(Hive Bee)
09-04-02 17:56
No 352602
      Get good results with Red-Al
(Rated as: excellent)

Red-Al [sodium bis(2-methoxyethoxy)aluminum hydride] is a very good reducing agent and a good substitute for lithium aluminum hydride (LAH). Compared to LAH, red-al is not sensitive to oxygen (but to moisture) and is very soluble in ether, THF, 1,4-dioxane, toluene and benzene.

There is a write-up on Rhodiumīs page which describes the reduction of 1-(2,4,5-trimethoxyphenyl)-2-nitropropene to TMA-2 using red-al and toluene. What this write-up lacks is some necessary advice in order to get as good, or better, yields as with LAH.

In theory each mol of a phenyl-2-nitroalkene needs 4 mols red-al to produce the amine. Which can lead one to think that a slight excess (0.2-0.5 mol, due to the contact with moisture in the air and some in the substrate) red-al is enough to get some smashing +95% yields. Sorry!!
To get the good yields, in my experience between 85-97%, one must use quite an excess red-al due to some wierd side reactions which both can and will take place otherwise.

As a rule of thumb 8mol red-al/mol nitrostyrene gives good results, BUT the thing to look for is the color of the reaction mixture. It should remain colorless or just slightly colored. If it becomes strongly colored I can almost guarantee the yield will not be higher than 30-40%.

Example of the procedure.

100 mmol (25.3g) 1-(2,4,5-trimethoxyphenyl)-2-nitropropene dissolved in 500ml toluene (toluene dissolves 50g/L) is added dropwise to 800 mmol red-al (225 ml of a 65% solution in toluene) diluted with 200ml toluene. Adjust the addition so the temperature doesnīt exceed 70 deg C.
when about 2/3 of the nitroalkene has been added itīs time to keep an eye on the color. Up until now the yellow color from each drop of the nitroalkene solution has dissapered at once it hits the hydride solution. This is the way it should behave until all is added. If not, immediately one sees a remaining color, stop the addition and add 50-100 mmol red-al then continue the dripping.
When all nitroalkene has been added allow the reaction mixture to stir and come back to room temperature during 1-2 hours. When the gas evolution has ceased the reaction is over. The work-up is done according to the procedure at Rhodiumīs page.
(Chief Bee)
09-04-02 18:13
No 352609
      Red-Al  Bookmark   

Great, this has now been amended to my ../rhodium/chemistry /red-al.html document.