Highly activity asymmetric hydrogenation of enones catalyzed by iridium complexes with chiral diamines and achiral phosphines

Article abstract of DOI:10.1016/j.tetlet.2020.151661

A selective asymmetric hydrogenation of enones has been well established by using an iridium complex composed of cheap phosphine ligands and cinchona alkaloids derivatives as catalyst. A wide range of allylic alcohol products could be obtained in high chemoselectivities (up to 99.6%), enantioselectivities (70.1% ee) and high activities (up to 3.64 × 10⁴(1/h) TOF). This catalytic system opens a new way of selective asymmetric hydrogenation and the method can be of practical value.

Full text of DOI:10.1016/j.tetlet.2020.151661

Journal Pre-proofs
Highly activity asymmetric hydrogenation of enones catalyzed by Iridium com-
plexes with chiral diamines and achiral phosphines
Xunhua Lu, Mengna Wang, Ling Zhang, Jian Jiang, Li Li, Xiuli Gao, Lin Zhang,
Chun Li, Hua Chen
PII:
S0040-4039(20)30080-0
DOI:
https://doi.org/10.1016/j.tetlet.2020.151661
TETL 151661
Reference:
Tetrahedron Letters
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Revised Date:
Accepted Date:
26 December 2019
18 January 2020
21 January 2020
Please cite this article as: Lu, X., Wang, M., Zhang, L., Jiang, J., Li, L., Gao, X., Zhang, L., Li, C., Chen, H., Highly
activity asymmetric hydrogenation of enones catalyzed by Iridium complexes with chiral diamines and achiral
phosphines, Tetrahedron Letters (2020), doi: https://doi.org/10.1016/j.tetlet.2020.151661
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Tetrahedron Letters
journal homepage: www.elsevier.com
Highly activity asymmetric hydrogenation of enones catalyzed by Iridium complexes
with chiral diamines and achiral phosphines
Xunhua Lua, Mengna Wanga, Ling Zhanga, Jian Jianga, Li Lia, Xiuli Gaoa, Lin Zhanga, Chun Lia,*, and Hua
Chenb,*
aState Key Laboratory of Functions and Applications of Medicinal Plants & School of Pharmaceutical sciences , Guizhou Medical University, 550004, Guiyang,
China.
bKey lab of Green Chemistry and Technology, Ministry of Education; College of Chemistry, Sichuan University, 610064, Chengdu, China.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A selective asymmetric hydrogenation of enones has been well established by using an iridium complex
composed of cheap phosphine ligands and cinchona alkaloids derivatives as catalyst. A wide range of allylic
alcohol products could be obtained in high chemoselectivities (up to 99.6%), enantioselectivities (70.1% ee) and
high activities (up to 3.64×104(1/h) TOF). This catalytic system opens a new way of selective asymmetric
hydrogenation and the method can be of practical value.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Enones
Asymmetric hydrogenation
Iridium
Chiral diamines
Introduction
asymmetric hydrogenation of benzalacetone. A highly efficient
Ru-C3 -TunePhos catalyst was reported by Zhang and
*
Both academia and industry have paid much attention to
developing effective methods for the synthesis of optically active
α, β-unsaturated alcohols, as the allylic alcohol is privileged
structural subunits commonly found in pharmaceuticals and
natural products with a wide range of biological activities1-4. One
effective method for carrying out the allylic alcohol is use of the
transition-metal selective reduction hydrogenation catalysts. Over
the past two decades, the transition metals in catalytic
asymmetric hydrogenation have attracted increasing attention
owing to their low cost and sustainability5-7. In 1995, Noyori and
coworkers13 in 2009, 97.4% ee values was observed for
hydrogenation of trans-4-phenly-3-butenone, highly selectively
reducing the ketone without reduction of the C=C bond. Recently,
Huang et al.14 designed and synthesized a ruthenium catalyst
prepared from a series of rigid chiral diamine ligands and a
simple achiral diphosphine, which demonstrated high
enantioselectivities (up to 96% ee) and activities for
methylenebenzylacetone. However, compared with the ruthenium,
iridium catalyst was less explored in the asymmetric
hydrogenation of benzalacetone. In 2003, Pierre A. Jacobs et al.15
developed the carbonyl affinity of metallic iridium combining
with the promotion effect of the H-β zeolite which enables the
asymmetric benzylacetone hydrogenation to carry out with
excellent conversions and high selectivities. Zhou and co-
co-workers8 reported
a homogeneous ruthenium-phosphate-
diamine catalytic system with up to 99.8% chemoselectivity and
70% enantioseletivity. Furthermore, the combined use of RuCl2
(xylbinap) (1, 2-diamine) and K2CO3 was developed and
increased the enantioseletivity of allylic alcohols by up to 97%9.
