silenziox (Newbee)
11-21-03 17:11
No 472362
       Large scale nitroalkene --> amide     

I'm referring on the Large-scale MDA from piperonal (../rhodium /mda.txt)

Could the "200 g of LiAlH4 dissolved in 10 L of ice cold anhydrous THF" ratio be suppressed to f.ex. 5L of THF?
Never done LAH reductions, but I think this is the way to go..
There is only one thing I can't tolerate and that's intolerance! 		 
 
 
 
    hest
(Hive Adickt)
11-22-03 04:46
No 472476
       THF     

Yes, no problem. Bu the 200g LiAlH4 might bee hard to get ;-) 		 
 
 
 
    silenziox
(Hive Bee)
11-22-03 10:01
No 472508
       No it isn't. It's just damn expensive ...     

No it isn't. It's just damn expensive (~160usd/200g).

But back to the question, what's the minimum amount of the solvent "best" to use on LAH-reductions?
There is only one thing I can't tolerate and that's intolerance! 		 
 
 
 
    Rhodium
(Chief Bee)
11-22-03 11:50
No 472533
       Large-scale LAH nitrostyrene reduction safety
(Rated as: good read) 		    

The maximum solubility of LiAlH4 in THF at 25°C is 130g/kg, and with the density of THF being 0.89 this is equivalent to ~115g/L. However, many sources reccommend that for safety reasons, the maximum concentration used in practice should be limited to around 1 mole/liter, which is 38g LiAlH4 per liter of solution. Therefore I don't reccommend that you go any lower than 5 liters of THF to dissolve 200g LiAlH4.

Also, as both the dissolution of LiAlH4 in THF as well as the addition of the nitrostyrene to the LiAlH4 solution are very exothermic processes, I wholeheartedly reccommend that you use an ice-bath to cool the reaction flask and the THF to 0-5°C before adding the LiAlH4, and that you do it slowly in portions of at most 10-20g. Use a two-necked reaction flask, or if you must use a single-neck setup, don't allow the powder funnel to cover the entire neck when adding each LiAlH4 portion, as there will always be a positive pressure in the flask during addition. Allow the temperature to return to 0-5°C before adding the next portion. The concentrated nitrostyrene solution in THF is to be added dropwise so that the evolved heat gets a chance to dissipate, otherwise a runaway reaction may result.

See the following documents:
Post 376864 (Rhodium: "Lab-size Mushroom Cloud", Methods Discourse)
Post 465429 (Rhodium: "LAH will fizz even with dry THF...", Newbee Forum)
http://www.dehs.umn.edu/whatshot/LithiumAH.html