Improved Methodology for the Preparation of Chiral Amines

More recently, 2008, he described the use of S-Chiral Bissulfinamide catalyst for imine
reduction. [44] Using 10 mol % of the catalyst (catalyst 25, figure 2.4), 2.0 equiv Cl 3 SiH, 0.3
equiv 2,6-lutidine, CH 2 Cl 2 , -20 °C, 24 h, the ee was 96% with 91% yield for phenyl methyl
N-aryl imine. (structure 1, figure 2.3). p-OMe (95% ee, 83% yield), p-Br (95% ee, 92%
yield), p-NO 2 (93% ee, 90% yield) and p-CF 3 (95% ee, 95% yield) phenyl methyl N-aryl
imines were reduced (structure 3, figure 2.3).
2.4. Reduction of Miscellaneous Imines:
Other imine derivatives were reduced as N-benzyl and N-tosyl imines. These substrates were
less investigated compared to the previous discussed substrates in the last 10 years. For
further reading please consult the following literatures. [45] Oximes and hydrazones were also
tested but with fewer examples over the last 10 years. For further reading please consult the
following literatures. [46] Imines with different chiral auxiliaries were reduced in good to high
enantioselectivity. For further reading please consult the following references. [47] Cyclic
imines were extensively investigated and extensively reviewed in book chapters and
published reviews. For further reading please consult the following references. [48]
2.5 Conclusion.
Imine reduction has been extensively investigated by many groups over the last forty years.
During the last two decades several milestones have been achieved in asymmetric imine
reduction. Imines are reduced with high enantioselectivity and yield. This methodology is
highly efficient for obtaining α-chiral amines in 99% enantioselectivity but the over all yield
from the starting material to the final product is usually low and below 50%. Removal of the
auxiliary usually requires harsh acidic conditions which may not compatible with different
acid sensitive groups. Despite these drawbacks the high enantioselectivity obtained makes
this method attractive for further improvements.
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