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Prometheus23
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[*] posted on 8-6-2012 at 21:56
Metal/acid reduction of benzoin


I've been tinkering with a way to reduce benzoin to deoxybenzoin. The most reliable source for a procedure is from orgsyn which uses tin powder and hydrochloric acid (Reference 1). The actual write up is concerning the reduction of anisoin to deoxyanisoin but the authors state in note 3 that the procedure is also applicable to other symmetrical benzoins and they give the general procedure for benzoin itself. I attempted this reduction myself recently and ran into one major problem. I discovered that the mesh of my tin powder was far too high (too fine) and resulted in much of the tin conglomerating at the bottom of the flask and not reacting. The authors themselves warn about this, but it was the only tin powder I had sitting around. Workup gave mostly unreacted benzoin.

I had hoped to resolve this problem by substituting the tin powder for stannous chloride as the reducing agent. In many tin/hcl reductions stannous chloride can serve the same function. But I have some across a paper (J Chem Soc, Vol 92, Part 1, Pg 537) which states that benzoin is not reduced by stannous chloride at all. Does anyone have an idea why this is? Even a theory would be welcome.

My next attempt will be with zinc dust and hydrochloric acid. I have found one procedure (J Chem Soc, Vol 54, Part 1, Pg 703) which might be able to be adapted, but I will likely be working through a lot of trial and error. If any of you have a suggestion for modifications (such as adding the zinc dust slowly to ensure its complete reaction perhaps?) please feel free so speak up. When I have some results I'd be happy to post them here for any interested.

[Edited on 6/9/2012 by Prometheus23]
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[*] posted on 8-6-2012 at 22:56


Quote: Originally posted by Prometheus23  
I attempted this reduction myself recently and ran into one major problem. I discovered that the mesh of my tin powder was far too high (too fine) and resulted in much of the tin conglomerating at the bottom of the flask and not reacting. The authors themselves warn about this, but it was the only tin powder I had sitting around. Workup gave mostly unreacted benzoin.

What kind of a stirrer did you use? You might want to use some overhead stirrer and add the tin in portions, instead of all of it from the start as done in the procedure.
Quote:
But I have some across a paper (Reference 2) which states that benzoin is not reduced by stannous chloride at all. Does anyone have an idea why this is? Even a theory would be welcome.

Please give a normal citation for that J. Chem. Soc. article. Google books links are location dependent and I don't have the time to search for which article that URL points to.
Quote:
My next attempt will be with zinc dust and hydrochloric acid. I have found one procedure (Reference 3) which might be able to be adapted, but I will likely be working through a lot of trial and error.

Same problem. But beware that the Clemmensen reduction of benzoin gives stilbene. The amalgamation might cause the difference.




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[*] posted on 8-6-2012 at 23:15


Stirring wasn't an option given that my magnetic stirrer wouldn't do the job and I do not have an overhead stirrer unfortunately. I believe rapid stirring would be very helpful but under the circumstances I am going to try adding the tin in portions. My biggest problem with this procedure is the extremely long reflux time (24 hours), and adding the tin in portions will only prolong this. However when it is complete I will post my results and hopefully my yield will be high enough to at least quantify this time. Thanks for your time and suggestions.

Also the second and third references from my first post have been changed
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[*] posted on 9-6-2012 at 06:14


I have too considered trying to reduce benzoin to desoxybenzoin sometime. I found this procedure with Zinc / Acetic acid, it seemed to be the easiest.

J. Am. Chem. Soc., 1930, 52 (10), pp 4128–4139 http://pubs.acs.org/doi/abs/10.1021/ja01373a058

Quote:

Desoxybenzoin.-Two lots, each of 346 g. of benzoin, 600 g. of glacial acetic acid
and 105 g. of zinc dust contained in a flask provided with a mechanical stirrer and an air
condenser, were heated in an oil-bath a t the boiling point of the liquid until a sample no
longer gave a flocculent precipitate when i t was dropped into water (four t o five hours).
The benzoin was first dissolved in the acid heated t o about 115", then the zinc dust
was added as rapidly as possible without causing frothing, and finally the temperature
was raised t o the boiling point.
The solution was poured into hot water, the mixture boiled until the oil collected
on the bottom, then cooled and filtered. The solid was washed with water and dissolved
in alcohol. After removal of the zinc, the alcoholic solution was distilled under ordinary
pressure until free from alcohol, then fractionated under diminished pressure. The
yield was 534 g. of desoxybenzoin, equal t o 82'%.