Zanotti-Gerosa assessed the utility of different asymmetric
diphosphine/diamine combinations in the Noyori reduction
system and PhanePhos-ruthenium-diamine complexes were used
to catalyze the asymmetric hydrogenation of α, β-unsaturated
ketones with high activity and 97% enantioselectivity10. Chan
workers16 demonstrated
a
high efficient asymmetric
hydrogenation of α-substituted α, β-unsaturated acyclic ketones
catalyzed by chiral spiro iridium complexes with ee up to 99.7%.
Mechanistically, the high enantioselectivity afforded in the
asymmetric hydrogenation of benzylacetone was believed to be a
result of the excellent chiral environment imparted by the
synergistic effect of the chiral diphosphine and chiral diamine
ligands. However, as is well known, most chiral phosphines and
chiral diamines used in homogeneous selective reduction are
often expensive owing to difficulties in synthesis and
instability17-21. In contrast, much less effort has been dedicated to
develop new catalysis system that combined with commercially
and co-workers11 prepared
a
new family of chiral
dipyridylphosphane ligands and established their effectiveness in
the ruthenium-catalyzed asymmetric hydrogenation of
benzalacetone with an S/C ratio of 10000 furnished the
corresponding allylic alcohol in 97.1% ee. Meanwhile, Zhou et
al.12 developed novel chiral diphosphine ligands with spiro
biindane as a new chiral scaffold, 96% ee obtained for the

2
Tetrahedron Letters
Table 1. The effect of different metal precursor for asymmetric hydrogenation of benzalacetone.a
Selectivity (%)
Con.
(%)
99.9
99.8
6.2
Ee
(%)
22.5
60.2
48.7
-
Entry
Metal precursor
Config.
SK
0.2
0.1
15.9
26.9
SA
36.1
7.7
60.8
73.1
UA
63.6
92.2
23.3
-
1
2
3
4
[Ir(COD)Cl]2
[Ir(COD)OCH3]2
Ru (TPP)3Cl2
S
S
S
S
Ru (COD) Cl2
6.4
a Substrate/metal/chiral diamine A/o-MOTPP = 1000/1/3/2, benzalacetone: 1.07 mol/L, LiOH: 0.1mol/L, EtOH: 2 ml, 30 °C, 6 MPa, 2 h.
available chiral diamine ligands and achiral phosphine ligands to
construct an efficient Ir complex for the asymmetric
hydrogenation of enones. Herein, we report the continuation of
N
H N
2
such studies, in which [Ir(COD)OCH3]2/chiral diamine/o-
MOTPP was used in situ to explore the selective reduction of
various synthesized enones, with the aim to screen the chemical
selectivity, and enantioselectivities related to the molecular
structure of the enones to be reduced. High chemoselectivities
(up to 99.6%), enantioselectivities (70.1% ee) and high activities
(up to 3.64×104(1/h) TOF) were obtained.
