I'm referring on the
Large-scale MDA from piperonal (https://www.thevespiary.org/rhodium/Rhodium/chemistry/mda.txt)
(https://www.thevespiary.org/rhodium/Rhodium/chemistry/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..
Yes, no problem. Bu the 200g LiAlH4 might bee hard to get ;-)
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?
The maximum solubility of LiAlH
4 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 LiAlH
4 per liter of solution. Therefore I don't reccommend that you go any lower than 5 liters of THF to dissolve 200g LiAlH
4.
Also, as both the dissolution of LiAlH
4 in THF as well as the addition of the nitrostyrene to the LiAlH
4 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 LiAlH
4, 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 LiAlH
4 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 (https://www.thevespiary.org/talk/index.php?topic=8867.msg37686400#msg37686400)
(Rhodium: "Lab-size Mushroom Cloud", Methods Discourse) Post 465429 (https://www.thevespiary.org/talk/index.php?topic=10858.msg46542900#msg46542900)
(Rhodium: "LAH will fizz even with dry THF...", Newbee Forum)http://www.dehs.umn.edu/whatshot/LithiumAH.html (http://www.dehs.umn.edu/whatshot/LithiumAH.html)