Chiral Pyrrolidine Bridged Polyhedral Oligomeric Silsesquioxanes as Heterogeneous Catalysts for Asymmetric Michael Additions

Article abstract of DOI:10.1007/s10562-017-2286-z

Abstract: A chiral pyrrolidine bridged polyhedral oligomeric silsesquioxane (SQ) was synthesized, characterized, and used as an effective heterogeneous catalyst. The synthesis involves two simple steps: nucleophilic substitution between benzylchloride fun

Full text of DOI:10.1007/s10562-017-2286-z

Catalysis Letters
https://doi.org/10.1007/s10562-017-2286-z
Chiral Pyrrolidine Bridged Polyhedral Oligomeric Silsesquioxanes
as Heterogeneous Catalysts for Asymmetric Michael Additions
Torsak Luanphaisarnnont1 · Sasikarn Hanprasit1 · Vetiga Somjit1 · Vuthichai Ervithayasuporn1
Received: 9 November 2017 / Accepted: 21 December 2017
© Springer Science+Business Media, LLC, part of Springer Nature 2017
Abstract
A chiral pyrrolidine bridged polyhedral oligomeric silsesquioxane (SQ) was synthesized, characterized, and used as an
effective heterogeneous catalyst. The synthesis involves two simple steps: nucleophilic substitution between benzylchloride
functionalized SQ and imidazoyl pyrrolidine carboxylate and subsequent deprotection. The catalyst was isolated by simple
filtration. The SQ-supported chiral pyrrolidine catalyst was used as a heterogeneous catalyst in an asymmetric Michael addi-
tion into nitrostyrenes under room temperature and neat condition, giving the product in excellent yields (85–91%), diaste-
reoselectivities (up to 99:1) and enantioselectivities (95–98%). The catalyst can be recycled by a simple filtration without a
significant loss in its reactivity and selectivity.
Graphical Abstract
Keywords Silsesquioxane · POSS · Asymmetric catalysis · Michael reaction · Heterogeneous catalysis
1 Introduction
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-017-2286-z) contains
Development of efficient catalysis for organic transforma-
tions has been among major challenges in chemistry [1].
The use of inorganic support for heterogeneous catalysis has
gained attention over organic materials due to their outstand-
ing mechanical and thermal stabilities [2]. Although silica
gel has been the most widely used support for chiral cata-
lysts [3–5], recent research investigations have shown that an
ordered nanostructure of mesoporous silica can also be used
supplementary material, which is available to authorized users.
* Vuthichai Ervithayasuporn
vuthichai.erv@mahidol.ac.th; maldiniandg@hotmail.com
1
Department of Chemistry, Center of Excellence
for Innovation in Chemistry (PERCH-CIC), and Center
for Inorganic and Materials Chemistry, Faculty of Science,
Mahidol University, 272 Rama VI Road, Ratchathewi
District, Bangkok 10400, Thailand
Vol.:(0123456789)
1 3

