I have attempted the reduction of a certain imine by use of NaBH4 in EtOH (200 proof). I have been able to increase the yield from ~ 15% to about 50% (4 eq. NaBH4 w.r.t the imine). I have also attempted the same reaction with STAB in DCE but for reasons quite unclear to me, the reaction failed to give me the corresponding amine. Can someone advise on the exact no. of equivalents of NaBH4 for similar reductive amination reactions, the mode of addition of the hydride (as solid / as a suspension / at R.T / at 0 C etc) and the actual method of quenching.
Currently I add 6M HCl to quench the reaction mixture and the amine is isolated as the hydrochloride salt.
Please could you also suggest the no. of equivalents of NaBH4 to be used.
Thanks!
Use sodium borohydride in dioxane (if your imine dissolves in this solvent)
1.6 eq. NaBH4 at room temp for 10-15 min
A. F. Abdel-Magid, K. G. Carson, B. D. Harris, C. A. Maryanoff, R. D. Shah, Reductive Amination of Aldehydes and Ketones with Sodium Triacetoxyborohydride. Studies on Direct and Indirect Reductive Amination Procedures. J. Org. Chem., 1996, 61, 3849-3862.
Thank you all for the information. A point I failed to highlight in my question is the presence of phenolic groups in my system (ortho to the imine; imine). This is why I have had to use higher equivalents of NaBH4.
I have as yet to try cyanoborohydride or other similar reagents capable of effecting reductive amination.
Thank you all once again.
Sudarsan, I think that there is no problem with your phenolic group. Simply a large excess of NaBH4 is usual to reduce imines to amines (5 - 6 equivalents). Plus, if you do not need the amine salt, you can quench your reaction with NaCO3 aq. and extract with ether or other suitable solvent.
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