Improved Methodology for the Preparation of Chiral Amines

2.3.2. Different Substrates Categories
Several organometallic and organocatalytic systems were developed for the reduction of
phenyl methyl (ethyl) N-aryl imines. The hydrogen source is either molecular hydrogen or
hydride reagents. Transition metals having chiral ligands on Ir, Ru, Rh and Ti were used and
resulted in high yield and selectivity.
Pfaltz and Leitner used cationic Ir complexes with chiral phosphinodihydrooxazoles modified
with perfluroalkyl groups. [18] Using 0.09 mol % of the catalyst (catalyst 1, figure 2.4), 30 bar
(435 psi) H 2 , supercritical carbon dioxide (scCO 2 ) at 40 °C an ee of 80% was accomplished
with complete conversion (structure 1, figure 2.2). The choice of counter ion dramatically
influenced selectivity with tetrakis-3,5-bis(trifluoromethyl)phenylborate anion (BARF),
resulting in the highest selectivity. They later developed the use of scCO 2 with ionic liquids
and obtained the same result. [19]
Zhang and Xiao reported one of the earliest examples for the efficient reduction of aryl
methyl N-aryl imines utilizing iridium. They introduced the use of air stable Irbisphospahnoferrocene
catalyst (catalyst 2, figure 2.4). [20] Using 2.0 mol % of the catalyst
(catalyst 2, figure 2.4), 70 bar (1015 psi) of H 2 , CH 2 Cl 2 , 25 °C over 44 h, 99% ee with 77%
conversion for phenyl methyl N-aryl imines (structure 1, figure 2.3). For p-OMe-phenyl
methyl N-aryl imines the ee was 98% with 77% conversion and for p-CF 3 -phenyl methyl N-
aryl imine the ee was 99% with 80% conversion (structure 3, figure 2.3).
Claver and Castillón introduced the use of sugar derived diphosphite ligands. [21] Using 1.0
mol % of the iridium catalyst (catalyst 3, figure 2.4), 10 bar (145 psi) H 2 , CH 2 Cl 2 , 25 °C, 18 h
an ee was 57% with 83% conversion for phenyl methyl N-aryl imine (structure 1, figure 2.3).
The use of 4.0 mol % Bu 4 NI improved conversion (100%) but lowered the ee (46%) at 70 bar
(1015 psi) of H 2 . Later they reported the use of other diphosphinite ligands (catalyst 4, figure
2.4). Using 1.0 mol % of the catalyst, 70 bar ( 1015 psi) H 2 , CH 2 Cl 2 , 25 °C, 16 h, the ee was
70% with complete conversion. [22]
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