Heterogeneous asymmetric hydrogenation of aromatic ketones enhanced by silanols on highly monodispersed silica spheres

Article abstract of DOI:10.1016/j.catcom.2014.05.009

A triphenylphosphine-stabilized Ir/SiO₂ catalyst modified by a chiral diamine was synthesized with four silica spheres as support for the asymmetric hydrogenation of aromatic ketones. The H-bond between substrate and silanols and the interaction between substrate and modifier commonly affected the steric configuration of asymmetric hydrogenation of aromatic ketones. With the silanols increasing, the activity of asymmetric hydrogenation significantly increased. This is the first report of heterogeneous asymmetric hydrogenation of aromatic ketones enhanced by silanols on highly monodispersed silica spheres with > 99.9% ee and > 99% conversion.

Full text of DOI:10.1016/j.catcom.2014.05.009

Catalysis Communications 54 (2014) 27–30
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Heterogeneous asymmetric hydrogenation of aromatic ketones
enhanced by silanols on highly monodispersed silica spheres
Chun Li, Lin Zhang, Hailong Liu, Xueli Zheng, Haiyan Fu, Hua Chen ⁎, Ruixiang Li
Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, 610064 Chengdu, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A triphenylphosphine-stabilized Ir/SiO2 catalyst modified by a chiral diamine was synthesized with four silica
spheres as support for the asymmetric hydrogenation of aromatic ketones. The H-bond between substrate and
silanols and the interaction between substrate and modifier commonly affected the steric configuration of
asymmetric hydrogenation of aromatic ketones. With the silanols increasing, the activity of asymmetric
hydrogenation significantly increased. This is the first report of heterogeneous asymmetric hydrogenation of
aromatic ketones enhanced by silanols on highly monodispersed silica spheres with N99.9% ee and N99%
conversion.
Received 19 February 2014
Received in revised form 4 May 2014
Accepted 6 May 2014
Available online 25 May 2014
Keywords:
Heterogeneous
Asymmetric hydrogenation
Silanols
© 2014 Elsevier B.V. All rights reserved.
Iridium
Ketones
1
. Introduction
Since 1968, silica materials have been widely used in high-tech
2. Experimental
2.1. General information
applications owing to their excellent physicochemical, mechanical and
optical properties [1]. Silica-supported mesoporous heterogeneous
catalysts for asymmetric hydrogenation have attracted a great deal of
interest [2–4]. Chuang and Maciel have recently found that approxi-
mately 60% of the single silanols and 50% of the geminal silanols in the
silica surface were non-hydrogen bonded (isolated species) at 25 and
Tetraethoxysilane (TEOS, Aldrich), cinchonine (Acros), triphenyl-
phosphine (tpp, Aldrich), aromatic ketones (Aldrich) and H IrCl ·6H O
2 6 2
(Institute of Kunming Noble Metals, China) were used as-received
without further purification. Other reagents were of analytical grade.
The purity of hydrogen was over 99.99%. High-resolution transmission
electron microscopy (HTEM) graph was recorded by FEI Tecnai G2
F20 S-Twin TEM with an accelerating voltage of 200 kV. X-ray
photoelectron spectroscopy data was taken on Kratos XSAM 800 XPS.
Inductively coupled plasma (ICP) data was tested by Intrepid IIXSP
ICP. Scanning electron microscopy (SEM) graph was obtained by JEOL
2
00 °C [5]. Katz [6] found that surface silanol groups affected heteroge-
neous catalysis activity, and Lin [7] reported higher reaction rates in
cyanosilylation than AEP–MSN alone owing to the increased number
of surface silanol groups in cooperative catalysis reactions. In our recent
work, we had also shown that high performance of heterogeneous
catalysts was related to surface hydroxyl [8–11]. We attributed this to
developing highly active and enantioselective catalytic systems
enhanced by silanols on support. Herein we report the synthesis of a
2
JSM-7500F SEM. N adsorption–desorption isotherms were tested
on a Micrometitics ASAP 2020 M + C analyzer at 77 K. The BET
(Brunauer–Emmett–Teller) method was adopted to calculate the
special surface areas using adsorption data. The pore size distributions
were derived from the adsorption branches of isotherms using the
BJH (Barret–Joyber–Halenda) model. Products were analyzed by GC
instrument with an FID detector and β-DEX120 capillary column.
2
TPP-stabilized and chiral diamine modified Ir/SiO catalyst with four
silica spheres as support which provided N99.9% ee and N99% conver-
sion for heterogeneous asymmetric hydrogenation of simple ketones.
The experimental results demonstrated that the surface silanol in the
silica significantly enhanced the activity and enantioselectivity of this
reaction through the hydrogen bonding between substrates and silanol.
To our knowledge, this is the first report of the heterogeneous
asymmetric hydrogenation of simple aromatic ketones enhanced by
silanols on highly monodispersed silica spheres.
2.2. The preparation of four monodispersed silica spheres
Firstly, ethanol (6 mL), aqueous ammonia (10, 20, 30 and 40 mL)
and TEOS (6 mL) were added into a round flask of 100 mL and stirred
for 8 h at 35 °C. Then the mixture was filtrated and washed with ethanol
and water a few times. The solid was dried under vacuum at room
temperature for 24 h.
⁎
Corresponding author. Tel./fax: +86 28 85412904.
E-mail address: scuhchen@163.com (H. Chen).
http://dx.doi.org/10.1016/j.catcom.2014.05.009
566-7367/© 2014 Elsevier B.V. All rights reserved.
1

