Heterogeneous selective hydrogenation of trans-4-phenyl-3-butene-2-one to allylic alcohol over modified Ir/SiO2 catalyst

Article abstract of DOI:10.1016/j.apcata.2012.02.002

The heterogeneous selective hydrogenation of trans-4-phenyl-3-butene-2-one to allylic alcohol, catalyzed by Ir/SiO₂ stabilized with phosphines and modified by cinchona alkaloids, was described herein. Under the optimized conditions, the chemoselectivity to α,β-unsaturated alcohol was more than 99% with enantioselectivity up to 46%.

Full text of DOI:10.1016/j.apcata.2012.02.002

Applied Catalysis A: General 421–422 (2012) 86–90
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Heterogeneous selective hydrogenation of trans-4-phenyl-3-butene-2-one to
allylic alcohol over modified Ir/SiO catalyst
2
a,∗
a
a
b
He-yan Jiang , Bin Sun , Xu-xu Zheng , Hua Chen
a
Research Center of Pharmaceutical Chemistry and Chemical Biology, Key Laboratory of Catalysis Science and Technology of Chongqing Education Commission,
Chongqing Technology and Business University, Chongqing 400067, PR China
Key Lab of Green Chemistry and Technology, Ministry of Education, The Institute of Homogeneous Catalysis, College of Chemistry, Sichuan University,
b
No. 29 Wang Jiang Road, Chengdu, Sichuan 610064, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The heterogeneous selective hydrogenation of trans-4-phenyl-3-butene-2-one to allylic alcohol, cat-
alyzed by Ir/SiO2 stabilized with phosphines and modified by cinchona alkaloids, was described herein.
Under the optimized conditions, the chemoselectivity to ␣,␤-unsaturated alcohol was more than 99%
with enantioselectivity up to 46%.
Received 9 December 2011
Received in revised form 31 January 2012
Accepted 1 February 2012
Available online 10 February 2012
©
2012 Elsevier B.V. All rights reserved.
Keywords:
Selective catalysis
Iridium
Alkaloids
Trans-4-phenyl-3-butene-2-one
Supported catalysts
1
. Introduction
Ketoisophorone can be reduced with high chemoselectivity and
4% ee over cinchona-modified Pt/alumina [16]. In above lim-
1
The quest for economic and green methods to prepare enan-
ited cases, the chemoselective and enantioselective reactions have
only been demonstrated for specific substituted substrates. We
focus on the selective hydrogenation of ketones, an important
organic transformation, since the resulting alcohols are versatile
precursors to many natural products and drug molecules. We suc-
ceed in heterogeneous enantioselective hydrogenation of aromatic
ketones [20–22], with enantioselectivity up to 98%. More than
99% allylic alcohol selectivity was obtained in the chemoselective
hydrogenation of trans-4-phenyl-3-butene-2-one employing 1,2-
diphenylethylene-1,2-diamine- and PPh -modified Ru/␥-Al O
tiomerically pure ␣,␤-unsaturated alcohols continues as a result of
the important role these intermediates serve in drug design [1,2].
Generally, they are obtained by the enantioselective reduction of
C
O in ␣,␤-unsaturated carbonyl compounds. These hydrogena-
tion reactions have been remarkably developed in homogeneous
catalytic reactions since the groundbreaking discovery disclosed
by Noyori and co-workers [3,4]. However, the recuperation of
homogeneous catalysts is still a problem. Moreover, the selec-
tive hydrogenation of the C O group of an unsaturated ketone
to chiral allylic alcohol on heterogeneous catalysts remains a
formidable scientific challenge [5,6]. The heterogeneous chemos-
elective reduction of unsaturated ketones to allylic alcohols has
been studied occasionally on metal Au [7–12], Ir [13], Ru [14],
Pt [15,16], Pd [16,17] or Ni [18,19]. The enantioselective reduc-
tion of unsaturated ketones to chiral allylic alcohols is much
more difficult, only a few such reactions have been described.
Enone testosterone and cholestenone undergo reduction to the
corresponding allylic alcohols with almost completely chemose-
lectivity and high diastereoselectivity (d.r. > 97:3) over Ir/H-␤ [13].
3
2
3
as catalyst [14]. As part of our ongoing research, we herein
firstly report the heterogeneous enantioselective hydrogenation
of trans-4-phenyl-3-butene-2-one, catalyzed by phosphines stabi-
lized Ir/SiO modified by cinchona alkaloids, with chemoselectivity
2
to unsaturated alcohol more than 99% and enantioselectivity up to
46%.