P
3
R
A
N
R = H, o-CH , m-OCH , p-OCH , p-CH , p-CF
3
3
3
3
3
9-Amino(9-deoxy)epi-cinchonine
N
H N
2
N
N
H N
2
H N
2
O
O
Results and Discussion
C
B
D
N
N
N
Initially, as shown in Table 1, four different metal precursors
combining with chiral diamine A and o-MOTPP were evaluated
for the asymmetric hydrogenation of benzalacetone employed as
a standard substrate. Compared with the ruthenium, iridium gave
the products with excellent conversion, and a higher ee of 60.2%
was obtained by [Ir(COD)OCH3]2 (Table 1, entry 2). The
asymmetric hydrogenation of benzalacetone was studied in detail
catalyzed by [Ir(COD)OCH3]2.
9-Amino(9-deoxy)epi-cinchonidine
9-Amino-(9-deoxy)epi-quinine
9-Amino-(9-deoxy)epi-dihydroquinine
Figure 1. Achiral phosphines and chiral diamines.
molar ratio of chiral diamine A/o-MOTPP/iridium was 2/2/1.
Increasing or decreasing the ratio was unfavorable for the activity,
chemoselectivity and enantioselectivity.
The steric and electronic properties of the ligands are two
critical factors that influence the reactivity and selectivity of the
Table 2. The effect of chiral amines and achiral phosphines
catalyst for the asymmetric hydrogenation of carbonyl groups22-
for asymmetric hydrogenation of benzalacetone.a
23
.
Encouraged by our previous studies on asymmetric
Selectivity (%)
Con.
(%)
9.3
10.5
24.3
95.7
9.0
Ee
Con
fig.
S
-
S
Entry
N
P
hydrogenation of ketones24-25, we also investigated cinchona
alkaloids derivatives combining with a survey of achiral
phosphines (Figure 1) in the asymmetric hydrogenation of the
model substrate. The results were summarized in Table 2.
Initially, only a single ligand was used in the asymmetric
hydrogenation of benzalacetone, and neither chiral diamine nor
achiral phosphine examined here gave us good results (Table 2,
entries 1, 2). However, a preferable result was obtained with the
combination of chiral diamine A and TPP. This system gave rise
to 24.3% conversion, delivering the expected allylic alcohol in
71.8% chemoselectivity (Table 2, entry 3). Considering the
electronic effect and steric hindrance, hindered triarylphosphine-
bearing substituents on the para position of the benzene ring,
such as MOTPP, MTPP and TFTPP, were evaluated in the
asymmetric hydrogenation of the model substrate. As it can be
seen, enantioselectivity slightly increased with the electron-with
donating group substituted triarylphosphine, although both
conversion and chemoselectivity decreasing (Table 2, entries 6-
7). Meanwhile, chemoselectivity and enantioselectivity decreased
dramatically when the substituents changed from the electron-
withdonating group to the electron-withdrawing group (Table 2,
entry 8). Excitedly, the combination of chiral diamine A with o-
MOTPP was investigated and a breakthrough result was obtained
with 95.7% conversion, 97.1% chemoselectivity and 58.4%
enantioselectivity (Table 2, entry 4). Contrary to o-MOTPP, m-
MOTPP examined here gave us disappointing results (Table 2,
entry 5). Screening of other chiral amines B-D showed good
90.8%-95.0% chemoselectivity, equivalent activity but inferior
enantioselectivity (Table 2, entries 9-11).
SK
SA
65.6
56.5
18.7
1.2
UA
(%)
22.7
-
1
2
3
4
5
6
7
8
9
A
-
-
8.0
40.1
9.5
26.4
3.4
o-MOTPP
TPP
A
A
A
A
A
A
B
C
D
71.8
97.1
23.8
67.1
54.0
27.4
90.8
95.0
95.0
24.4
58.4
39.2
32.3
34.3
12.4
36.1
42.6
33.6
o-MOTPP
m-MOTPP
p-MOTPP
p-MTPP
p-TFTPP
o-MOTPP
o-MOTPP
o-MOTPP
1.7
S
S
5.2
11.3
9.7
19.6
6.0
3.0
71.0
21.6
36.3
47.2
3.1
23.2
15.3
25.5
68.4
92.2
82.2
S
S
S
R
R
R
10
11
2.0
1.5
3.5
a Substrate/Ir/chiral diamine/ achiral phosphines = 1000/1/3/2, benzalacetone:
1.07 mol/L, LiOH: 0.1mol/L, EtOH: 2 ml, 30 °C, 6 MPa, 1 h.
