Author Topic: Reductions with metal borohydrides -Strontium  (Read 1341 times)

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Reductions with metal borohydrides -Strontium
« on: April 19, 2000, 07:16:00 PM »

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Author  Topic:   Reductions with metal borohydrides 
Strontium
Hive Bee   posted 01-15-2000 09:47 AM          
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I have to admit that I´m totally puzzled here. In TSII, Rhodium gives a example of a reduction of phenyl-2-nitropropene to phenyl-2-aminopropane, with in situ generated Ni2B and sodium borohydride, in MeOH. Well that´s all fine and dandy.
My quiz here is about what´s really going on in that reaction flask.
When 2 mol sodium borohydride is added to 6 mol MeOH, 2 mol sodium trimethoxyborohydride is quickly generated, and 3 H2 is given off.

2NaBH4 + 6MeOH --> 2Na(OMe)3BH + 3H2

So when this nitropropene is around it´s being reduced to an aminopropane. But is it an ordinary H2 reduction with that Ni2B as catalyst, similar to H2 over Pd/C reductions?
Or is it that funky sodium trimethoxyborohydride that acts as the reducing agent with Ni2B as catalyst?
Or is it a combination of them both?

I´ve asked around quite a lot. But no one seems to know actually.

How stable is really that sodium trimethoxyborohydride? Could a 1M soln in MeOH be made and stored for a period
(one week)?

Strontium


Rhodium
Pimp Master   posted 01-15-2000 01:01 PM          
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Check out JACS 1982, 104, 6801-6802.
It deals with the mechanism of CoCl2/NaBH4 reduction (in which reaction Co2B is formed), and I believe the mechanism is the same with Nickel.

It appears that the metal boride complexes with the substrate in some fashion, and thus making it easier to reduce by NaBH4.

It is not an ordinary H2 reduction, as the authors above found that stirring a nitrile with Co2B under a H2 atmosphere gave no reduction, even after 11 days....


rev drone
Hive Bee   posted 01-19-2000 10:57 AM          
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Strontium,
I have to disagree with the Pimp Master. Actually, from our discussions, and from the literature I've read, Ni2B acts as a heterogeneous hydrogenation catalyst. Its applications, as well as its catalytic activity, seems to indicate it as such. In fact, there are examples in the literature of it being used with H2 rather than NaBH4 for reducing various organic compounds. In fact, I know you've even read these papers (I'll did out the pertinant ones tonight.)

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-the good reverend drone

Ipsa scientia potestas est


 
Rhodium
Pimp Master   posted 01-19-2000 06:16 PM          
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Ok, but I don't understand wehy Co2B would behave so radically different from Ni2B?
 
rev drone
Hive Bee   posted 01-27-2000 07:02 AM          
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Rhodium,
The source of the difference lies primarily in the nature of the complexes formed.

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-the good reverend drone

Ipsa scientia potestas est


 
LaBTop
PimpBee   posted 01-28-2000 01:05 PM          
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-quote- How stable is really that sodium trimethoxyborohydride? Could a 1M soln in MeOH be made and stored for a period
(one week)? -end quote-
Strontium, in practice it's tried out with the NaBH4 one pot method, and observed that when you add the NaBH4 to a very cold (-30 C) MeOH solution, only sparse bubbling of H2 starts, and keeps on going for a long time, more then 2 hours.(when deep cooling is provided).
How far it really would keep going at that very low boiling rate is not practicised, the choise was made for adding the NaBH4 in powder form.
However, I still am curious if that whole pain in the ass adding spoonfulls per 5 minutes could be substituted by controlled dropping in of a freezing (-30C) cold NaBH4/MeOH solution via a deep cooled ss drip funnel.
In my opinion the longest you could save such a freezing cold solution would be around 5 to 7 hours, and you should have to add extra NaBH4 to compensate for losses over time. But that would be enough for the automated addition.
It would however simplify the whole procedure extremely and give much better reproducable yields in multiple batches.
( humans tend to get bored after 4 to 5 hours adding spoons of NaBH4 and start to smuggle with the time and quantities!)

Why you would want to store such a solution for a week ? You can make it on the spot.

And why should this procedure not work (with or without the Ni2B) following my method?
(I have not read the reduction mentioned, so forgive my ignorance if this is obviously impossible). LT/

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WISDOMwillWIN


 
rev drone
Hive Bee   posted 02-04-2000 05:13 AM          
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...Then again, automated dry addition funnels are definately an off-the-shelf process engineering item. Who needs humans anyways?
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-the good reverend drone

Ipsa scientia potestas est


 
LaBTop
PimpBee   posted 02-04-2000 11:34 AM          
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Made several in the past, every fuckin one of them ended up in malfunction, because the NaBH4 attracked humidity from the air and became hard as concrete within 2-3 hours.
Thinking of an airtight dry addition process every day for the past 14 years. LT/
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WISDOMwillWIN


 
LaBTop
PimpBee   posted 02-04-2000 11:37 AM          
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To elaborate: it's not the air around the device, it's the air/methanol vapors which enter when the diafragm has to open every 5 minutes! LT/
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WISDOMwillWIN


 
LaBTop
PimpBee   posted 02-04-2000 11:42 AM          
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Am now thinking of pressing 10 cm diameter and 2 cm thick NaBH4 pills, put them in a airtight tower, and use a second chamber at the bottom to collect one of them after the previous one is fallen down in you know what. LT/
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WISDOMwillWIN


 
Strontium
Hive Bee   posted 02-04-2000 12:15 PM          
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Labby
"Why you would want to store such a solution for a week ? You can make it on the spot."

Of course, there is a bunch of things which can made on the spot. But it would be nice to know whether or not one could make and store say 25L 1M Na(OCH3)3BH/MeOH, or not.

I´m always toying around with the idea of scaling up a good method, from labscale to a scale of pure erotica
Then it´s nice to know if such a soln. is stable enough for longer storage times or not.

Strontium


Teonanacatl
Hive Bee   posted 02-04-2000 10:54 PM          
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The only related reducing agent I'm aware of with a non-negligible shelf-life is the sodium amino borohydrides...but I have seen very few references to their reducing power, and the nitrostyrenes were not one of these references...on the other hand I haven't seen any definate references against this...