efficiency, and 1,2-diphenylethane-1,2-diol gave the highest enantioselectivity
(Scheme 11). Treatment of the imines in toluene, at 80
C, over 24 h, with 3 equivalents of this alcohol, in the presence of 5 mol% of complex 48 and 5 mol% of
phosphoric acid afforded the corresponding amines with yields from 60 to 90% and
e.r.’s from 87/13 to 98/2. Based on experimental studies and DFT calculations, a
mechanism involving an iridium alkoxide as the reducing agent was proposed [86].
2.2 Transfer Hydrogenation of CO 2
With the aim of reducing the concentration of CO 2 in the atmosphere, methods to
convert it into valuable chemicals have been the subject of intense investigation in
recent years [87]. Among them, catalytic methods for CO 2 direct hydrogenation to
one-carbon molecules such as formic acid or methanol using homogeneous metallic
O
O
P
O
OH
iPr
iPr
Pr
iPr
iPr
Pr
i
i
Scheme 10 Chiral phosphoric acid used as additive in the TH of imines
Cat* =
OH
Me
Ph
OH
Ph
Ph
OH
Ph
OH
OH
OH
OH
Ph
Ph
OH
OH
OH
Ph
OH
Ph
92%
90/10 e.r.
87%
88/12 e.r.
31%
91/9 e.r.
73%
92/8 e.r.
16%
75/25 e.r.
90%
95/5 e.r.
70%
91/9 e.r.
Toluene, 80 ºC, 24 h
HN
Ir
Cp*
N
48
Ph
Ph
SO 2 C 6 Me 5
N
Me
Bn
HN
Me
Bn
E/Z = 12:1
Hydrogen
donor
=
Hydrogen donor 3 equiv.
cat* 5% mol
Phosphoric acid 5% mol
Scheme 11 Screening of hydrogen donors for the TH of imines
88
M. Pilar Lamata et al.