Heterogeneous asymmetric hydrogenation of heteroaromatic methyl ketones catalyzed by cinchona-modified iridium catalysts

Article abstract of DOI:10.1016/j.tetasy.2014.04.013

A heterogeneous iridium catalyst was synthesized with silica particles as support for the hydrogenation of heteroaromatic methyl ketones. The catalyst and support were characterized by solid-state NMR, HTEM, SEM, XPS, and BET. A series of heteroaromatic methyl ketones were investigated at room temperature. The catalytic system was effective and more than 99% conversion and up to 83.6% enantioselectivity were obtained in the hydrogenation of heteroaromatic methyl ketones.

Full text of DOI:10.1016/j.tetasy.2014.04.013

Tetrahedron: Asymmetry 25 (2014) 821–824
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Heterogeneous asymmetric hydrogenation of heteroaromatic methyl
ketones catalyzed by cinchona-modified iridium catalysts
⇑
Chun Li, Lin Zhang, Congye Zheng, Xueli Zheng, Haiyan Fu, Hua Chen , Ruixiang Li
Key Lab 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 heterogeneous iridium catalyst was synthesized with silica particles as support for the hydrogenation
of heteroaromatic methyl ketones. The catalyst and support were characterized by solid-state NMR,
HTEM, SEM, XPS, and BET. A series of heteroaromatic methyl ketones were investigated at room temper-
ature. The catalytic system was effective and more than 99% conversion and up to 83.6% enantioselectiv-
ity were obtained in the hydrogenation of heteroaromatic methyl ketones.
Received 28 February 2014
Accepted 24 April 2014
Available online 29 May 2014
Ó 2014 Published by Elsevier Ltd.
1
. Introduction
Chiral heteroaryl alcohols containing nitrogen, oxygen or sulfur
supported Ru catalysts, which were modified with triphenylphos-
phine and (1R,2R)-DPEN, was investigated by Yuan et al. for the
8
enantioselective hydrogenation of activated ketones and the cata-
in the heterocyclic ring have numerous applications as important
intermediates in the synthesis of biologically active molecules
and also act as chiral ligands/auxiliaries in a number of asymmetric
lyst system of Ru/MgO-TPP-(1R,2R)-DPEN afforded the highest ee
9
10
of 74.0% and 71.6%. In 1999, Sugimura and Szöllösi reported
on the enantioselective heterogeneous catalytic hydrogenation of
prochiral olefins over cinchona modified Pd catalysts in which chi-
ral heteroaromatic compounds were obtained in high yields and
with good enantiomeric purities. Moreover, the asymmetric hydro-
genation of heteroaromatic ketones to give chiral heteroaryl alco-
hols on heterogeneous catalysts remains a formidable scientific
challenge. In earlier studies, we succeeded in the heterogeneous
asymmetric hydrogenation of aromatic ketones with enantioselec-
1
reactions. The asymmetric hydrogenation of heteroaromatic
ketones is an important process that provides useful chiral second-
ary alcohols from inexpensive starting materials. Over the last few
decades, the asymmetric hydrogenation of C@O bonds was gener-
ally affected by a variety of chiral transition metal catalysts such
as Ru, Rh, and Pd which provided highly active and selective cata-
2
lysts. The most important breakthrough was made by Noyori et al.
1
1
in 1995; the homogeneous catalyst RuCl
2
(diphosphine)(diamine)
tivities of up to 96%. As part of our ongoing research, we herein
report the heterogeneous asymmetric hydrogenation of heteroaro-
has become one of the best reduction systems for both academia
3
and industry. Gao et al. demonstrated that chiral diaminodiphos-
2
matic methyl ketones catalyzed by 2%Ir/SiO /2tpp modified by
phine–Ru complexes were excellent catalysts for the asymmetric
hydrogenation of heteroaromatic ketones.4 Wills synthesized an
Ru/TsDPEN catalyst containing an ether-based linking tether,
which showed excellent activity and enantioselectivity in the
chiral diamines. The catalyst was synthesized with prepared mon-
odispersed silica particles as support and displayed more than 99%
conversion and with up to 83.6% enantioselectivity in the hydroge-
nation of heteroaromatic methyl ketones.