T. Luanphaisarnnont et al.
as a support for catalyst, exhibiting high catalytic activities
in both symmetric and asymmetric organic reactions [6–12].
Polyhedral oligomeric silsesquioxanes (POSS) or cage-like
silsesquioxanes (SQ) with a well-defined nanostructure,
composed of a silica (Si–O) core with the highest organic
functionalities [13–15], have also been studied and used as
sensors [16], electronic materials [17], and alternative cata-
lyst supports [18–20]. Recent works showed that SQ could be
applied in C–C bond coupling reactions [21–23], oxidations
[24], multicomponent coupling reactions [25], carbon dioxide
fixation reactions [26], and polymerizations [27, 28]. Despite
many reports on the use of SQ as a catalyst support, the devel-
opment of SQ-supported chiral catalysts for heterogeneous
asymmetric reactions has been limited. An organorhodium-
functionalized SQ, which was used as a bifunctional hetero-
geneous catalyst for an asymmetric transfer hydrogenation
of aromatic ketones, has been reported [29]. To the best of
our knowledge, there is no report on the use of chiral SQ as
a heterogeneous catalyst in asymmetric C–C bond forming
reactions.
spectrometer. HPLC analysis was performed on Waters 2695
Separation Modules with Waters 2487 Dual λ Absorbance
Detector using a Chiralpak AD-H column purchased from
Daicel Chemical Industries, Ltd.
The 13C and 29Si CP/MAS NMR spectra were acquired at
60 MHz frequency with AVANCE 300 MHz Digital NMR
Spectrometer (Bruker Biospin; DPX-300). A 7 mm triple
resonance probe, a sample spinning rate of 5 kHz with a con-
tact time of 2 ms, and pulse delay of 5 s were used. The pulses
and the spectra were calibrated using commercially available
samples of 2,2-dimethyl-2-silapentan-5-sulfonate, glycine and
adamantine, for 29Si and 13C, respectively. Fourier transform
nuclear magnetic resonance spectra in solution phase were
obtained by using a Bruker-Ascend™ 400 and Bruker-AV
500 high-resolution magnetic resonance spectrometers for 1H
(400 MHz) and (500 MHz) nuclei, respectively. Powder X-ray
diffraction was performed by Bruker D8 Advance with a mon-
ochromatic Cu Kα radiation at λ=0.154 nm. Applied voltage
and current were used at 40 kV and 30 mA, respectively. The
mode of recorded system was operated with 2θ=5.00–40.00
(scan rate=0.005° s−1). Thermogravimetric analyses (TGA)
were performed on a TG 209F3 instrument in Al2O3 pans and
carried out under oxygenated atmosphere on heating at 10 °C/
min in a range of 35 and 800 °C.
Catalytic asymmetric Michael reactions are one of the
most important and useful asymmetric C–C bond forming
reactions [30, 31]. Many chiral pyrrolidine catalysts have
been developed for asymmetric homogeneous Michael addi-
tion into nitrostyrenes [32, 33]. Developments of heteroge-
neous catalysts by attachment of chiral pyrrolidines on modi-
fied organic and inorganic materials have also been studied
[34–39]. However, a chiral heterogeneous pyrrolidine-modi-
fied SQ catalyst has not been well investigated. Herein, a chi-
ral pyrrolidine bridged polyhedral oligomeric SQ was syn-
thesized, characterized, and used as a heterogeneous catalyst
for asymmetric Michael additions of cyclohexanones into
nitrostyrenes. Furthermore, the chiral pyrrolidine loading
of SQ catalyst were determined to be much greater than any
previous silicon-based materials.
2.2 Preparation of Catalysts
2.2.1 Synthesis of SQ‑Supported Chiral Pyrrolidine
Procatalyst (2)
Under argon atmosphere, octa(4-(chloromethyl)styryl)octa-
silsesquioxane [40, 41] (0.30 g, 0.18 mmol) and tert-butyl
(S)-2-((1H-imidazol-1-yl)methyl)pyrrolidine-1-carboxylate
[42] (1.13 g, 4.52 mmol) were mixed in 4 mL of dried tolu-
ene. The solution was kept at 70 °C and stirred for 24 h. The
precipitate was filtered and washed with CH2Cl2, methanol,
and ethyl ether, respectively, and dried under vacuum. The
Boc-protected SQ-supported product was obtained as a pale
yellow solid (0.51 g, 0.135 mmol, 75% yield). Procatalyst 2
was further quantified through CHN microanalysis and found
to be 2.14 mmol/g of pyrrolidine-based imidazolium cation
based on nitrogen percentage. 29Si CP/MAS NMR: δ − 67.89,
− 77.43 ppm; 13C CP/MAS NMR: δ 154.8, 148.0, 136.9,
129.9, 127.4, 123.0, 79.4, 56.9, 52.4, 46.8, 28.4, 23.4 ppm.
2 Experimental
2.1 Chemicals and Instruments
Di-tert-butyl dicarbonate was purchased from Tokyo
Chemical Industry, while imidazole, l-proline, trans-β-
nitrostyrene, trans-4-methoxy-β-nitrostyrene, trans-4-fluoro-
β-nitrostyrene, trans-4-chloro-β-nitrostyrene, and anhydrous
toluene were purchased from Sigma Aldrich and used with-
out additional purification. The commercial grades of ethyl
acetate, methylene chloride, methanol and hexane were fur-
ther distilled. Precoated silica gel 60 F254 plates and silica
gel (No. 60) used for chromatography were purchased from
Merck & Co., Inc.
2.2.2 Synthesis of SQ‑Supported Chiral Pyrrolidine Catalyst
(3)
The Boc-protected SQ-supported catalyst 2 (0.51 g,
0.135 mmol) was deprotected using 5 M HCl in ethanol
(10 mL) with vigorously stirring at room temperature for
4 h. The reaction was subsequently neutralized with NEt3
(3 mL). The reaction was filtered, and the solid catalyst
The FT-IR spectra were collected using an attenu-
ated total reflectance technique with Bruker model Alpha
1 3