Otherwise I also found this patent for reducing with tin / hydrochloric acid:

Quote:

US4263311

The benzoin (0.075 mole) was dissolved in 80 ml. of alcohol and 20 g. of tin (20 mesh) was added. The mixture was gently heated and a solution containing 28 ml. of concentrated hydrochloric acid, 0.8 g. of anhydrous cupric sulfate and 0.5 ml. of water was added. The reaction mixture was refluxed until thin layer chromatography no longer detected the presence of starting material. The granular tin was removed by filtration and the filtrate was evaporated to an oil which was diluted with water to 400 ml. and was extracted with chloroform. The organic extract was washed with 5% aqueous sodium carbonate solution (45 ml.×3), with water (45 ml.×2) and with brine; it was dried over anhydrous magnesium sulfate and evaporated to obtain desoxybenzoin.


Please report how it went!
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Prometheus23
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[*] posted on 9-6-2012 at 06:33


Thank you Bronstein for posting the procedure from your reference as I cannot access it directly. I too came across a similar procedure using zinc dust and glacial acetic acid (J Chem Soc, Vol 60, pg 1492) but it involved reacting 40g benzoin with 40g zinc dust and 200g acetic acid. No further details are given such as workup conditions, reaction time/temperature, or even yield. Interesting that in your example they don't use zinc in excess. Anyways now that I have a more specific example as a starting point I will have to try using this method also and compare it to others. Looks like it might be time to make some more benzoin after all.

I also stumbled on the patent you mention, but since it relies on the use of granular tin metal (I only have very fine mesh tin powder) I assume the results would be the same as my first attempts.
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[*] posted on 9-6-2012 at 07:03


If you intend to try using zinc dust and hydrochloric acid for the reduction I wonder if the fine zinc powder won't conglomerate together just as the tin did. Obviously adding it in portions and/or stirring would help things as Nicodem says. Maybe you could try running the procedure in your first reference again but substituting an equimolar amount of zinc dust for the tin?
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[*] posted on 19-6-2012 at 19:15


A scaled down version of the procedure posted by Bronstein from the J. Am. Chem. Soc. was attempted (5g benzoin). The zinc dust seems to react a bit with the acetic acid when first added but then stops after a couple of minutes. After that the refluxing was continued and hour after hour after hour the zinc dust remains. Almost none of it seems to react and disappear, indicating it is not reducing the benzoin.

Thoughts anyone?
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[*] posted on 20-6-2012 at 11:14


Quote: Originally posted by Prometheus23  
Almost none of it seems to react and disappear, indicating it is not reducing the benzoin.

Thoughts anyone?

What you say does not necessarily make sense. What makes you say that the reduction is not occurring? Surely, you don't base that only on the visual observation of the zinc powder? Tell us what you get on TLC? What is the amount of zinc you used? And so on...
You will need to give at least the minimum data required for a useful answer, assuming you want a useful answer. My suggestion for you is to provide a proper experimental of your work, so that all the data is at hand.




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[*] posted on 20-6-2012 at 17:27


I am using the visual observation of the zinc powder as a gauge of the reaction progress because as the reduction occurs the elemental zinc is converted to zinc acetate. Since the zinc is used in a 1:1 molar ratio to the benzoin (as is made clear in the procedure listed just a couple of posts above yours), logic follows that if there is no noticeable decrease in the amount of zinc in the flask then there has been no significant amount of reduction of the benzoin. If you look you'll see that I stated in my previous post the procedure was the one Bronstein posted and it was scaled down to 5g of benzoin from 346g.
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[*] posted on 21-6-2012 at 22:31


Just wanted to report on the success of this procedure. The zinc/acetic acid reduction was tried again but this time with an added portion of 95% ethanol (3mL ethanol for every 7mL acetic acid). The ethanol seemed to keep the reaction going quite nicely and after workup and recrystallization I got approximately 3g deoxybenzoin from 5g benzoin. The crude product was recrystallized from methanol.
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[*] posted on 23-6-2012 at 12:01


Sorry, I was not paying much attention, but it sounded so strange when you said that after hours of reflux the zinc remained unreacted. In my experience, zinc powder refluxing in pure acetic acid (without anything else) dissolves in few hours. Thus it appears to me that perhaps your zinc is of poor quality or your stirring was inefficient. Have you activated your zinc prior the reaction (with 1M HCl or otherwise)? For reductions in acetic acid can make quite a difference.