5
asymmetric transfer hydrogenations of ketones. In 2008, a new
⁄
type of Cp Ir(OTf)(MsDPEN) catalyst was developed for the asym-
2
2
. Experimental
metric hydrogenation of aromatic heterocyclic ketones by Ohkuma
6
et al. and 93–99% ee was obtained. Since the recovery of homoge-
.1. The preparation of monodispersed silica spheres
The monodispersed silica particles were synthesized by a one-
neous catalysts remains a problem, some heterogeneous hydroge-
nation catalysts have been developed in recent years to match
economic and environmental requirements. Zhao et al. reported
1
2
step process (Stöber process). Firstly, 6 mL of ethanol, 10 mL of
aqueous ammonia, and 6 mL of TEOS were added into a 50 mL
round bottomed flask. The mixture was then stirred for 8 h at
on a DPEN and phosphine modified Ru/Al
hydrogenation of acetophenone with 60.5% ee. A series of
2 3
O for the asymmetric
7
3
5 °C. The slurry was then filtered and washed a few times with
ethanol and water. The solid was dried under vacuum at room
temperature for 24 h to afford the support.
⇑
Corresponding author. Tel./fax: +86 28 85412904.
E-mail address: scuhchen@163.com (H. Chen).
http://dx.doi.org/10.1016/j.tetasy.2014.04.013
957-4166/Ó 2014 Published by Elsevier Ltd.
0

8
22
C. Li et al. / Tetrahedron: Asymmetry 25 (2014) 821–824
2
.2. The preparation of 2%Ir/SiO
2
/2tpp
and BJH average pore sizes of 18.3 Å. The 29Si NMR spectrum
showed that plentiful hydroxyl groups (silanols), Si(OSi) OH, and
Si(OSi) (OH) , were present on the surface of the silicas (Fig. 2).
The catalyst 2%Ir/SiO /2tpp was also investigated by nitrogen
adsorption, which had BET surface areas of 38.7 m g and a BJH
pore size of 17.2 Å. Compared to silica spheres, the surface area of
3
/2tpp was prepared according to the literature.11,13
14
2
%Ir/SiO
Under an argon atmosphere, 1 g of silica spheres, 0.104 mmol of
IrCl , 0.208 mmol of tpp, and 30 mL of i-PrOH were added into
2
2
2
2
2
À1
H
2
6
a 100 mL round bottomed flask and stirred for 24 h at room tem-
perature. Next, 5 mL of formaldehyde was added and the mixture
heated at 110 °C for 5 h. The mixture was then cooled and the solid
was filtered and washed a few times with methanol. Finally, the
solid was dried under vacuum for 6 h to give the catalyst.
2%Ir/SiO
indicated the attachment of the Ir-tpp to the SiO
HRTEM picture of 2%Ir/SiO /2tpp showed that iridium nanoclusters
were highly dispersed on the SiO . The particle size distribution
Fig. 3) showed that the most probable iridium particle size was
2
/2tpp was reduced. The reduction of the surface areas
2
surface. The
2
2
(
2
.3. General procedure for the asymmetric hydrogenation
approximately 3.0 nm, which was consistent with the analysis of
HRTEM. Moreover, 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), and the binding energy
(103.3 eV) of Si2p is similar to the expected value (103.4 eV) for
reaction
Definite quantities of catalyst, chiral diamine, base, solvent, and
heteroaromatic methyl ketones were placed into a 60 mL stainless
steel autoclave equipped with a magnetic stirrer bar. The autoclave
was purged with hydrogen three times and the hydrogen pressure
was increased to the desired level. The mixture was then stirred at
room temperature for a suitable time. At the end of the reaction,
the reactor was decompressed. Finally, the products were sepa-
rated by centrifugation and analyzed by a GC instrument with an
FID detector and b-DEX120 capillary column. The ee value was cal-
culated from the equation: ee (%) = 100 Â (S À R)/(S + R).
1
5
the corresponding SiO
the amount of Ir supported on the SiO
2
The full range XPS spectra indicated that
was 2%wt/wt. ICP was also
2
used to test the real iridium loading. No iridium ions were detected
in the reaction solution after the catalyst was prepared. These show
that all of the iridium ions were absorbed on the silica spheres.
