Klute - 7-4-2007 at 18:13
Alot of common procedure for anhydrous reductive aminations of ketones with NaBH4 use silica gel during the imine formation, but decant/filter it
before the reduction/adiition of the hydride... Why is that?
Silica gel wouldn't cause any interfernece with the reduction, no? Actually, it could be helpfull : as the formed imine is reduced, any unreacted
ketone would form a imine as equilibrium is deplaced by the consomation of the imine (even in anhydrous conditions, I doubt the imine formation is a
totaly complete..), so more water is produced. Leaving the silica gel to absorb that water would deplace the equilibrium even more, leaving less
ketone to be possibly reduced by the hydride... no?
I don't understand the reason for filtering/decanting a smelly MeNH2 solution before reduction if the silica doesn't interfer with the reaction..
bio2 - 8-4-2007 at 14:56
Silica gel looses water at relatively low temperature starting
IIRC somewhere over 40 or 50 degrees.
Maybe this is why, except the reaction in question is
usually performed under room temperature or ice temperature.
Molecular sieves could be used as they will hold water
at over 100 degrees.
Klute - 8-4-2007 at 18:56
I guess that filtering the silica before acid hydrolysis of excess hydride and distillation should take care of the water release problem... We will
see if it really has an impact or not...
Nicodem - 9-4-2007 at 04:34
Silica gel is a weak acid and thus promotes the decomposition of NaBH4 to H2. Perhaps this is the reason why it is removed before the addition of
NaBH4.
Klute - 9-4-2007 at 13:43
Good point Nicodem, I didn't think of that.... It makes sense, the hydride is already been partially degraded by the solvant when using alcohols...