Noyori and co-workers8 found that the combined effects of
diamine and KOH decelerate olefin hydrogenation catalyzed by
RuCl2[P(C6H5)3]3 and in turn accelerate carbonyl hydrogenation.
The presence of KOH played a very important role. Herein, the
influence of bases on the asymmetric hydrogenation of
benzalacetone was also examined, and the results were collected
in Table 3. As can be seen, the reaction could not be proceeded
without any base (Table 3, entry 1). However, benzalacetone
converted with base added to the catalysis system. Better results
were given when the reaction was carried with NaOH as the base
(Table 3, entry 3). 99.0% conversion, 98.5% chemoselectivity
and 57.3% enantioselectivity were obtained, respectively. With
other bases added, such as LiOH and KOH (Table 3, entries 2
and 4), both enantioselectivity and chemoselectivity were
equivalent to the best data, but lower activity achieved. The
catalysis system showed much lower performance, only 14.8%
conversion achieved, with t-BuOK as the base (Table 3, entry 5).
Then we investigated the effect of molar ratios of chiral
diamine A/o-MOTPP/iridium on the reaction. The optimum

3
Since the influence of alcohols on the asymmetric
hydrogenation was prominent, five different alcohols were
investigated in this reaction with results summarized in Table 4.
Better performance was achieved in ethanol, with up to 75.9%
conversion, 97.4% chemoselectivity and 57.9% ee (Table 4, entry
2). The lower-middle conversions were obtained with 79.2-
87.1% chemoselectivity when the asymmetric hydrogenation was
carried out in methanol and n-propanol (Table 4, entries 1 and 3).
Only less than 1% conversions were obtained when the
asymmetric hydrogenation was carried out in n-Butanol and i-
PrOH (Table 4, entries 4 and 5).
electron-donating group, especially at the ortho position, on the
aromatic ring have lower reactivity. However, there was an
Table 5. Asymmetric hydrogenation of enones catalyzed by
[Ir(COD)OCH3]2/chiral diamine A/o-MOTPP.a
O
R
OH
O
Saturated Ketone (SK)
R
OH
R
Saturated Alcohol (SA)
Substrate
R
Unsaturated Alcohol (UA)
Table 3. The effect of bases for asymmetric hydrogenation of
R = H, para-F, para-Cl, para-Br, para-CF , para-CH , para-OCH , ortho-OCH
benzalacetone.a
3
3
3
3
Selectivity (%)
Con.
(%)
<1
80.2
99.0
64.6
14.8
Ee
(%)
-
58.7
57.3
58.8
58.8
Con
fig.
-
S
S
Entry
Base
Substrate
R
Selectivity(%)
Con.
(%)
Ee
(%)
Con
fig.
S
S
S
S
S
S
S
SK
-
1.4
0.5
1.3
3.8
SA
UA
Entry
SK
0.5
1.5
2.4
0.3
3.0
2.7
2.3
4.2
SA
1.1
1.1
1.6
0.1
1.0
2.8
2.3
3.3
UA
98.5
97.4
96.0
99.6
99.6
95.5
95.4
92.5
1
2
3
4
5
-
-
-
1b
2c
3
4
5
6
7
8
H
H
99.0
75.9
99.6
95.7
95.6
80.3
61.6
31.5
57.3
57.9
48.2
48.6
51.7
38.3
23.2
61.9
LiOH
NaOH
KOH
1.4
1.1
2.3
97.2
98.5
96.3
74.4
para-F
para-Cl
para-Br
para-CF3
para-CH3
para-OCH3
ortho-
OCH3
S
S
t-BuOK
21.8
a Substrate/Ir/chiral diamine A/ o-MOTPP = 2000/1/2/2, benzalacetone: 2.15
mol/L, base: 0.1mol/L, EtOH: 2 ml, 30°C, 6 MPa, 5min.