2
.4. Characterization
The 29Si NMR spectrum was obtained in a Bruker AVANCE III
00 MHz solid NMR spectrometer. High-resolution transmission
5
electron microscopy (HTEM) graph was recorded by FEI Tecnai
2
G
F20 S-Twin TEM with an accelerating voltage of 200 kV. X-ray
photoelectron spectroscopy data was taken on Kratos XSAM 800
XPS. The scanning electron microscopy graph was obtained by JEOL
JSM-7500F SEM. N
on a Micromeritics ASAP 2020M+C analyzer at 77 K. The BET
Brunauer–Emmett–Teller) method was adopted to calculate the
2
adsorption–desorption isotherms were tested
(
special surface areas using adsorption data. The pore size distribu-
tions were derived from the adsorption branches of isotherms
using the BJH (Barret–Joyber–Halenda) model. Inductively coupled
plasma (ICP) data was tested by Intrepid IIXSP ICP.
Figure 2. 29Si NMR spectra (black lines) recorded by direct excitation at 8 kHz MAS
frequency from the set of silica samples. The peak components as obtained from
3
. Results and discussion
spectral deconvolutions are shown by blue lines and assigned to their respective
n
Q
Si environments as shown at the top.
According to the SEM graph, the silica spheres were monodi-
spersed and monosized with the average particle size of approxi-
mately 560 nm (Fig. 1). Nitrogen adsorption data indicated that
2
À1
the silica spheres had a medium BET surface areas of 69.2 m g
2
Figure 3. HRTEM graph of 2%Ir/SiO /2tpp, the Ir nanoclusters are marked with blue
Figure 1. SEM graph of high monodispersed silica spheres.
lines.

C. Li et al. / Tetrahedron: Asymmetry 25 (2014) 821–824
823
The modifier played an important role with regard to the
activity and enantioselectivity in most heterogeneous catalytic
Table 2
The effect of different solvents on the asymmetric hydrogenation of heteroaromatic
methyl ketones
a
1
6
reactions. Different modifiers (Fig. 4) were evaluated in the
hydrogenation of 2-acetylthiophene. Results are summarized in
Table 1. More than 99% conversion and 78.4% enantioselectivity
were obtained when modifier 1 9-amino(9-deoxy)epicinchonine
was used in the asymmetric hydrogenation of 2-acetylthiophene
Entry
Solvent
Con. (%)
ee (%)
Config.
1
2
3
4
MeOH
EtOH
n-PrOH
i-PrOH
99.0
5.5
3.2
6.0
78.4
55.4
64.7
56.3
(S)
(S)
(S)
(S)
(
Table 1, entry 1). By contrast, the conversion decreased to 63.5%
with an ee value down to 19.4% for the system containing modifier
-amino(9-deoxy)epicinchonidine 2 (Table 1, entry 2). As can be
a
Substrate/Ir = 300/1,2-acetylthiophene: 0.856 mol/L, diamine 1: 2.8Ã10À3 mol/
L, LiOH: 0.05 mol/L, 25 °C, 6 MPa, 2 h.
9
seen, a small change in the structure of the modifiers had a great
influence on the activity, enantioselectivity, and the configuration
of the product. In addition, another two enantiomers (1R,2R)-1,
Table 3
2
-diphenylethylenediamine 3 and (1S,2S)-1,2-diphenylethylenedi-
The effect of different bases on the asymmetric hydrogenation of heteroaromatic
methyl ketones
a
amine 4 were tested and showed the worst activity (Table 1,
entries 3 and 4). When modifier (1S,2S)-dimethyl diaminocyclo-
hexane 5 was used, it exhibited just 7.3% conversion, and medium
Entry
Base
Con. (%)
ee (%)
Config.
1
2
3
4
5
t-BuOK
LiOH
NaOH
KOH
88.3
99.0
98.2
99.4
17.2
77.8
78.4
77.0
76.7
72.8
(S)
(S)
(S)
(S)
(S)
5
8.8% ee in the reaction. These results indicated the high specific
correlation between the catalyst and modifier.
Since the choice of solvent, especially alcohols, had a strong
effect on the enantioselectivity of the asymmetric hydrogenation
in our previous work, four different alcohols were evaluated in this
reaction. As can be seen, the catalytic system showed the best per-
formance in methanol, with up to 78.4% ee and a conversion of
Ca(OH)
2
a
Substrate/Ir = 300/1,2-acetylthiophene: 0.856 mol/L, diamine 1: 2.8Ã10À3 mol/
L, base: 0.05 mol/L, MeOH: 2 mL, 25 °C, 6 MPa, 2 h.
