Chemistry Reference
In-Depth Information
TABLE 7.19. Asymmetric Hydrogenation of
α
- Aryl
N
- Arylimines
Ar
Ar
HN
N
Ir catalysts
∗
H
2
R
R
Conv.
(%)
Catalyst
R
Ar
S
/
C
Reaction Conditions
ee %
Reference
Ir - PHOX
H
Ph
1000
CH
2
Cl
2
, 5 ° C, 100 atm
100
89
371
Ru - Et - DuPhos
H
Ph
i
PrOH, 65 ° C, 15 atm,
69 h
97
94
360
Ir - f - BINAPHANE
H
Ph
100
100
94
372
CH
2
Cl
2
, I
2
,
5 ° C,
1000 psi, 40 h
−
Ir - f - BINAPHANE
MeO
Ph
100
CH
2
Cl
2
, I
2
,
5 ° C,
1000 psi, 24 h
−
100
95
372
Ir - f - BINAPHANE
H
2,6 - Me
2
Ph
100
CH
2
Cl
2
, rt, 1000 psi,
44 h
77
>
99
372
Ir - f - BINAPHANE
MeO
2,6 - Me
2
Ph
100
CH
2
Cl
2
, rt, 1000 psi,
44 h
77
98
372
Ir - f - BINAPHANE
CF
3
2,6 - Me
2
Ph
100
CH
2
Cl
2
, rt, 1000 psi,
44 h
88
99
372
A Pd-BINAP catalyst has been tested in the hydrogenation of a series of
β
- fl uorinated
α
-imino esters. Up to 91% ee was achieved in 2,2,2-trifl uoroethanol (Eq. 7.51 ) [376] .
The Zhang group expanded this family of substrates to
- imino esters
[377]. With an Rh-TangPhos catalyst, 90-95% ee and over 800 turnover numbers were
achieved under reasonable conditions (Eq. 7.52).
α
- alkyl or aryl
α
0.5 mol %
Rh-(
R
,
R
)-DeguPhos
O
NHBn
+
BnNH
2
MeOH, rt, 60 bar H
2
COOH
COOH
99%
R
-(+)-, 98% ee
(7.53)
The related reductive amination of ketones without isolating and purifying the imine
intermediates serves as a concise and convenient approach for the preparation of chiral
amines. Using an Ir-f-Binaphane complex as the catalyst, complete conversions and high
enantioselectivies (up to 96% ee) were achieved in the asymmetric reductive amination
of aryl ketones in the presence of Ti(O
i
Pr)
4
and I
2
[378]. The hydrogenation product can
be deprotected by ceric ammonium nitrate (CAN) to give the chiral amine product with
ee intact (Scheme 7.16). Among a large number of catalyst screened, Rh-DeguPhos
(PyPhos) has shown high enantioselectivity for reductive amination of α - keto acids, and
a series of α-amino acids were produced with up to 98% ee (Eq. 7.53) [379].