S
The effect of H2 pressure on the hydrogenation of
benzalacetone was also investigated. Significant changes of the
conversion was observed with the H2 pressure changing from 5
MPa to 7 MPa, but equivalent chemoselectivity (93.0-97.4%) and
enantioselectivity (57.9-58.4%) was estimated. When the
reactions were run at 6 MPa hydrogen pressure, higher
conversion 75.9% was maintained (S/C = 4000/1, 5 minute
reaction).
9
1.2
1.2
97.6
99.8
70.1
S
a Substrate/Ir/chiral diamine A/o-MOTPP = 500/1/2/2, benzalacetone: 0.535
mol/L, NaOH: 0.1mol/L, EtOH: 2 ml, 30 °C, 6 MPa, 2 h;
Substrate/Ir/chiral diamine A/ o-MOTPP = 2000/1/2/2, 5 min;
Substrate/Ir/chiral diamine A/ o-MOTPP = 4000/1/2/2, 5 min, TOF =
b
c
3.64×104(1/h).
interesting phenomena found. When the position of electron-with
donating group, -OCH3, was changed from para-position to
ortho-position, the hydrogenation reactions showed higher ee
value (70.1%) and higher conversion (Table 5, entry 9).
Table 4. The effect of solvent for asymmetric hydrogenation
of benzalacetone.a
Selectivity (%)
Con.
(%)
36.4
75.9
14.2
<1
Ee
(%)
50.0
57.9
57.7
-
Con
fig.
S
S
S
Entry
Base
SK
1.5
1.5
2.0
-
SA
11.4
1.1
18.8
-
UA
87.1
97.4
79.2
Conclusion
1
2
3
4
5
MeOH
EtOH
n-PrOH
n-Butanol
i-PrOH
In summary, we have demonstrated that asymmetric
hydrogenation of enones bearing different substituent groups on
the phenyl ring to chiral allylic alcohols is feasible. The reaction
was catalyzed by an iridium catalyst combined a simple achiral
-
-
-
-
-
-
<1
-
a Substrate/Ir/chiral diamine A/o-MOTPP = 4000/1/2/2, benzalacetone: 4.29
phosphine and
a chiral diamine, which exhibited high
mol/L, NaOH: 0.1 mol/L, Solvent: 2 ml, 30°C, 6 MPa, 5 min.
chemoselectivities (up to 99.6%), enantioselectivities (70.1% ee)
and high activities (up to 3.64×104(1/h) TOF).
To explore the synthetic utility of this catalysis system, a series
of enones bearing different substituent groups on the phenyl ring
was synthesized and subjected to this hydrogenation process
under the optimum condition. The results were summarized in
Table 5. As it can be seen, the substituent groups on the phenyl
ring of enones made significant influence on the activity and
enantioselectivity, but the chemoselectivity was more
independent. In comparison with other substituent groups, it
seems that the electron-withdrawing group at the para position is
little influence on both activity and selectivity (Table 5, entries 3
- 5). However, the lower 80.3% conversion and 38.3% ee were
obtained for para-CF3-substituted benzalacetone (Table 5, entry
6). The substrates with electron-with donating group, such as -
CH3, -OCH3, at the para position on the phenyl ring were also
tested (Table 5, entries 7-8). Obviously, the para-position
electron-with donating group deactivated the reaction completely.