>
99% being achieved (Table 2, entry 1). Just 5.5% and 3.2% conver-
sions were obtained when the asymmetric hydrogenation was car-
ried out in ethanol and n-propanol (Table 2, entries 2 and 3). The
solvent i-propanol was also ineffective in the asymmetric hydroge-
nation, with just 6.0% conversion being obtained (Table 2, entry 4).
Methanol was a unique solvent while the other alcohol solvents
were almost ineffective for the reaction.
enantioselectivities were achieved when the reaction was carried
with LiOH, NaOH, or KOH as the base (Table 3, entries 2–4). A con-
version of 88.3% with 77.8% enantioselectivity was obtained when
t-BuOK was added (Table 3, entry 1). The catalytic system showed
2
the worst performance with Ca(OH) as the base with only 17.2%
conversion being achieved, but with similar enantioselectivity of
72.8% (Table 3, entry 5). Similar to our previous research, the high
concentration of the base and the high concentration of the modi-
fier were necessary in the asymmetric hydrogenation of heteroar-
omatic methyl ketones.
The effect of different bases on the hydrogenation of 2-acetyl-
thiophene was also examined (Table 3). Equivalent activities and
A substrate screening was consequently performed under the
optimized conditions, where differently functionalized and substi-
tuted heteroaromatic methyl ketones were reduced. All of the
hydrogenations were investigated at room temperature and the
results are summarized in Table 4. The sulfur-containing heterocy-
cle did not disturb the catalytic activity, conversely, the catalysis
system showed the best performance when 2-acetylthiophene
was hydrogenated (Table 4, entry 1). A conversion of 99.0% was
obtained in just 2 h and the enantioselectivity of the asymmetric
hydrogenation of 2-acetylthiophene was up to 78.4%. Two furyl
ketones were used to test the catalyst. The highest enantioselectiv-
ity (83.6% ee) was obtained with 80.3% conversion when 2-acetyl-
furan was hydrogenated for 3 h (Table 4, entry 2). No conversion or
enantioselectivity was achieved in the asymmetric hydrogenation
of 5-methyl-2-acetylfuran compared to 2-acetylfuran (Table 4,
entry 3). Previous research showed that no reliable methods for
N
N
H
H
H
H
H2N
NH2
N
N
2
1
NH2
NH2
NH
NH
NH2
NH2
4
5
17
3
the asymmetric hydrogenation of 2-acetylpyrrole were available.
Similar to this research, no reaction took place in the attempted
hydrogenation of 2-acetylpyrrole (Table 4, entry 4). It should be
mentioned that attempts to reduce 1-methyl-2-acetylpyrrole with
the same reaction conditions led to 11.2% conversion and 44.0% ee
(Table 4, entry 5). The ortho-, meta-, and para-acetyl pyridines were
also investigated. The catalytic system did not give excellent activ-
ity or enantioselectivity for the three acetyl pyridine. Low and sim-
ilar conversions (6.4–7.4% in Table 4, entries 6–8) were obtained
after 3 h of reaction. When the reaction time was lengthened to
40 h, 2-acetylpyridine and 3-acetylpyridine were hydrogenated
with almost the same conversion, that is 95.3–95.5% (Table 4,
entries 6 and 7). A conversion of 82.1% was obtained in the hydro-
genation of 4-acetylpyridine in 20 h (Table 4, entry 8). This showed
that 4-acetylpyridine was easier to hydrogenate compared to
Figure 4. Chiral diamines.
Table 1
The effect of different modifiers on the asymmetric hydrogenation of heteroaromatic
methyl ketonesa
Entry
Modifier
Con. (%)
ee (%)
Config.
1
2
3
4
5
1
2
3
4
5
99.0
63.5
<1
<1
7.3
78.4
19.4
—
—
58.8
(S)
(R)
—
—
(R)
a
À3
Substrate/Ir = 300/1,2-acetylthiophene: 0.856 mol/L, diamine: 2.8Ã10 mol/L,
LiOH: 0.05 mol/L, MeOH: 2 mL, 25 °C, 6 MPa, 2 h.

8
24
C. Li et al. / Tetrahedron: Asymmetry 25 (2014) 821–824
Table 4
Asymmetric hydrogenation of the heteroaromatic methyl ketones catalyzed by 2%Ir/
SiO
/2tppa
References
2
1
.