Chiral Pyrrolidine Bridged Polyhedral Oligomeric Silsesquioxanes as Heterogeneous Catalysts…
was washed with ethanol and ethyl ether and dried under
vacuum at room temperature to afford the desired product as
a solid (0.39 g, 0.129 mmol, 96% yield). Catalyst 3 was fur-
ther quantified through CHN microanalysis and found to be
2.28 mmol/g of pyrrolidine-based imidazolium cation based
on nitrogen percentage. 29Si CP/MAS NMR: δ − 59.92,
− 68.14, − 78.03 ppm; 13C CP/MAS NMR: δ 147.8, 136.8,
127.8, 123.0, 58.4, 52.7, 46.6, 28.8, 24.1, 21.6 ppm.
(2), can be synthesized from a substitution reaction between
octa(4-(chloromethyl)styryl)octasilsesquioxane (1) [40, 41]
and tert-butyl (S)-2-((1H-imidazol-1-yl)methyl)pyrrolidine-
1-carboxylate [42] under thermal activation in 75% yield,
subsequently followed by a Boc deprotection under acidic
condition to obtain SQ-supported catalyst 3 in 96% yield. It
2.3 Catalytic Testing for Asymmetric Michael
Addition
The SQ-supported catalyst 3 (7.5 mg, 0.0029 mmol), nitros-
tyrene (75 mg, 0.5 mmol), and cyclohexanone (0.4 mL) were
added to a reaction flask. The reaction was stirred at room
temperature for 24 h. The reaction mixture was filtered and
washed with ethyl acetate. The combined filtrate was con-
centrated. Flash column chromatography (hexanes/ethyl
acetate=3:1) provided the ꢀ-nitroketone as a solid.
3 Result and Discussion
3.1 Preparation of Catalysts
The synthesis approach for SQ-supported (S)-1-((pyrrolidin-
2-yl)methyl)-1H-imidazolium chloride, the SQ-supported
catalyst (3), was depicted in Scheme 1. Firstly, SQ-supported
(S)-1-((1-(tert-butoxycarbonyl)pyrrolidin-2-yl)methyl)-
1H-imidazolium chloride, the SQ-supported procatalyst
Fig. 1 IR spectra of starting materials, procatalyst 2, and catalyst 3
Scheme 1 Synthesis of a chiral pyrrolidine bridged polyhedral oligomeric SQ, the SQ supported catalyst (3)
1 3

T. Luanphaisarnnont et al.
is noteworthy that the syntheses of the procatalyst 2 and the
catalyst 3 require no templating reagents and the isolation
processes involve only a simple filtration because of their
insolubility in organic and aqueous solvents.
asymmetrical Si–O–Si stretching vibrations at 1100 and
1020 cm−1 arising for more disordered polyhedral SQ
cages, which differ from a perfect T8 cage of 1 with a very
strong broad band at 1030 cm−1 [43]. The presence of a
broad O–H band (3000–3700 cm−1) can be assigned to
SiOH units, which also confirmed the presence of some
cage-degradation resulting in open siliceous structures
with silanols groups such as T1 [R-Si(OSi)1(OH)2] and
T2 [R-Si(OSi)2(OH)]. These disordered cages from partial
cage opening could be explained by the strong nucleophi-
licity of the sp2 nitrogen on the imidazole, which increased
the chance of the cleavage of Si–O bonds at a SQ core
[44].
3.2 Catalyst Characterizations
The formations of procatalyst 2 and the catalyst 3 were
confirmed by IR, NMR, TGA, XRD, and CHN methods.
The FT-IR spectra of 2 as shown in Fig. 1 showed a key
C=O stretching vibration band at 1683 cm−1 correspond-
ing to the presence of Boc–pyrrolidine moieties. This
absorption band is absent in the IR spectra of 3 (Fig. 1),
confirming the removal of Boc protecting group. Both 2
and 3 also exhibited overlapped C–N stretching vibra-
tions of pyrrolidine rings at 1150 cm−1among two distinct
The NMR results (Fig. 2) also agreed well with the IR
spectra. To confirm a successful substitution of substituted
imidazole on 1, 13C CP/MAS NMR spectrum of 2 (Fig. 2a)
Fig. 2 a 13C and b 29Si CP/
MAS NMR spectra of procata-
lyst 2 and catalyst 3
1 3