Anyway, now you got it working, so thanks for sharing your experience.

How did your characterize the product? The melting point and the melting interval of the product? Have you checked the purity with TLC or other means?
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[*] posted on 26-6-2012 at 02:37


The product was identified by melting point determination. It gave a melting point of 55-56°C. I suspect the yields are lowered by the wasteful side reaction of the zinc with the acetic acid to give zinc acetate.

This got me wondering what other reducing agents might be able to affect the reaction. Perhaps oxalic acid could serve as the electron donor as well as the proton donor? I'm not familiar with any reduction mechanisms for oxalic acid, so I could be completely off base here.
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[*] posted on 26-6-2012 at 07:43


Quote: Originally posted by Prometheus23  
This got me wondering what other reducing agents might be able to affect the reaction. Perhaps oxalic acid could serve as the electron donor as well as the proton donor? I'm not familiar with any reduction mechanisms for oxalic acid, so I could be completely off base here.

Well, organic reductions with oxalic acid are pretty much the same as the better researched reductions with formic acid which itself is a decomposition product of oxalic acid thermolysis. Formic acid can be used for reductive amination-like reactions, or the reduction of imines and pyridinium salts (the Leuckart-Wallach and the Eschweiler–Clarke reactions, and similar ones). Other unusual reductions with formic acid are also known, but I don't remember any examples that would relate directly to benzoin->desoxybenzoin type of transformations (though I never searched!). The closest that I can think of is the reduction with dehydration of glycerol to allyl alcohol by using formic acid or oxalic acid.

In my opinion, the idea is well worth evaluating and if you have the time to check the literature, please do so. If you find nothing, you can try refluxing benzoin in 85% formic acid or heating the benzoin / oxalic acid melt. A stream of CO2 bubbles that slowly subsides as the reaction goes on is an indication that there might be something useful going on.
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[*] posted on 29-6-2012 at 08:52


I checked a bit of the literature about reductions with formic acid, because I found nothing useful on oxalic acid. Finding articles on this topic is not easy due to the flood of catalytic hydrogen transfer reductions using formic acid as a hydrogen source making searching difficult.

Still there are quite some articles on the topic. For example, the reduction of imines and enamines with formic acid has been well studied (for example: Tetrahedron 29, 57-64).

I found no example on benzoins and related benzyl alcohols. However, the easily homolysed benzylic alcohols can be reduced by formic acid. It is known that the benzylic carbocations formed from benzylic alcohols and styrenes in acidic media get reduced by formic acid, but most examples are on trityl alcohols and similar tertiary benzylic alcohols (these easily form carbocations). Some electron poor aldehydes and electron poor conjugated alkenes can also get reduced. There are other functional groups that can also be reduced (even carboxy groups!), but there is not enough information for a general perspective. I would say that the idea of trying formic or oxalic acid on benzoin is not to be discarded.

The reductions are most efficient with the so called "activated formic acids" which are mixtures of tertiary amines with several-fold excess of formic acid. The most commonly used is the azeotropic trimethylamine : formic acid mixture which is easily prepared by the reaction of hexamine, formaldehyde and formic acid (the product is distilled from the reaction mixture). Triethylamine : formic acid mixtures also work equally well.

All this and more can be read in the two attached reviews (Chem. Rev. 69, 673–692 and Angew. Chem. Internat. Edit. 9, 50-54). If you understand Japonese, you can also try with this presumably excellent review ギ酸による還元反応 which, even if you can't read it, is nevertheless an excellent collection of references on the topic.

Then there is the patent US3397963 which is a must read on the topic, particularly on the reduction of aldehydes with formic acid.