28
C. Li et al. / Catalysis Communications 54 (2014) 27–30
2
.3. The prepareation of 2%Ir/SiO
2
/2tpp
Table 1
a
The effect of surface silanols for asymmetric hydrogenation of acetophenone.
Under an argon atmosphere, silica spheres (1 g),
0.104 mmol), tpp (0.208 mmol) and iPrOH (30 mL) were added into a
H
2
IrCl
6
Entry Size/nm Surface Average X/mmol/g Conv. Ee
b
Config.
2
−1
(
area/m g
pore/Å
(%)
(%)
round flask of 100 mL and stirred for 24 h at room temperature. Formal-
dehyde (5 mL) was then added and the mixture was heated at 110 °C for
1
2
3
4
46
290
400
560
109.2
22.9
13.2
69.2
63.1
35.4
24.6
18.3
14.5
11.7
12.5
15.0
63.7
38.9
58.4
71.8
90.0
89.7
90.3
90.0
S
S
S
S
5
h. The mixture was cooled to room temperature and the solid was
filtrated and dried under vacuum at room temperature for 6 h to afford
the catalyst.
a
Substrate/Ir = 300/1, acetophenone: 0.856 mol/L,9-amino(9-deoxy)epicinchonine:
−
3
2
.8 × 10 mol/L, LiOH: 0.025 mol/L, 25 °C, 6 MPa, 3 h.
b
3
2
%Ir/SiO /2tpp.
2
.4. General procedure for the asymmetric hydrogenation reaction
Asymmetric hydrogenation of aromatic ketones was performed in a
0 mL stainless steel autoclave with a magnetic stirred bar at room tem-
The amount of surface silanols on the silica particles was estimated
6
by chemical titration [13]. The surface silanol amount of four silicas
was respectively 14.5, 11.7, 12.5 and 15.0 millimole/g (Table 1). The
results indicate that there were substantial amounts of silanols in
the silica, and the silanol amount of four silicas was in the order: (d) N
perature, by using 9-amino(9-deoxy)epicinchonine as modifier, which
is derived from cinchonine. In a typical run, the catalyst, chiral diamine,
solvent, base and acetophenone were placed in the autoclave, followed
by five purges hydrogen. The hydrogen pressure was thereafter in-
creased to desired level. The mixture was stirred at room temperature
for the appropriate duration.
(
a) N (c) N (b). Nitrogen adsorption isotherms indicated that the
silica particles had a low BET surface area being 109.2, 22.9, 13.2 and
2
−1
6
6
9.2 m g and their corresponding BJH average pore diameters were
3.1, 35.4, 24.6 and 18.3 Å respectively.
3
. Results and discussion
2
3%Ir/SiO /2tpp was prepared with four silicas as supports according
to our previously reported method [14–19]. The results of asymmetric
hydrogenation were summarized in Table 1. In general, the large surface
areas and pore diameters of support were beneficial for the activity in
Initially, four monodispersed silica spheres were prepared as a sup-
port for the asymmetry hydrogenation of aromatic ketones according to
Chen's research [12]. The four diameters of the silica particles were
synthesized depending on the reagent concentrations used. Increased
concentration of ammonia correlated with the decrease in average par-
ticle sizes of the silica spheres. The SEM graph of silica spheres ((a), (b),
heterogeneous catalytic systems. However, 3%Ir/SiO
formed 63.7% conversion lower than 3%Ir/SiO (d)/2tpp, although the
former had the largest surface areas and the largest average pore diam-
eters (Table 1, entries 1, 4). In addition, 3%Ir/SiO (c)/2tpp with the low-
est surface areas and slighter average pore diameters exhibited 58.4%
2
(a)/2tpp per-
2
2
(
c) and (d)) is shown in Fig. 1. The silica spheres are monosize with the
average particle size about 46, 290, 400 and 560 nm respectively.
2
conversion higher than 3%Ir/SiO (b)/2tpp (Table 1, entries 2, 3). In
(a)
(b)
(
c)
(d)
Fig. 1. SEM graph of high monodispersed silica spheres.