Chiral Pyrrolidine Bridged Polyhedral Oligomeric Silsesquioxanes as Heterogeneous Catalysts…
exhibited two key signal sets corresponding to imidazolium
(148.0 ppm) and Boc–pyrrolidine rings (154.8, 79.4, 56.9,
52.4, and 28.4 ppm). The absence of the signals at 154.8,
79.4, and 28.4 ppm in the 13C CP/MAS NMR spectrum of
3 (Fig. 2a) confirmed the complete removal of the Boc pro-
tecting group. The 29Si CP/MAS NMR spectra were used to
probe into the core structures of 2 and 3 (Fig. 2b). The sig-
nals of catalyst 3 at −77, −68, and −59 ppm corresponded
to the T3 [R-Si(OSi)3] and some partially cage-opening silse-
quioxanes (T2 and T1), respectively.
TGA of 2 and 3 (Fig. 3) confirmed the existence of the
hydrophilic imidazolium and silanol moieties, which can
strongly absorb moisture, on both materials. The TGA
curves showed 5% weight loss between 25 and 120 °C;
therefore, the reformulated ceramic yields of 2 and 3 were
13.7 and 17.9 wt%, respectively, versus the calculated values
of 14.9 and 18.3 wt%, respectively. The derivative weight
loss of 2 (Fig. 3) showed an additional peak at 150–215 °C
confirmed the degradation of Boc groups on 2. This peak
was absent in the curve of 3 (Fig. 3).
Fig. 4 XRD spectra of a procatalyst 2 and b catalyst 3
is decreased to about 1.70 Å. This shift confirmed a suc-
cessful removal of the Boc protecting group on 2. In order
to confirm the number of active sites of pyrrolidine-based
imidazole on SQ, catalyst 3 was simply quantified through
CHN microanalysis. Its carbon-to-nitrogen (C:N) mass ratio
was found to be 6.09, compared to a calculated value of 5.95
for the SQ with six chiral pyrrolidine rings. This result sug-
gested that the number of active chiral pyrrolidine rings on
the SQ core in 3 is approximately 6.
According to the XRD spectra of 2 and 3 in Fig. 4, both
materials exhibited an amorphous nature with a key charac-
teristic reflection at 2θ=21.0°, which can be converted to
a 4.22 Å of d-value. This value indicated the existence of a
SQ (Si–O–Si) cage [27]. When compared with that of the
SQ-supported procatalyst 2, the reflection of the SQ cata-
lyst 3 was shifted from 2θ = 7.3°–8.5°, and the d-spacing
3.3 Catalytic Testing for Asymmetric Michael
Addition
To test for the effectiveness and efficiency of the SQ scaf-
fold as a support for chiral catalysts, the SQ-supported chi-
ral catalyst 3 was used in an asymmetric Michael addition
reaction of cyclohexanone into nitroolefins (Table 1). The
amount of catalyst 3 is 0.57 mol%, which is approximately
equal to 3.4 mol% active chiral pyrrolidine rings. Both
electron-rich and electron-poor nitrostyrenes work well in
the reaction, giving the corresponding products in excellent
yields, diastereoselectivities and enantioselectivities. The
slightly lower yield of electron-rich substrate may be due to
the lower electrophilicity of the substrate. The high yields
and selectivities were similar to those obtained from the
homogeneous catalysis [32] and silica–gel-supported cata-
lyst [35], suggesting that the SQ core did not interfere with
the active chiral pyrrolidine ring. It is noteworthy that the
SQ-supported catalyst was more efficient at catalyzing the
reaction, yielding similar yields and selectivities even with
a lower amount of the active chiral pyrrolidine rings and a
shorter reaction time. When compared with the homogene-
ous catalyst, the SQ-supported catalyst was also easier to
Fig. 3 TGA and derivative weight loss curves of a procatalyst 2 and
b catalyst 3
1 3