(a) Hayoz, P.; Zelewsky, A. V.; Stoeckli-Evans, H. J. Am. Chem. Soc. 1993, 115,
111–5114; (b) Kawano, S.; Horikawa, M.; Yasohara, Y.; Hasegawa, J. Biosci.
Biotechnol. Biochem. 2003, 67, 809–814.
2. (a) Braunstein, P.; Graiff, C.; Naud, F.; Pfaltz, A.; Tiripicchio, A. Inorg. Chem.
000, 39, 4468–4475; (b) Naili, S.; Suisse, I.; Mortreux, A.; Agbossou, F.; Ali, M.
5
O
OH
Het (S)
2
%Ir/SiO2/2tpp
H2, rt
Het
2
A.; Karim, A. Tetrahedron Lett. 2000, 41, 2867–2870; (c) Arx, M. V.; Mallat, T.;
Baiker, A. Tetrahedron: Asymmetry 2001, 12, 3089–3094.
(a) Matsumura, K.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1997,
1
2
3
4
5
6
7
8
: Het = 2-thienyl
: Het = 2-furyl
3.
1
19, 8738–8739; (b) Hashiguchi, S.; Fujii, A.; Haack, K.-J.; Matsumura, K.;
: Het = 2-(5-methyl)furyl
: Het = 2-pyrrolyl
Ikariya, T.; Noyori, R. Angew. Chem., Int. Ed. 1997, 36, 288–290; (c) Uematsu, N.;
Fujii, A.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1996, 118, 4916–
4917; (d) Fujii, A.; Hashiguchi, S.; Uematsu, N.; Ikariya, T.; Noyori, R. J. Am.
Chem. Soc. 1996, 118, 2521–2522; (e) Ohkuma, T.; Koizumi, M.; Yoshida, M.;
Noyori, R. Org. Lett. 2000, 2, 789–792.
: Het = 2-(1-methyl)pyrrolyl
: Het = 2-pyridyl
: Het = 3-pyridyl
: Het = 4-pyridyl
4
5
.
.
Shen, W.-Y.; Li, Y.-Y.; Dong, Z.-R.; Gao, J.-X. Synthesis 2009, 14, 2413–2417.
Parekh, V.; Ramsden, J. A.; Wills, M. Catal. Sci. Technol. 2012, 2, 406–414.
Entry Substrate
Time (h) Con. (%) ee (%)
Config.
1
2
3
4
5
2-Acetylthiophene
2-Acetylfuran
5-Methyl-2-acetylfuran
2-Acetylpyrrole
1-Methyl-2-
2
3
3
3
3
99.0
80.3
—
—
11.2
78.4
83.6
—
—
44.0
(S)
(S)
—
—
(S)
6. Utsumi, N.; Tsutsumi, K.; Watanabe, M.; Murata, K.; Arai, N.; Kurono, N.;
Ohkuma, T. Heterocycles 2010, 80, 141–147.
7. Cheng, H.; Hao, J.; Wang, H.; Xi, C.; Meng, X.; Cai, S.; Zhao, F. J. Mol. Catal. A:
Chem. 2007, 278, 6–11.
8
9
.
.
Ye, L.; Lin, H.; Zhou, H.; Yuan, Y. J. Phys. Chem. C 2010, 114, 19752–19760.
Misaki, T.; Otsuka, H.; Uchida, T.; Kubota, T.; Okamoto, Y.; Sugimura, T. J. Mol.
Catal. A: Chem. 2009, 312, 48–52.
acetylpyrrole
6
7
8
2-Acetylpyridine
3-Acetylpyridine
4-Acetylpyridine
3/40 7.1/95.3 49.7/50.1 (S)
3/40 6.4/95.5 66.3/67.0 (S)
3/20 7.4/82.1 50.2/50.0 (S)
1
1
0. Hermán, B.; Szöllösi, G.; Felföldi, K.; Fülöp, F.; Bartók, M. Catal. Commun. 2009,
0, 1107–1110.
1. Jiang, H. Y.; Yang, C. F.; Li, C.; Fu, H. Y.; Chen, H.; Li, R. X.; Li, X. J. Angew. Chem.,
Int. Ed. 2008, 47, 9240–9244.
