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Author: Subject: Reduction of substituted 2-phenylnitroethenes to 2-phenylnitroethanes with NaBH4 - Maybe the greatest way to make tar

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[*] posted on 23-4-2018 at 20:53

Finally a great success to share:
A suspension of 5g substrate in a solvent-mixture composed of IPA and DCM (sry I didn't remember ratios and can't find the paper anymore the idea came from which used chloroform+IPA) and a huge excess (~maybe 40g if I remember correctly) of silical gel (made from silica gel* cat litter!!! which was finely powdered using an electric coffee grinder and kitchen metal sieve) was added in small portions to a suspensions of a teaspoon of homemade homemade "Raney-Ni T-4" to about 10g very old NaBH4 (catalyst and especially NaBH4 were used in extreme excess, the amount of NaBH4 should be a MASSIVE OVERKILL, a fraction could be used for sure) in little cold absolute EtOH. Everything was stirred using a magnetinc stirrer. Reaction was cooled slightly using an water bath and a few ice cubes - reaction at this scale was very smooth, it didn't get a very warm, cooling might be actually optional.

After addition the reaction stirrend for another one or two hours, the catalyst+silica gel were filtered off by suction filtration and washed with EtOH.

-->Don't don't destroy the NaBH4 as long as Ra-Ni is present, as it will yield into a very voluminous sludge which is very hard to filtrate and wash (and dangerous due the massive hydrogen evolutin and the pyrophoric nature of Ra-Ni).

The excess of NaBH4 was extracted two times with 40% aq. NaOH (--->maybe aq. potassium carbonate might be better but I'm not sure as 2C's do react rapidly with CO2 and maybe aswell with carbonates to form carbmates(!?), maybe someone has an idea to this point?<---), the organic phase was dried using fused and powdered MgSO4 (choosed it instead of waterfree Na2SO4, as it could partially neutralise traces of remaing NaOH which might hydrolyse the a part of the product during the destillation process), decanted from the drying agent and drying agent was washed three more times with EtOH, solvents were destilled and evaporated under aspirator vakuum using a boiling water bath as it source, the water bath was switched to an ~160°C oil bath, the revieving flask was switched to an 25mL oven-dried RBF and pump was switched to a two-stage rotary vane pump -->

The remaining oil was destilled** using the two strage rotary vane pump with a cold trap to yield 83%(!!!!) of theory of completly colorless 2C-H which was successfully identified by TLC and by making the HCl salt (by HCl-gas made from fusing NaCl and pre dried NaHSO4 in a flask on a hotplate , gassing the HCl into a little toluene+base, yielding 79% of constant dried very pure 2CH*HCl, directly hitting the literature metling point without further recristallsation)

Without the silica gel and absolute EtOH as only solvent the yield drops to about 50-60% "only" (still better than everything else tried except LAH).

*the silica gel acts as a weak proton donor to improve yield of the in-situ produced phenylnitroethane and avoid micheal-addition side product

**also tried A/B extraction with DCM, but DCM tends to form emulsions and you need very huge amounts (at least four extractions with a lot solvent) to get a similar yield with lower purity, so direct destillation is prefered

-Cat litter silica gel seems to be a usable source of cheap silica gel which is pure enough for this purpose, it can be easily grinded using a electric coffee grinder. There's no need of using expensive chromatography-grade silica gel.

-The idea to this was developed after 10+ more or less unsessful tries (mostly based on the writeups from "Barium", using EtOAc or IPA as only solvent an isolating the intermediate) but got only very bad yields due side products forming - but now finally a good yield was reached mixing several methods - The reaction isn't optimized yet, I'm almost sure the yield can reach 90%+ using the perfect conditions and solvents, as already with such a sloppy work a ~80% yield was reached. Phenyl-2-nitropropenes maybe even reach 95%+ as they should produce even less side products!

-Reusage of the Ra-Ni + silica gel sludge wasn't tried but should be possible a few times.

-Most likely Ni2+ (as nickel acetate or NiCl2) precipitated onto the silica gel in MeOH or EtOH as solvent with NaBH4 directly onto the silica gel matrix and then using DCM/IPA as solvent system should work just as well as Raney-Nickel T4, maybe It's giving even giving higher yields as the intermediate should react faster. And this specialised catalyst might be reusable several times aswell.
Though "even better yields" are only speculation so far.