Attachment: Reactions with Addition Compounds Containing Activated Formic Acid.pdf (165kB)
This file has been downloaded 183 times

Attachment: Chemistry of formic acid and its simple derivatives.pdf (467kB)
This file has been downloaded 13733 times

[Edited on 29/6/2012 by Nicodem]




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[*] posted on 29-6-2012 at 09:29


Thank you Nicodem for your time and effort spent on this research. It is always nice to have another pair of eyes on the look out. It is interesting that they mention that a carbocation intermediate is necessary for the reduction to take place. I would expect that in the case of benzoin the adjacent carbonyl would hinder such a carbocation from forming by withdrawing electron density. As soon as I have a chance to (I'm currently spending most of my time trying to optimize the AcOH/Zn reduction a little) I may get some more oxalic and formic acids and test out their effectiveness. Even if deoxybenzoin is not produced it seems at least possible that hydrobenzoin may be formed, which would still be interesting. It's also possible that an array of products may be formed in which case it might be difficult to separate and identify them with my current setup. Am I correct in thinking that with oxalic acid the reaction would best be carried out neat using the dihydrate?
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[*] posted on 1-7-2012 at 04:09


If hydrobenzoin is produced, I guess it would be possible to rearrange it with acid, in a pinacol-like rearrangement to desoxybenzoin?

Unless you get something similar to the benzilic acid rearrangement instead. I haven't researched it.

[Edited on 1-7-2012 by Bronstein]
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[*] posted on 1-7-2012 at 04:55


If I remember correctly hydrobenzoin in a pinacol rearrangement should give chiefly diphenylacetaldehyde actually, not deoxybenzoin.
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[*] posted on 21-10-2012 at 11:29


I'd like to confirm Nicodem's suggestion earlier in this thread that activating the Zn powder prior to this reduction makes a lot of difference. In my hands the reaction was finished in less than an hour, all zinc powder being consumed.

In a 250ml round-botton flask fitted with a reflux condensor, 10.4 grams of benzoin (98%, Acros Organics) was suspended in 25ml 80% acetic acid (technical grade from the hardware store) and the mixture was heated with strong stirring using an egg-shaped stirring magnet. When the temperature of the mixture reached 115°C a clear solution was obtained.

3.2 grams of zinc powder was activated by immersing in a 3% HCl solution for 15 minutes. The dilute acid was decanted and washed with an aliquot of distilled water, and the zinc powder was added portionwise to the hot, strongly stirring solution of benzoin, care being taken that the resulting frothing was kept under control. After this the temperature was raised so the mixture was boiling. After one hour all zinc was consumed and an oily precipitate was noted when the stirring was temporarily halted.

The contents of the flask were poured into 100ml hot water in a beaker and there was observed the formation of an oil layer on the bottom. After cooling overnight the oil solidified to white crystals with only a faint odour, distincly different than the almond-like odour of benzoin.

The crystals have yet to be weighed and recrystallized, but a quick test gave a melting point of 50°C. Considering that the main product of this reaction, desoxybenzoin, melts at 55-56°C and benzoin melts at 132-137°C, it can be reasonably assumed that the product is mainly desoxybenzoin (2-phenylacetophenone).


EDIT (Erratum): A 3% HCl solution was used, not a 1% as originally stated.

[Edited on 21-10-2012 by Sydenhams chorea]




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[*] posted on 22-10-2012 at 16:36


Update: After recrystallising from methanol (4ml for every gram) there was obtained 7.35 grams desoxybenzoin (2-phenylacetophenone), which corresponds to a 76% total yield.



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[*] posted on 22-10-2012 at 17:49


Thank you for your contribution to this thread Sydenhams chorea. When I have time I will perform this synthesis again using your procedure, and post the results here to compare.
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[*] posted on 24-10-2012 at 20:03


Please do so, I was surprised that the reaction finished so quickly. Yesterday I proceeded to synthesize what I assume is 1-dimethylamino-1,2-diphenylethane. I obtained a basic oil with amine characteristics as expected, but the hydrochloride salt is still crystallizing in the freezer, so I haven't been able to characterize my product. The procedure was adapted from similar ketimine reductions found online, and went as follows:

In a flatbottomed flask was placed 10 grams dimethylformamide (DMF) and 40 grams of a 30% NaOH solution was added, upon which the solution turned a yellow colour. The FBF was closed with a rubber stopper fitted with a glass tube, plastic tubing attached led from this tube to another glass tube immersed in 90ml methanol in another FBF.

The FBF containing the DMF/NaOH mixture was placed on a hotplate and the temperature was gradually increased until gas generation started (only gentle heating was needed, around 50°C). All dimethylamine dissolved readily in the methanol and there was no odour discomfort arising from this. When gas evolution ceases the yellow colour of the original mixture suddenly faded and became clear. At this point the alkaline hydrolysis of DMF has finished, so beware for suckback that will occur if one is careless!