1
a
Substrate/Ir = 300/1, heteroaromatic methyl ketones: 0.856 mol/L, diamine 1:
.8Ã10À3 mol/L, LiOH: 0.05 mol/L, MeOH: 2 mL, 25 °C, 6 MPa.
12.
3. (a) Huang, Y.; Chen, J.; Chen, H.; Li, R.; Li, Y.; Min, L.; Li, X. J. Mol. Catal. A: Chem.
001, 170, 143–146; (b) Huang, Y.; Li, Y.; Hu, J.; Cheng, P.; Chen, H.; Li, R.; Li, X.;
Chen, S.-L. Colloids Surf. A 1998, 142, 59–63.
2
1
2
Yip, C. W.; Chan, A. S. C. J. Mol. Catal. A: Chem. 2002, 189, 219–224; (c) Ma, H.;
Chen, H.; Zhang, Q.; Li, X. J. Mol. Catal. A: Chem. 2003, 196, 131–135; (d) Xiong,
W.; Ma, H.; Hong, Y.; Chen, H.; Li, X. Tetrahedron: Asymmetry 2005, 16, 1449–
2
-acetylpyridine and 3-acetylpyridine. Excellent enantioselectivity
1
452; (e) Liu, D.; Xiong, W.; Fang, G.; Chen, H.; Li, R.; Li, X. Appl. Catal. A 2008,
39, 93–98; (f) Tang, B.; Xiong, W.; Liu, D. R.; Jia, Y.; Wang, J. B.; Chen, H.; Li, X.
J. Tetrahedron: Asymmetry 2008, 19, 1397–1401.
14. (a) Chuang, I.-S.; Kinney, D. R.; Maciel, G. E. J. Am. Chem. Soc. 1993, 115, 8695–
705; (b) Liu, C. C.; Maciel, G. E. J. Am. Chem. Soc. 1996, 118, 5103–5119; (c)
(
67% ee) was obtained when 3-acetylpyridine was hydrogenated.
3
From these experimental data, we can conclude that the different
activities and enantioselectivities are due to the electronic and ste-
ric effects of the substrates.
8
Christiansen, S. C.; Zhao, D.; Janicke, M. T.; Landry, C. C.; Stucky, G. D.; Chmelka,
B. F. J. Am. Chem. Soc. 2001, 123, 4519–4529.
1
1
5. (a) Castaño, P.; Pawelec, B.; Fierro, J. L. G.; Arandes, J. M.; Bilbao, J. Appl. Catal. A
2006, 315, 101–113; (b) Marzialetti, T.; Fierro, J. L. G.; Reyes, P. Catal. Today
4
. Conclusion
2005, 107–108, 235–243.
6. (a) Mallat, T.; Orglmeister, E.; Baiker, A. Chem. Rev. 2007, 107, 4863–4890; (b)
Meemken, F.; Maeda, N.; Hungerbühler, K.; Baiker, A. Angew. Chem., Int. Ed.
2012, 51, 8212–8216; (c) Balázsik, K.; Szöri, K.; Szöllösi, G.; Bartók, M. Chem.
Commun. 2011, 1551–1552; (d) Szöllösi, G.; Busygin, I.; Hermán, B.; Leino, R.;
Bucsi, I.; Murzin, D. Y.; Fülöp, F.; Bartók, M. ACS Catal. 2011, 1, 1316–1326; (e)
Szöllösi, G.; Makra, Z.; Kovács, L.; Fülöp, F.; Bartók, M. Adv. Synth. Catal. 2013,
In conclusion, we have demonstrated that the heterogeneous
asymmetric hydrogenations of heteroaromatic methyl ketones
can be catalyzed by an efficient heterogeneous catalyst via the
immobilization of iridium onto monodispersed silica particles. A
series of heteroaromatic methyl ketones have been investigated
at room temperature and high activity and good enantioselectivity
were obtained.
3
55, 1623–1629; (f) Schmidt, E.; Bucher, C.; Santarossa, G.; Mallat, T.; Gilmour,
R.; Baiker, A. J. Catal. 2012, 289, 238–248; (g) Baddeley, C. J.; Jones, T. E.; Trant,
A. G.; Wilson, K. E. Top. Catal. 2011, 54, 1348–1356.
1
7. Ohkuma, T.; Koizumi, M.; Yoshida, M.; Noyori, R. Org. Lett. 2000, 2, 1749–1751.