[Edited on 24-4-2018 by Bottle]
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Chemi Pharma
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[*] posted on 24-4-2018 at 01:24

Congratulations @Bottle. Finally my point of view was proved. Nickel Boride made "in situ" from raney nickel or nickel salts and NaBH4 is far superior than other methods to realize a low cost and high yielding hydrogenation.

The surprise for me was the usage of silica gel as you've proposed and DCM/IPA as a solvent media. Very interesting. I will try this kind of optimization next time.

Well done. Nice job!

[Edited on 24-4-2018 by Chemi Pharma]
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S.C. Wack

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[*] posted on 24-4-2018 at 17:55

Quote: Originally posted by Bottle  
2N HCl was then carefully2 added until pH 4 was reached followed by enough solid NaCl to cause the IPA to form a separate layer containing the product.


I used ~30% HOAc instead of HCl to bring the pH to 4.00 and used a 300ml erlenmeyerflask with ice-bath-cooling instead of RT to avoid dimeric sideproducts (maybe a mistake?)

A bit late reading the thread just now...BTW there was something posted as an attachment somewhere at the-hive saying "This document presents a writeup of his final work on the topic. Some modifications of the original procedure has been included in this document."'s a Rhodium page (Easy borohydride reduction of beta-nitroalkenes to nitroalkanes) but it isn't in Rhodium and wasn't then AFAIK and after 14 years I forget what's up with that, but notice how acetic acid appears and somehow everything before and after is the same.

Dilute 80% acetic acid was added untill no more fizzing from the remaining borohydride occured. This is followed by addtion of enough solid NaCl to cause the IPA to form a separate upper layer containing the product. The mixture was suction filterd and rinsed with a little IPA.


General Procedure

Use the following amounts of reagents:

1 mole eq. of any arylnitrostyrene
1,25 mol eq. potassium or sodium borohydride1
Approximately 5*wt. of the nitrostyrene in mL's of IPA (i.e. 10 g P2NP would require 50 mL IPA)
2/5 the vol of IPA used of dH2O

Mix the IPA and water in a sufficiently large beaker. Add the borohydride in one portion and commence stirring. Begin adding the nitrostyrene in small portions after a minute or two. This will cause a rise in temperature and moderate hydrogen evolution. Add the substrate in small portions to avoid any nasty volcano reactions. The rate of addition should be such that a lively evolution of hydrogen can be observed in the beaker, but slow enough to avoid the reaction shooting out of the flask.

Once all of the substrate has been added, keep stirring the mixture for 30 mins. The colour of the mixture should be alot more pale than that of the nitrostyrene2. Add dilute (32-80%) acetic acid drop wise untill fizzing stops. Add solid non-iodized table salt while stirring heavily, untill no more dissolves. Suction filter the mixture, to remove any remaining salt and borates. Rinse the filter cake with a little fresh IPA. The IPA layer, containing the product will float on top of the water. Isolate the IPA layer, and discard the water.

At this point the reaction is over, and the IPA layer contains a product which is sufficiently pure for a CTH, Zn/Formate, SnCl2 nitro reduction or whatever your preference might be. So simply use this IPA direcly.


[1]The larger the scale of the reaction is, the larger molar eq. of boroydride is required. From emperical knowledge potassium borohydride requires a larger molar eq. Perhaps KBH4 decomposes faster in the IPA/water environment.

[2]Please use as pure nitrostyrene as possible in this reduction. If there are too many coloured impurities left, it is very hard to tell if the compound has been reduced properly, once the reaction is over.

[Edited on 25-4-2018 by S.C. Wack]

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[*] posted on 30-4-2018 at 07:07

The discussion is over, I guess, although @S.C. Wack oppinion.

The thread is exhausted. The two papers I brought to the topic teaching how to reduce nitrostyrenes with zinc and HCl at 0ºC to phenylethylamines and phenyl-2-nitropropenes to amphetamines with nickel boride made in situ with nickel salts (or the more expensive raney nickel) and NaBH4 is enough in my oppinion.

@bottle testimony above proved what I've been saying all time long about the better techniques to do this kind of reductions with satisfactory yields.

[Edited on 30-4-2018 by Chemi Pharma]
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