2. Experimental
2.1. Materials
The
trans-4-phenyl-3-butene-2-one
(Acros),
cinchona
∗ Corresponding author. Tel.: +86 23 6276 9652; fax: +86 23 6276 9652.
E-mail address: orgjiang@163.com (H.-y. Jiang).
alkaloids (Acros), triphenylphosphine (PPh3) (Aldrich), and
H IrCl ·6H O (Institute of Kunming Noble Metals, China)
2
6
2
0
926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2012.02.002

H.-y. Jiang et al. / Applied Catalysis A: General 421–422 (2012) 86–90
87
N
N
N
H
H N
H
NH2
H
NH2
H
H
H
2
H CO
3
N
N
N
1
2
3
O
A. saturated ketone
OH
O
3%Ir/phosphine/SiO
2
/
cinchona alkaloid
+
H2
CH OH
3
B. saturated alcohol
HO H
H OH
or
C. unsaturated alcohol
Scheme 1. Modifier used and the general reaction.
were used as-received without further purification. Other
reagents were analytical grade. The purity of hydrogen was
3. Results and discussion
2
−1
over 99.99%. The surface area of SiO2 was 135.9 m g . tppts
tris(sodium-m-sulfonatophenyl) phosphine], motpp [tris(4-
3.1. Effect of different stabilizers and P/Ir ratio on selective
hydrogenation
[
methoxyphenyl)phosphine], tftpp [tris(4-trifluoromethylphenyl)
phosphine], bdpx [1,2-bis(diphenylphosphinomethyl)benzene]
Stabilizer focuses on the precise molecular definition of the
catalytic materials. This strategy potentially allows optimiza-
tion of the parameters that govern the efficiency in catalytic
reactions, including enantioselective ones [24]. Stabilizers,
including amines [25], phosphines [26,27], and thiols [28],
are widely used in the preparation of heterogeneous cata-
lysts. From our previous research [20,21], it was obvious that
stabilizer not only stabilized the metal particles in the prepa-
ration of the catalyst, but also served to additionally modify
the supported metal in the asymmetric hydrogenation. Phos-
phines, such as triphenyl phosphine (tpp), tris(4-methoxyphenyl)
ꢀ
ꢀ
and bisbi [2,2 -bis(diphenylphosphinomethyl)-1,1 -biphenyl] were
synthesized according to known methods in our laboratory [21].
Phosphine stabilized Ir/SiO2 catalysts were prepared according to
our previously reported methods [20], and catalysts were char-
acterized by high-resolution transmission electron microscopy,
X-ray photoelectron spectroscopy, FTIR, 31P solid state NMR and
BET method [20]. Cinchona alkaloid derivatives were synthesized
according to literature [23].
2
.2. Catalytic hydrogenation
phosphine
tftpp),
(motpp),
tris(4-trifluoromethylphenyl)phosphine
(
tris(sodium-m-sulfonatophenyl)phosphine
(tppts),
ꢀ
The hydrogenation was performed in a 20 ml stainless autoclave
1,2-bis(diphenylphosphinomethyl) benzene (bdpx) and 2,2 -
bis(diphenylphosphinomethyl)-1,1 -biphenyl (bisbi), were tested
ꢀ
with a magnetic stirrer bar. The desired amounts of catalyst (20 mg),
cinchona alkaloid, LiOH, solvent and trans-4-phenyl-3-butene-2-
one were added into the autoclave and then the autoclave was
sealed and purged with pure hydrogen several times. After the reac-
tants were heated to the desired temperature, the reaction timing
began.
in selective hydrogenation of trans-4-phenyl-3-butene-2-one
(Table 1). Without the stabilizer, the hydrogenation resulted in a
conversion as low as 12.0% and a chemoselectivity to saturated
ketone up to 96.5%. In the presence of phosphine stabilizers, the cat-
alytic activity and selectivity for the hydrogenation of C O bound
drastically increased. Simple PPh3 stabilized 3%Ir/SiO2 showed
the best catalytic performance. In comparison to 3%Ir/SiO /2tpp,
2
2
.3. Analysis
3%Ir/SiO2/2motpp and 3%Ir/SiO2/2tftpp showed similar chemose-
lectivity but lower activity and enantioselectivity (Table 1, entries
1 and 4–5). When tppts was employed as a stabilizer in the catalyst
preparation, high activity and enantioselectivity were achieved,
however, the chemoselectivity was just 71.0% (Table 1, entry 6).
