Sodium triacetoxyborohydride
The boron-hydrogen bond is stabilized by the steric and electron-withdrawing effects of the acetoxy groups, making sodium triacetoxyborohydride a mild reducing agent.
Sodium triacetoxyborohydride is especially suitable for reductive aminations. Since the reaction rate for the reduction of iminium ions is much faster than for ketones or even aldehydes, the reductive amination can be carried out as a one-pot procedure by introducing the reducing agent into a mixture of the amine and carbonyl compound. The presence of a stoichiometric amount of acetic acid, which catalyzes the imine formation and provides the iminium ion, doesn't present any problem under these conditions.
Reductive amination (simplified)
Secondary amines also undergo this reaction. This leads to the possibility that primary amines can be converted to the dialkylated products (tertiary amines), although the reaction rate is quite slow. Alternatively, the imine may be formed in a prior step and then reduced by NaBH4:
A. F. Abdel-Magid, K. G. Carson, B. D. Harris, C. A. Maryanoff, R. D. Shah, J. Org. Chem., 1996, 61, 3849-3862.
Recent Literature
In comparison with other
reductive amination procedures such as NaBH3CN/MeOH,
borane-pyridine, and catalytic hydrogenation, NaBH(OAc)3 gave
consistently higher yields and fewer side products. In the reductive amination of
some aldehydes with primary amines where dialkylation is a problem, a
stepwise procedure involving imine formation in MeOH followed by
reduction with NaBH4 was developed.
A. F. Abdel-Magid, K. G. Carson, B. D. Harris, C. A. Maryanoff, R. D. Shah, J. Org. Chem., 1996, 61, 3849-3862.
A one-pot, tandem reductive
amination-transamidation-cyclization reaction produces substituted
piperazin-2-ones in good yields.
D. C. Beshore, C. J. Dinsmore, Org. Lett., 2002, 4, 1201-1204.
