tube running from a thermometer adapter in the straight arm of a Claisen adapter (reflux condenser on the other) down into the stirring vortex of the solution, through which a solution of ethyl bromide was added about a milliliter at a time over a few hours. It worked well, at least according to TLC and a small evaporated sample of the extraction (I screwed up and scorched the product after distilling the solvent off in that run), but it took several hours, whereas this way, with a high boiling polar aprotic solvent, the reaction proceeds very quickly, albeit with lower yields, it would seem. DMSO is clearly best for methylations, as the side product can methylate phenols as well. I think it needs higher than 100 degrees C for that to happen, though.

You are right that DMF or another amide solvent would be better. I just work with what is available and affordable to me. I can get DMF, but just cannot afford it currently. And I still have yet to find a good source of NMP.

[Edited on 10-10-2014 by Crowfjord]
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[*] posted on 10-10-2014 at 23:35


Quote: Originally posted by Crowfjord  
Yes, I am aware of the side reaction; that probably did consume at least some of my ethyl bromide. I wanted to see for myself exactly how it would pan out. I would expect the ethyldimethylsulfonyl bromide (that is one that forms, yes?) to react with the phenolate to add a methyl rather than ethyl group.

Methylation is not possible with O-alkyl-DMSO cations where only the O-alkyl group is electrophilic, utmost it might be possible with S-alkyl-DMSO cations, but as far as I know this does not occur under normal conditions. It is hard to predict how relevant the side reactions with DMSO as the solvent for alkylations are as this depends from case to case (it is this unpredictability that makes chemists prefer DMF which also gets O-alkylated, but reversibly as the product does not decompose).

Quote:
I found, with syringaldehyde at least, that forming the phenolate in a separate step gave better results when using DMSO as the reaction solvent.

The potassium phenolate of syringaldehyde is extraordinarily insoluble even in DMF, so perhaps the precipitation from ethanol is more close to quantitative.

Quote:
You are right that DMF or another amide solvent would be better. I just work with what is available and affordable to me. I can get DMF, but just cannot afford it currently. And I still have yet to find a good source of NMP.


I mentioned NMP because it is a good OTC substitute for DMF. At least in most European countries it is sold to the public as a solvent for cleaning paint from the walls (like graffiti remover, etc.). I'm not sure it will stay available for long, because some bureaucrat added it on the shooting list of the REACH initiative.

Anyway, the literature is full of examples of O-ethylation of vanillin and syringaldehyde in a variety of solvents. Even water can be used (see Example 7 in US4883798). All kinds of ethylation reagents have been used (there is even an example with EtOSO3K at Hyperlab, post No. 494316). Other examples not listed in reference databases are the ethylation of syringaldehyde in MeOH using KOH and EtI (PIHKAL #25). This reaction is also described at Hyperlab, post No. 551471 to give a 99% yield using EtI and K2CO3 in DMF (I'm sure it would apply to vanillin as well).




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[*] posted on 11-10-2014 at 05:34


soory if this is a dumb question but when using acetone and K2CO3, is there any chance of an aldol condensation

also can acetonitrile be used as an alternative to DMF

i am surprised that you find it so comfortable working with DMSO considering that it is flammable and also stinks:(
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[*] posted on 11-10-2014 at 08:07


Aldol isn't much of a worry, as most if not all phenols are stronger acids than acetone.

Acetonitrile may be a useful alternative; it is quite polar and aprotic. I have not had the chance to work with it. It also has a much lower boiling point than DMF, so this must be taken into account.

Flammability isn't much of an issue, as most lab solvents are flammable to some degree. It is something a chemist learns to deal with and becomes accustomed to early on. And DMSO is actually odorless, or nearly so. It is sulfides and thiols (which can sometimes be degradation products of DMSO, but so far I have not encountered them) that stink. The main risk with DMSO worth serious consideration is it's ability to carry solutes directly through the skin and into the bloodstream. For this reason, I am always extremely careful when working with the stuff.

[Edited on 11-10-2014 by Crowfjord]
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[*] posted on 11-10-2014 at 08:18


I've seen DMSO wash ball point ink right through the skin. I can imagine what it does with other substances.
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[*] posted on 12-10-2014 at 01:59


Quote: Originally posted by Crowfjord  
Aldol isn't much of a worry, as most if not all phenols are stronger acids than acetone.

could you explain why being a stronger acid helps to prevent aldol



Quote:
And DMSO is actually odorless, or nearly so. It is sulfides and thiols (which can sometimes be degradation products of DMSO, but so far I have not encountered them) that stink.

yes ,DMSO is sometimes contaminated with dimethyl sulphide

[Edited on 11-10-2014 by Crowfjord]
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