Bariums wet reductive-amination, has gained great use throughout this forum. Although it gives nice yields - they are always lower than when using anhydrous conditions (mostly because some of the ketone is reduced to the alcohol), because the imine is formed in equilibrium with the ketone. Under anhydrous conditions the imine is in a rather "pure" state.
In order to push the limits on this reaction, i decided to turn to my old love - molecular sieves
, and test the usefullness in this reaction.
para-fluoro-P2P was aminated with a 40% commercial aqueous methylamine solution in the following manner:
25 mmole pfP2P was dissolved in 30 mL's IPA (predried with MgSO4
). 40 mmole methylamine (3.1 g 40% aqueous solution) was put in the same beaker. Subsequently 15 g's1
3A molecular sieves was added, and the whole mess stirred for two hours at room temperature.
The sieves where filtered off2
and rinsed with an additional small amount of dry IPA. The combined IPA/imine was put in a flask and stirred. 1 g NaBH43
was added in one portion, to this and stirred violently overnight at room temperature. The flask was covered with some foil during the night.
A control experiment was made with the exact same amounts, using the "normal" wet-amination scheme, in order to get some usefull results4
. This was also reduced overnight.
The following day, both flasks where quenched with acetic acid (untill pH = 2). Totally 50 mL's of water was added to each. The IPA was stripped at atmospherical pressure. The water in the "dry" flask was completely clear after stripping, while the control flask was somewhat unclear with some yellowish oil remaining(probably pfP2Pol). The acidic water phases where washed with 2*20 mL's toluene. Basifying with NaOH caused a white oil to precipitate. This was extracted with 3*20 mL's DCM. The DCM phases where dried over epsom salt, and the solvent stripped.
Both the experiments gave a sligthly yellow tinted oil (GC-MS has not been conducted yet). The "dry" flask gave 3.4 g's of p-fluoro-methamphetamine, and the wet one gave 2.4 g's. This corresponds to a 81% and 58% yield
. Thus using molecular sieves (which are dirt-cheap and reuseable, its possible to greatly improve the yields on this easy reaction.
) The reaction is still "wet", meaning that the need for alcoholic methylamine has been eliminated. Methylamine generated in-situ from the hydrochloride and an alkali hydroxide can most likely be used just as easy.
I'd advice everyone to get some sieves - they truly rock.
Now ill go flush that evil N-methylated amphetamine down some drain - its probably evil
Theoretically 12 g's should be used (20% absorbance) - just added a litte extra to be sure.
To reuse them, they where rinsed with dilute HCl, and then pure water a few times. After a few hours in the oven, they are ready to go again.
Way to much borohydride - half is more than enough. Was just to lazy to crush those tablets.
I am rarely able to get the same yields as Barium, due to my somewhat more "sloppy" nature. Thus i ran a control experiment, to check things out. It was also done in order to get some usefull data - and not just another drug candidate