T. Luanphaisarnnont et al.
Table 1 Asymmetric Michael additions of cyclohexanone catalyzed by the SQ-supported chiral catalyst 3
Entrya
Product
Yieldb (%)
90
drc
eed (%)
98
1
98:2
2
3
85
90
97:3
99:1
97
97
4
91
99:1
95
aNitroolefin (0.5 mmol), cyclohexanone (0.4 mL) and 3 (0.57 mol%) under neat reaction condition at room temperature for 24 h
bIsolated yields
cDiastereomeric ratio, dr (syn:anti), determined by 1H NMR spectroscopy of the crude product
dDetermined by HPLC using a Chiralpak AD-H column
handle with a simple filtration to separate the catalyst from
the reaction mixture and without need for solvents.
active species leached out form the SQ-supported catalyst 3
during the reaction. A preliminary study on the basic kinetic
behavior of the catalyst was performed by monitoring the
product yield at different times (Fig. S5). The formation
of the product plateaued after around 24 h. The turnover
number of the catalyst after 24 h could be calculated to be
approximately 158. A further kinetic investigation to study
the detailed behavior of the SQ-supported catalyst, par-
ticularly at the beginning of the reaction, is currently being
investigated.
The recyclability of the SQ-supported catalyst 3 was
investigated by subjecting it into a new reaction for mul-
tiple cycles (Table 2). The catalyst can be used for at least
four cycles with a minimal loss in yields and selectivities.
This result showed the high robustness of the SQ as a cata-
lyst support. To further study the durability of the SQ-sup-
ported catalyst 3, a leaching test and a filtration test were
performed, and their results suggested that no catalytically
1 3