In contrast, bidentate phosphines stabilized catalysts showed less
effective catalytic activity, chemoselectivity and enantioselectivity
(Table 1, entries 2–3). Above results might be concluded that the
effects of the steric bulk and the electronic nature of stabilizers
influence the activity and selectivity of the hydrogenation, and the
The hydrogenation products were analyzed by GC and HPLC.
The conversion and chemoselectivity were determined by GC
with a Chrompack Chirasil-DEX column (25 m × 0.25 mm). The
ee values were determined by HPLC with a Chiralcel OD-H col-
umn (25 cm × 4.6 mm). The ee values were calculated from the
equation: ee (%) = 100 × (R − S)/(R + S). With the exception of prod-
ucts in Scheme 1, no other products were detected (GC–MS and
NMR).

8
8
H.-y. Jiang et al. / Applied Catalysis A: General 421–422 (2012) 86–90
Table 1
Effect of different stabilizers on selective hydrogenation of trans-4-phenyl-3-butene-2-one employing compound 1 as a modifier.
a
Entry
Catalyst
Conversion (%)
Selectivity (%)
ee (%)b
A
B
C
1
2
3
4
5
6
i
ii
iii
iv
v
99.6
16.2
20.0
69.0
72.9
99.3
0.2
16.4
12.0
1.3
2.7
11.8
0.5
99.3
43.9
75.5
95.5
93.0
71.0
46.0
25.7
25.2
32.7
27.0
43.2
39.7
12.5
3.2
4.3
17.2
vi
a
◦
Reaction was carried out at 35 C. Substrate: in a 2 ml methanol solution at [0.32 M], PH2:6.0 MPa. Substrate/Ir/modifier = 200:1:2, and [LiOH] = 0.125 mol/l. Reaction time:
h. i = 3%Ir/SiO2/2tpp, ii = 3%Ir/SiO2/bisbi, iii = 3%Ir/SiO2/bdpx, iv = 3%Ir/SiO2/2motpp, v = 3%Ir/SiO2/2tftpp, and vi = 3%Ir/SiO2/2tppts.
Configuration: S.
5
b
Table 2
Effect of different P/Ir ratio on selective hydrogenation of trans-4-phenyl-3-butene-2-one employing compound 1 as a modifier.
Entry
P/Ir
Conversion (%)
Selectivity (%)
ee (%)a
A
B
C
1
2
3
4
5
6
7
8
0
12.0
23.7
63.0
99.0
99.6
31.0
19.0
11.0
96.5
39.0
17.6
3.0
0.2
2.0
3.0
8.8
5.9
1.0
0.5
1.4
1.4
3.1
0.5
–
0.1
0.5
1
2
3
52.2
76.5
96.0
99.3
96.6
96.2
92.0
31.2
35.6
43.8
46.0
40.0
35.5
30.2
4
5
2.4
4.9
The reaction conditions are the same as in Table 1 (3%Ir/SiO2/tpp) except the P/Ir ratio.
a
Configuration: S.
Table 3
Effect of different modifiers on selective hydrogenation of trans-4-phenyl-3-butene-2-one.
Entry
Modifier
Conversion (%)
Selectivity (%)
ee (%)
Config
A
B
C
1
2
3
4
5
6
Cinchonine
Cinchonidine
Quinine
1
2
3
6.2
8.1
5.4
99.6
99.9
12.0
73.8
72.3
75.6
0.2
1.5
15.1
24.6
25.9
22.9
0.5
25.6
54.0
1.6
1.8
1.5
99.3
72.9
30.9
–
–
–
46.0
9.0
13.4
–
–
–
S
R
S
The reaction conditions are the same as in Table 1 (3%Ir/SiO2/2tpp). 1 = 9-amino(9-deoxy)epicinchonine, 2 = 9-amino(9-deoxy)epicinchonidine, and 3 = 9-amino(9-
deoxy)epiquinine.
steric hindrance has greater influence on the selectivity than the
electronic effect in this heterogeneous catalytic hydrogenation.
Furthermore, the effect of stabilizer/metal ratio on selective
hydrogenation is listed in Table 2. The highest activity, chemos-
electivity and enantioselectivity were obtained when the initial
molar ratio of PPh3 to Ir used in the preparation of the catalyst was
hydrogenation activity, chemoselectivity and enantioselectivity to
O bond although the activity and chemoselectivity decreased in
some content. The result indicated that the soluble metal Ir com-
plex was not formed in this catalytic system during the reaction
and the hydrogenation occurred on solid surface of the catalyst.
