Isomeric_Fred
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new/old nitroalkene reduction method
Dear sciencemadnessers,
We would like to share with you our latest findings as promised some time ago.
We have been searching and testing various reduction methods for nitroalkenes to amines. We have tried various teks, including but not limited to
Steve's Reduction (nabh4/zinc+formic), nabh4/CuSO4, al/hg, and various others.
We have come up with what in our eyes seems to be the best, most benign and not dangerous way to reduce phenylnitroalkenes.
NaBH4/BF3xEt2O reduction of Nitroalkenes to amines (verification of Varma&Kebalka's method) - insitu borane reduction.
400mmol of solid NaBH4 is charged in a roundbottom three-neck flask. 550mmol of BF3xEt2O is charged in a pressure equalized addition funnel and fitted
on the rb flask, and stoppered. 500ml THF is charged in a second pressure equalized addition funnel, fitted to the rb flask and stoppered.
RB flask is immersed in an ice/water bath, third neck stoppered, and slow addition of THF and BF3xEt2O is started (THF rate should be such as to
finish at the same time as the BF3). ice bath is removed and stirring is continued
When both has been added, a 500ml THF solution of 100mmol nitroalkene (PURE!) in a third pressure equalized is fitted, stoppered, and dripped at such
a rate as not to raise the temp so much (about 1-5 drops per second). When all nitroalkene solution has been added, the reaction mixture is further
stirred for 2 hours. If by that time the color of the starting nitroalkene has not faded completly to a white solution, then an extra 50-100mmol of
NaBH4 and BF3xEt2O are added, and solution is left to stir for 10 more hours. All in all the reaction mixture should be stirred for no less then 12
hours. Then, ice is added slowly (prefrebly in a fume-hood, as some remaining borane will come out) until it is not reacting any more. To be sure all
reducing agent has been quenched water is added and watched for any reaction taking place.
Then most of the THF is removed by distillation, and 140-150ml of concentrated HCl in 200ml water is added, and solution heated and stirred at
85-90deg in an oil bath for 2 hours (again prefrebly in the fume hood, as the smell can be annoying). By the time it is done, the solution should be
totally clear, with very little color.
It is then basified, extracted 3 times with toluene, toluene washed with water, saline solution, and water again, dried, and concentrated to afford
quite pure amine at 75-85% yield.
This has been tested on several nitroalkenes and has yielded almost always very well.
It is in our eyes the best most convenient reduction method we have tried, and most consistent.
Photos will follow soon...
any comments are welcome, but please refrain from talking about banned substances, as it is not our intention for producing these materials.
NOTES :
reaction time should be at least 12 hours! if like written after 2 hours the color has not faded a boost of nabh4/bf3xet2o will result in complete
reduction.
Solvents should be relativly dry as bf3xet2o is sensitive to water.
care must be taken when ice is added, as it is done to decompose the reducing agent.
solvent volumes should not be minizmed to such a level where you get a very concentrated solution. this will result in lower yields.
If addition is not done with stoppered pressure equalizied addition flasks borane gas will escape. this is a NASTY gas, and is harmful. if not
stoppering the reaction flask, then it must be done under a fume hood or outside.
ISOMERIC_FRED!
[Edited on 10-12-2006 by Isomeric_Fred]
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Sandmeyer
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What's new with this? And what's all this "Steve's (TM) reduction" bullshit when the method has been developed by Bandil at the hive. You claimed
CuSO4/NaBH4 sucsess*, but have not posted useful info, nor have you referenced to the Tetrahedron paper I posted from which you got the idea to use
NaBH4/CuSO4 couple in the first place (in case someone else wants to try it and actually post the results and not just make useless claims).
*: https://sciencemadness.org/talk/viewthread.php?tid=6733
[Edited on 11-12-2006 by Sandmeyer]
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Isomeric_Fred
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Dear sandmayer, in the topic it is named new/old, and in the post itself, it is said as "verification of varma&kebalka's reduction method" -
insitu borane reduction.
We have not seen any post with this reduction method, and have thought to share.
Thus far, it is the most consistent, providing good to excellent results on all occaisions when done under the conditions we have written. This is not
to say that we have not suffered failures with this, but after much trial and error, we believe we have come to optimized conditions to give
consistent results. This is "new" part. If you have posts from synthetikal, then you can see that this method was only tried by 2 posters (post #
10682, phenethylamines forum, oct 04, 2005, original post by Niobium) and both have had very low yields and very non pure product prior to
distillation.
The so-called "steve reduction" has indeed been developed in the begining by bandil, but the procedure he supplied was not giving consistent results.
I believe Stoichiometric Steve has optimized it for several nitroalkenes.
About NaBH4/CuSO4, we have tried it various times, with several refs at hand (Synthetic Communications, 24(4), 549-555 (1994) as well as SynLet, July
1990, 419-420). We have found that under various conditions there is some success, but it is non consistent. We have the habit of only posting full
workups when we reach consistent success or consistent failures.
Im sorry if I have offended you, it is surely not our intentions.
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Sandmeyer
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Please, post the Syn. Comun. article if possible!
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Isomeric_Fred
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here it is, as you requested. note that this is for azides, but alot can be learned from this regarding nitroalkAnes. They are similair to
nitrogroups, and usually reduction mechanisms that work for azides, work for nitrogroups.
Attachment: 000399263-Syn_Comm_24_Reduction_of_azides_with_NaBH4_and_CuSO4.pdf (405kB)
This file has been downloaded 1625 times
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