C. Li et al. / Catalysis Communications 54 (2014) 27–30
29
monodispersed silica spheres
N
Table 2
a
The effect of different iridium loadings for hydrogenation of acetophenone .
Entry
Load capacity/%
Conv./%
Ee/%
Config.
N
1
2
3
1
2
3
2.5
86.3
71.8
79.2
91.6
90.0
S
S
S
H
N
H
O
X
H
H
O
H
O
H
O
H
O
H
O
H
O
a
Substrate/Ir = 300/1, acetophenone: 0.856 mol/L,9-amino(9-deoxy)epicinchonine:
2.8 × 10− mol/L, LiOH: 0.025 mol/L, 25 °C, 6 MPa, 3 h.
3
iridium
3 3
Scheme 1. Proposed hydrogen model. X: F, Cl, Br, OCH , and CF .
that the enantioselectivity of acetophenone had little variation
(Table 1, entries 1–4) over the range of silanol concentration increasing
from 11.7 to 15.0 millimole/g. According to the proposed hydrogen
model, the steric effect was not different when the substrate was same.
That was the reason the amount of surface silanols had no significant ef-
fect on asymmetry of the same substrate hydrogenation. However, the
experiment data suggests that increased concentration of silanols im-
proved the activity.
addition, the activity of acetophenone significantly increased from
3
1
8.9% to 71.8% (Table 1, entries 1–4) with silanols increasing from
1.7 to 15.0 millimole/g. In particular, the catalyst 3%Ir/SiO (d)/2tpp
2
with the most silanols exhibited the highest activity (71.8%) (Table 1,
entry 4). These results suggest that the effect of silanols was higher
than the effect of surface areas and pore diameters. Interestingly, the
surface silanols vastly enhanced the activity but had no effect on the
enantioselectivity of asymmetric hydrogenation of acetophenone. This
prompted us to study the model of asymmetric hydrogenation.
As early as 1979, Orito et al. reported the first asymmetric hydroge-
nation of prochiral ketocarbonyls using chiral modified supported plat-
inum catalysts [20]. Subsequently, a number of mechanisms have been
proposed for the Orito reaction and recent surface science studies
indicate that a second H-bonding interaction may occur in the adsorbed
2
According to the catalytic activity from Table 1, the SiO (d) was cho-
sen to be the best support. The catalysts at various iridium loadings (1, 2
and 3%) were prepared. The reaction solution was tested by ICP and no
iridium ions were detected after the catalysts were prepared demonstrat-
ing that all iridium ions were adsorpted on the supports. The asymmetric
hydrogenation data was summarized in Table 2. The 2%Ir/SiO (d)/2tpp
2
performed the highest catalytic activity and enantioselectivity (Table 2,
entry 2) and was selected as the catalyst in the following examinations.
The HTEM picture of 2%Ir/SiO
nanoclusters were highly dispersed on the SiO
2
(d)/2tpp showed that iridium
. The average diameter
1
:1 modifier–substrate complex [21–26]. According to the generalized
2
two-point H-bonding model [27], we speculate that there was an inter-
action carried out through H-bond between substrate and silanols for
the heterogeneous asymmetric hydrogenation of aromatic ketones
of iridium nanoclusters was about 3.0 nm. XPS analysis indicated that
Ir 4f7/2 core level centered at 61.6 eV which is a reduced state compared
to Ir (0) (Ir 4f7/2 core level centered at 60.8 eV) [32]. The full range XPS
(Scheme 1). IR spectroscopy was one of standard methods to investigate
2
spectra indicated that the amount of Ir supported on the SiO is 2%wt./wt.
hydrogen bonds in the solid state [28]. 1-(2-Fluorophenyl)ethanone was
selected as subject of study because of its strong electronegativity. As
The effect of the different reaction factors on the asymmetric hydro-
genation of acetophenone was tested (see supporting information).
Methanol and LiOH were the best solvent and base respectively in the
reaction. N99% conversion and good enantioselectivity of up to 87.6%
were obtained while MeOH/LiOH was examined. The combination of
iPrOH and KOH was also investigated in our catalytic system. Just
shown in Fig. 2, the red line was the IR spectrum of 3%Ir/SiO
2
(d)/2tpp
adsorbing 1-(2-fluorophenyl)ethanone. A new weak sharp peak at
−
1
3
680 cm was confirmed by FTIR methods. The peak was due to the
H–F interaction between fluoro- and silanols compared to experimental
−
1
−1
(
3687 cm ) and theoretical frequencies (3640 cm ) [29–31]. This
11.0% conversion and 48.9% enantioselectivity were obtained, although
demonstrated that a hydrogen bond was formed between substrates
and catalyst. Due to the hydrogen bonding, steric effects influence the
asymmetry of hydrogenation as shown in Scheme 1. It is noteworthy
the iPrOH/KOH was identified as the best combination for hydrogena-
tion of aromatic ketones. A high concentration of the base and modifier
was needed in our catalytic system. The effect of hydrogen pressure on
the reaction was also examined. When the reactions were run at high
hydrogen pressure (6–7 MPa), the activity and enantioselectivity of
the catalytic system were maintained (N99% conversion, 87.6–88.9%
ee). However, when the hydrogen pressure decreased to 5 MPa, the
conversion dropped (79.4%) while the enantioselectivity (89.7%)
remained. When the pressure was further reduced to 4 MPa, the con-
version (47.3%) and the enantioselectivity (72.8%) were reduced. In ad-
dition, less than 1% conversion was observed with no hydrogen when
the base and the solvent were instead KOH and iPrOH. According to
the procedures reported by G. Szőllősi et al. [33], additional experiments
were carried out to clarify the effect of support acidity on activity and
enantioselectivity. The acetophenone was hydrogenated with the cata-
lyst treated by 5% aqueous NaOH solution and low conversion and low
enantioselectivity were obtained.
A representative range of simple aromatic ketones divided into three
categories (ortho-substituted, para-substituted and un-substituted
substrate) respectively was hydrogenated with the catalyst 2%Ir/
SiO
2
(d)/2tpp. The results obtained were shown in Table 3. In general,
excellent conversions and high enantioselectivities were obtained for
all tested aromatic ketones. When the steric bulk of substituent was
larger, the enantioselectivity and activity were lower. As previously
reported research, the steric and the electronic effect commonly
influences the activity and the enantioselectivity in the asymmetric
Fig. 2. The IR spectrum. A: The IR spectrum of 3%Ir/SiO
of 3%Ir/SiO (d)/2tpp adsorbing 1-(2-fluorophenyl)ethanone; and. C: the IR spectrum of
-(2-fluorophenyl)ethanone.
2
(d)/2tpp; B: the IR spectrum
2
1