C
2
:1. Increasing or decreasing the ratio of PPh3 to Ir was unfavor-
3.2. Effect of different modifiers and modifier concentration
able to the activity, chemoselectivity and enantioselectivity. When
the P/Ir ratio was low, the chemoselectivity obviously decreased,
which might be explained that part of the substrate progressed in
the conventional route (Table 2, entries 1–2). Similar to some pre-
vious reports [20,21,29], a poisoning phenomenon was observed
when P/Ir ratio was high.
In order to clarify whether the high activity, chemoselectiv-
ity and enantioselectivity of this catalyst for the hydrogenation
of trans-4-phenyl-3-butene-2-one resulted from heterogeneous
catalysis on the solid surface or homogeneous catalysis caused by
the soluble metal Ir complex, which might be formed in the hydro-
genation process, the solution and catalyst were separated at the
end of hydrogenation by centrifugation under argon atmosphere.
Both the solution and catalyst were used for the hydrogenation of
the substrate, respectively. It was found that the reacted solution
had no catalytic activity after some fresh substrate was intro-
duced in it. However, the reused catalyst obviously maintained the
Modifier has important influence on the activity, chemose-
lectivity and enantioselectivity in most heterogeneous catalytic
reactions [30,31]. Among modifiers tested (Table 3), natural cin-
chonas (cinchonine, cinchonidine, and quinine) exhibited low
catalytic activity and high chemoselectivity to saturated ketone
(Table 3, entries 1–3). Modifier 1 exhibited best chemoselectivity
and enantioselectivity with good catalytic activity. 9-Amino(9-
deoxy)epicinchonidine
2 showed high catalytic activity but
moderate chemoselectivity and low enantioselectivity. When 9-
amino(9-deoxy)epiquinine 3 was used as a modifier in the
hydrogenation of trans-4-phenyl-3-butene-2-one, the reaction
progressed slowly with low chemoselectivity and enantioselec-
tivity. It had to be stressed that the configuration of the product
switched in comparison to modifier 2 (Table 3, entries 5–6), the
phenomenon, small change in the structure of modifier led to the
switch in chirality, provided useful information for understanding

H.-y. Jiang et al. / Applied Catalysis A: General 421–422 (2012) 86–90
89
Table 4
Effect of modifier 1 concentration on selective hydrogenation of trans-4-phenyl-3-butene-2-one.
Modifier × 10− mol/l
3
Conversion (%)
Selectivity (%)
ee (%)a
A
B
C
0
9.0
54.0
79.0
99.6
86.4
78.3
82.1
2.6
0.4
0.2
1.2
2.4
16.8
1.4
0.6
0.5
0.7
2.2
1.1
–
0.8
1.6
3.2
6.4
9.6
96.0
99.0
99.3
98.1
95.4
45.0
45.0
46.0
43.8
36.6
The reaction conditions are the same as in Table 1 (3%Ir/SiO2/2tpp) except the concentration of modifier 1.
a
Configuration: S.
Table 5
Effect of different solvents on selective hydrogenation of trans-4-phenyl-3-butene-2-one.
Entry
Solvent
Dielectric constant
Conversion (%)
Selectivity (%)
ee (%)a
A
B
C
1
2
3
4
5
6
MeOH
EtOH
iPrOH
H2O
THF
Toluene
33.6
24.3
19.9
80.4
7.60
2.38
99.6
45.4
10.0
6.8
13.3
0
0.2
0.9
6.3
26.5
7.0
–
0.5
2.1
21.7
7.2
12.6
–
99.3
97.0
72.0
66.3
80.4
–
46.0
43.3
22.2
28.0
30.1
–
The reaction conditions are the same as in Table 1 (3%Ir/SiO2/2tpp) except the solvent.
a
Configuration: S.
Table 6
Effect of different bases on selective hydrogenation of trans-4-phenyl-3-butene-2-one.
Entry
Base
Conversion (%)
Selectivity (%)
ee (%)a
A
B
C
1
2
3
4
5
No
5.4
99.6
99.9
99.9
99.9
99.9
0.2
1.3
0.7
0.2
0.1
0.5
4.2
6.3
18.8
0.0
–
LiOH
NaOH
KOH
tBuOK
99.3
94.5
93.0
81.0
46.0
35.1
32.4
36.8
The reaction conditions are the same as in Table 1 (3%Ir/SiO2/2tpp) except the base.
a
Configuration: S.
the access of substrate to cinchona alkaloids modified solid surface.