After weighing the dimethylamine in methanol solution it was observed that the weight had increased by 4.75 grams (~77% yield or about 0.1 mol, ignoring the fact that the gas was not dried).[1]


The latter solution was placed in a 250ml RBF fitted with a reflux condensor, and a solution of 5.9 grams (0.03 mol) desoxybenzoin (2-phenylacetophenone) in 50ml methanol was added. The resulting solution was stirred for 2 hours (to ensure complete formation of the enamine), after which 3 grams (0.11 mol) shredded 18µ kitchen Al foil "nuggets" was added, followed with a dash (tip of the knife really) of HgCl2. The flask was fitted with its condensor, and strong stirring was performed throughout the entire reaction.

The first half hour the reaction proceeded sluggeshly, although all clear signs of amalgamating were present. Gentle heat was apllied with a waterbath for 5 minutes, which caused a moderately exothermic reaction to kick in which continued until most Al was consumed (another 30 min). It was left stirring overnight.

The next morning the flask was set up for distillation, part of the methanol was distilled off[2], and after cooling the solution was basified with concentrated NaOH solution. This was extracted twice with toluene, the pooled extracts washed 3x with a large amount of water and 2x with brine. After evaporation of the solvent 2.35 grams of a yellowish brown oil with a sharp smell and basic properties remained. On diluting the oil with 4 ml isopropanol, followed by neutralisation by dropwise addition of 30% HCl, a precipitate was formed. This is presumably 1-dimethylamino-1,2-diphenylethane, the solid product has yet to be isolated and characterised.


[1] Thanks to garage chemist for suggesting this extremely facile preparation of pure dimethylamine gas by alkaline hydrolysis of DMF in this thread:
http://www.sciencemadness.org/talk/viewthread.php?tid=9139

[2] Considering that there is some mercury metal present, however small the amount, it is better to do this outside or in the fume hood. I don't think this step is even necessary, providing that perhaps an extra extraction and extra washings are performed. About 60ml was distilled off.

[Edited on 25-10-2012 by Sydenhams chorea]




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[*] posted on 7-3-2013 at 07:52


The reduction of benzoin with formic acid was attempted using denatured ethanol as cosolvent. 5g of benzoin was added to a flask and denatured ethanol (5% methanol) was added with heating and stirring until all of the benzoin had dissolved and the solution was at reflux. 2mL of 90% formic acid was then added and the solution was refluxed for 1 hour. Fizzing of (presumably) CO2 was observed during reflux. After letting the mixture come back down to room temperature aqueous NaHCO3 was added to neutralize excess formic acid and an oil separated from the solution. It was extracted with 2x 10mL EtOAc and the organic extracts dried with Na2SO4. The EtOAc was evaporated to give a viscous oil which solidified on cooling.

TLC shows the presence of at least 3 major products, as well as a small amount of unreacted benzoin. Given the properties of the oil/solid (solubility in solvents, melting point range, etc) my initial suspicion is that it is a mixture of hydrobenzoin isomers, diphenylethanol, and bibenzyl. No deoxybenzoin was observed on TLC.
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[*] posted on 12-3-2013 at 00:17


That is an interesting result. It makes me wander, if it would be possible to obtain mainly one product by doing the experiment in the proper way, without ethanol. By the way, why did you use ethanol at all? Ethanol reacts with formic acid, reduces the reflux temperature and inhibits the solvation of benzylic carbocations, so I would expect it to greatly inhibit the reduction. Given that most of the benzoin nevertheless got converted, I would expect that refluxing it in 85% formic acid or in the Et3N / HCOOH mixture would lead to complete conversion and possibly push the reduction further to (hopefully) one single product.

Were there any TLC spots higher than benzoin? Like substantially higher as one would expect for bibenzyl? The reduction would be an interesting discovery, if it turns out preparatively useful.




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[*] posted on 12-3-2013 at 02:08


I carried out the procedure again but this time with only benzoin and 85% formic acid. The formic acid was used in excess and the two were refluxed together for an hour. However this time there was no reaction. None at all. The benzoin dissolved in the formic acid but there was no evolution of CO2. Workup revealed only unreacted benzoin. The only thing that comes to mind is that the benzoin used in the two reactions was from two different sources. The first was an old batch that was synthesized months ago, while the second was freshly prepared.

Perhaps there was an impurity in my benzoin in the first run causing the difference...
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