30
C. Li et al. / Catalysis Communications 54 (2014) 27–30
Table 3
Asymmetric hydrogenation of aromatic ketones catalyzed by 2%Ir/SiO
4. Conclusion
(d)/2tpp.a
2
In summary we have synthesized four monodispersed and rich
silanol silicas that are used as support and demonstrated that silanols
enhanced the activity and enantioselectivity via hydrogen bonding
between substrates and silanol in the heterogeneous asymmetric hydro-
genation of aromatic ketones, especially for ortho-substituted aromatic
ketones. By increasing the concentration of silanols, the activity of asym-
metric hydrogenation was significantly increased. This was the first
report of the heterogeneous asymmetric hydrogenation of aromatic
ketones enhanced by silanols on highly monodispersed silica spheres
with N99.9% ee and N99% conversion. The work reported herein
provides a new direction for asymmetric hydrogenation.
Entry
R
1
R
2
Con (%)
ee (%)
Config.
1
2
3
4
5
6
7
8
9
C
6
H
5
Me
Me
Me
Me
Me
Me
Me
Me
Et
99.3
50.1
52.9
66.9
81.2
6.3
90.1
90.7
91.7
92.2
79.5
N99.9
N99.9
82.6
90.2
42.7
S
S
S
S
S
S
S
S
S
S
2-FC
2-ClC
2-BrC
2-CF
2-CH
2-CH
4-CH
6 4
H
6 4
H
H
6 4
3
6
C H
4
3
3
3
OC
OC
OC
6
H
H
H
4
4
4
99.7b
99.8
98.9
35.8
6
6
Appendix A. Supplementary data
C
C
6
H
H
5
10
6
5
i-Pr
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2014.05.009. This data include MOL file and
InChiKey of the most important compounds described in this article.
a
Substrate/Ir = 300/1, acetophenone: 0.856 mol/L, 9-amino(9-deoxy)epicinchonine:
−
3
2
.8 × 10 mol/L, LiOH: 0.05 mol/L, 25 °C, 6 MPa, 3 h.
b
Reaction time: 48 h.
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