The lowest ee value (23.2%) was observed for para-CH3-
substituted benzalacetone and the lowest conversion (31.5%) was
obtained for para-OCH3-substituted benzalacetone. Generally,
the steric and the electronic effect influence the activity and the
selectivity in the asymmetric hydrogenation of carbonyl group.
Ketones with an electron-withdrawing substituent on the
aromatic ring have high reactivity, and the ketones with an
Acknowledgment
Research reported in this publication was supported by the
National Natural Science Foundation of China (21562010,
21601039).
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General experimental procedure: All reagents were of analytical
grade and used as-received without further purification. The purity of
hydrogen was over 99.99%. 1HNMR spectra were recorded on JEOL
NMR 400 MHz spectrometer with reference to TMS as the internal
standard. Product were determined by GC-9790 with a β-DEX120
column (25 m × 0.25 mm) and LC-16 with Chirasil OB-H column (25
cm × 4.6 mm), Chirasil IA column (25 cm × 4.6 mm) and Chirasil IB
column (25 cm × 4.6 mm). Iridium precursor, chiral diamine, achiral
phosphine, solvent, base and benzalacetone were simultaneously added
in situ to a 50 ml stainless autoclave with a magnetic stirrer. Then, the
autoclave was replaced with hydrogen three times, and the hydrogen
pressure was raised to desired level. The mixture was stirred at a
predetermined temperature for preset time. After the reaction was
completed, the conversion, chemical selectivity and enantioselectivity
values were analyzed by GC and HPLC, respectively. The selectivity
and ee values of unsaturated alcohol were calculated from the equations
chemical selectivity (UA, %) = UA / (SK + SA + UA) and
enantioselectivity values (ee, %) = |R - S| / |R + S|, respectively.
Click here to remove instruction text...
24. Li, C.; Zhang, L.; Du, Y.; Zheng, X.; Fu, H.; Chen, H.; Li, R. Catalysis
Communications 2012, 28, 5-8.

5
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The asymmetric hydrogenation of enones catalyzed by iridium
complexes was investigated in the presence of chiral damines
and achiral phosphines. The catalyst system exhibited high
chemoselectivities (up to 99.6%), enantioselectivities (70.1% ee)
and high activities (up to 3.64×104(1/h) TOF)
Highly activity asymmetric hydrogenation of
enones catalyzed by Iridium complexes with
chiral diamines and achiral phosphines
Xunhua Lu, Mengna Wang, Ling Zhang, Jian Jiang,
Li Li, Xiuli Gao, Lin Zhang, Chun Li*, and Hua Chen*
O
R
OH
O
Saturated Ketone (SK)
R
OH
R
Saturated Alcohol (SA)
Substrate
R
Unsaturated Alcohol (UA)
>99% chemoselectivities, 70.1% ee
4
3.64×10 (1/h) TOF
R = H, para-F, para-Cl, para-Br, para-CF , para-CH , para-OCH , ortho-OCH
3
3
3
3

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Tetrahedron Letters
Highlights：
1. An Iridium complex was used for enones
asymmetric hydrogenation;
2. Cheap phosphine ligands and cinchona alkaloids
derivatives were used;
3. A series of enones has been well asymmetric
hydrogenated under mild condition;
4. 99.6% chemoselectivity, 70.1% ee and
3.64×104(1/h) TOF were obtained.

7

8
Tetrahedron Letters
Declaration of interests
☒ The authors declare that they have no known
competing financial interests or personal relationships
that could have appeared to influence the work
reported in this paper.
☒The authors declare the following financial
interests/personal relationships which may be
considered as potential competing interests:
We declare that we have no financial and personal relationships with other people or organizations that
can inappropriately influence our work, there is no professional or other personal interest of any nature or
kind in any product, service and/or company that could be construed as influencing the position presented
in, or the review of, the manuscript entitled “Highly activity asymmetric hydrogenation of enones
catalyzed by Iridium complexes with chiral diamines and achiral phosphines”.