On these grounds, we have arrived at the conclusion that introduc-
tion of amino group to natural cinchonas is necessary to improve
the catalytic activity, chemoselectivity and enantioselectivity to
unsaturated alcohol; considering amino substituted cinchona alka-
loids 1, 2 and 3, the high specific correlation between modifier and
substrate has sharp influence on catalytic activity and selectivity.
The effect of cinchona alkaloid modifier 1 concentration on the
selective hydrogenation of trans-4-phenyl-3-butene-2-one is sum-
marized in Table 4. In the absence of modifier 1, the hydrogenation
of substrate occurred almost completely in the C C bond and the
activity was very low. However, both the catalytic activity and
selectivity for the C O hydrogenation drastically increased with
the increase of modifier 1 concentration. When the concentration
of modifier 1 was 3.2 × 10 mol/l, an excellent chemoselectivity
of more than 99% and enantioselectivity of 46% were achieved
with a high conversion of 99.6%. Further increase the concentra-
tion of modifier 1, a portion of catalytic center would be covered
and thus the catalytic activity, chemoselectivity and enantioselec-
tivity would decrease [32]. Above results reasonably indicated the
importance of competitive absorption between the modifier and
the substrate on the catalyst surface.
solvent used [30,31]. The effect of different solvents was investi-
gated and the data is listed in Table 5. The results indicated that
protonic solvent was more effective than aprotic one, in general.
The results in Table 5 showed that there was no clear corre-
lation between catalytic performance and the solvent polarity.
Considering alcohols tested (Table 5, entries 1–3), higher activ-
ity, chemoselectivity and enantioselectivity to unsaturated alcohol
could be achieved in solvent with higher dielectric constant. MeOH
was the most suitable solvent for the reaction, in which the trans-4-
phenyl-3-butene-2-one hydrogenation activity, chemoselectivity
and enantioselectivity were the highest (Table 5, entry 1). This
phenomenon is in agreement with our previous report [20].
3.4. Effect of different bases
−3
The importance of a basic additive in a homogeneous catalyst
system for the asymmetric hydrogenation of ketones has been
well-established [3,4]. Similar to homogeneous and some hetero-
geneous reactions [33–35], a basic additive is helpful to improve
the catalytic performance. In the absence of base (Table 6, entry 1),
the conversion was only 5.4% with chemoselectivity almost com-
◦
pletely to saturated ketone at 35 C for 5 h. The addition of different
bases, such as tBuOK, LiOH, NaOH, or KOH, could enhance the activ-
ity, chemoselectivity and enantioselectivity. For the alkali metal
hydroxides, the chemoselectivity and enantioselectivity decreased
in the order of Li > Na > K (Table 6, entries 2–4). This phe-
nomenon is consistent with our previous report [20], a possible
3
.3. Effect of different solvents on selective hydrogenation
+
+
+
In heterogeneous selective catalysis, the activity, chemose-
lectivity and enantioselectivity are always very sensitive to the

9
0
H.-y. Jiang et al. / Applied Catalysis A: General 421–422 (2012) 86–90
Table 7
Effect of hydrogen pressure on selective hydrogenation of trans-4-phenyl-3-butene-2-one.
P(H2)/MPa
Conversion (%)
Selectivity (%)
ee (%)a
A
B
C
2
4
6
8
72.0
90.0
99.6
99.9
1.6
0.6
0.2
0.5
0.2
0.4
0.5
1.3
98.2
99.0
99.3
98.2
43.0
44.7
46.0
45.3
The reaction conditions are the same as in Table 1 (3%Ir/SiO2/2tpp) except the hydrogen pressure.
a
Configuration: S.
explanation for this phenomenon is that the sterically hindered
modifier 1 prefers the base with smaller metal cation to produce
higher chemoselectivity and enantioselectivity on metal iridium
surface.
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4
. Conclusion
[
[
We demonstrated the heterogeneous enantioselective hydro-
genation of trans-4-phenyl-3-butene-2-one to corresponding
allylic alcohol with high chemoselectivity and moderate enantios-
electivity catalyzed by cinchona- and phosphine-modified iridium
catalysts. Additional work is currently in progress in this and related
areas.
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This work was financially supported by Natural Science Founda-
tion Project of CQ (No. CSTC, 2011BA5025), Ministry of Education
of Chongqing (No. KJ100701), Chongqing Technology and Business
University (No. 2010-56-14) and Chongqing Innovative Research
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