Chiral Pyrrolidine Bridged Polyhedral Oligomeric Silsesquioxanes as Heterogeneous Catalysts…
Table 2 Recycling experiments of the catalyst 3
Cycle
Yield (%)
dr
98:2
98:2
98:2
97:3
ee (%)
1
2
3
4
90
88
89
85
98
97
98
95
11. Rostamnia S, Gholipour B, Hossenini HG (2016) Process Saf
Environ Prot 100:74
4 Conclusion
12. Rostamia S, Hossieni HG, Doustkhah E (2015) J Organomet
Chem 791:18
The chiral pyrrolidine bridged cage-like SQ catalyst was
synthesized and characterized. The catalyst was used to
effectively and efficiently catalyze the Michael addition
reactions of cyclohexanone into nitroolefins with excel-
lent yields and selectivities at room temperature. The
catalyst could be recycled by a simple filtration with a
minimal loss in the yields and selectivities, which con-
firms the effectiveness and robustness of the SQ core as a
heterogeneous scaffold for chiral catalysts. Further appli-
cations of SQ cage as a support for other chiral catalysts
is undergoing and will be reported in due course.
13. Cordes DB, Lickiss PD, Rataboul F (2010) Chem Rev 110:2081
14. Hanprasit S, Tungkijanansin N, Prompawilai A, Eangpayung S,
Ervithayasuporn V (2016) Dalton Trans 45:16117
15. Chimjarn S, Kunthom R, Chancharone P, Sodkhomkhum R,
Sangtrirutnugul P, Ervithayasuporn V (2015) Dalton Trans 44:916
16. Chanmungkalakul S, Ervithayasuporn V, Hanprasit S, Masik M,
Prigyai N, Kiatkamjornwong S (2017) Chem Commun 53:12108
17. Ervithayasuporn V, Abe J, Wang X, Matsushima T, Murata H,
Kawakami Y (2010) Tetrahedron 66:9348
18. Zheng W, Lu C, Yang G, Chen Z, Nie J (2015) Catal Commun
62:34
19. Moreau JJE, Vellutini L, Man MWC, Bied C (2003) Chem Eur J
9:1594
20. Duchateau R (2002) Chem Rev 102:3525
21. Bivona LA, Giacalone F, Carbonell E, Gruttadauria M, Aprile C
(2016) ChemCatChem 8:1685
Acknowledgements This research was supported by Thailand Research
Fund (MRG5980118 for T.L.), the Central Instrument Facility, the Fac-
ulty of Science, Mahidol University, and the Center of Excellence for
Innovation in Chemistry (PERCH-CIC), Office of the Higher Education
Commision (OHEC), Ministry of Education. S.H. thanks a graduate
student financial support from TRF IRG5980007.
22. Zhang C, Leng Y, Jiang P, Lu D (2016) RSC Adv 6:57183
23. Mohapatra S, Chaiprasert T, Sodkhomkhum R, Kunthom R, Han-
prasit S, Sangtrirutnugul P, Ervithayasuporn V (2016) Chemis-
trySelect 1:5353
24. Sangtrirutnugul P, Chaiprasert T, Hunsiri W, Jitjaroendee T,
Songkhum P, Laohhasurayotin K, Osotchan T, Ervithayasuporn
V (2017) ACS Appl Mater Interfaces 9:12812
25. Safaei-Ghomi J, Nazemzadesh SH, Shahbazi-Alavi H (2016) Appl
Organomet Chem 30:911
References
1. Beller M, Renken A, van Santen RA (2012) Catalysis: from prin-
ciples to applications. Wiley, Weinheim
26. Bivona LA, Fichera O, Fusaro L, Giacalone F, Buaki-Sogo M,
Gruttaduaria M, Aprile C (2015) Catal Sci Technol 5:5000
27. Kunthom R, Jaroentomeechai T, Ervithayasuporn V (2017) Poly-
mer 108:173
2. Liu H, Zheng S, Nie K (2005) Macromolecules 38:5088
3. Soai K, Watanabe M, Yamamoto A (1990) J Org Chem 55:4832
4. Heckel A, Seebach D (2000) Angew Chem Int Ed 39:163
5. Fraile JM, Mayoral JA, Serrano J, Pericas MA, Sola L, Castellnou
D (2003) Org Lett 5:4333
28. Pochwala M, Bialek M, Franczyk A, Marciniec B, Czaja K (2016)
Eur Polym J 79:121
29. Tang S, Jin R, Zhang H, Yao H, Zhuang J, Liu G, Li H (2012)
Chem Commun 48:6286
6. Kawasaki T, Araki Y, Hatase K, Suzuki K, Matsumoto A, Yokoi
T, Kubota Y, Tatsumi T, Soai K (2015) Chem Commun 51:8742
7. Cheng T, Zhao Q, Zhang D, Liu G (2015) Green Chem 17:2100
8. del Pozo C, Corma A, Iglesias M, Sánchez F (2011) Green Chem
13:2471
30. Berner OM, Tedeschi L, Enders D (2002) Eur J Org Chem
2002:1877
31. Almasi D, Alonson DA, Najera C (2007) Tetrahedron: Asymmetry
18:299
9. Croissant JG, Cattoën X, Durand JO, Man MWC, Khashab NM
(2016) Nanoscale 8:19945
32. Ishii T, Fujioka S, Sekiguchi Y, Kotsuki H (2004) J Am Chem Soc
126:9558
10. Hosseini HG, Doustkhah E, Kirillova MV, Rostamnia S,
Mahmoudi G, Kirillov A (2017) Appl Catal A 548:96
33. Reyes-Rangel G, Vargas-Caporali J, Juaristi E (2017) Tetrahedron
73:4707
1 3

T. Luanphaisarnnont et al.
34. Du J, Shuai B, Tao M, Wang G, Zhang W (2016) Green Chem
18:2625
40. Harrison PG, Hall C (1997) Main Group Met Chem 20:515
41. Cheng G, Vautravers NR, Morris RE, Cole-Hamilton DJ (2008)
Org Biomol Chem 6:4662
35. Li P, Wang L, Zhang Y, Wang G (2008) Tetrahedron 64:7633
36. Luo S, Mi X, Zhang L, Liu S, Xu H, Cheng JP (2006) Angew
Chem Int Ed 45:3093
42. Wang L, Tang R, Yang H (2013) J Korean Chem Soc 57:591
43. Fina A, Tabuani D, Carniato F, Frache A, Boccaleri E, Camino G
(2006) Thermochim Acta 440:36
37. Tian J, Zhang C, Qi X, Yan X, Li Y, Chen L (2015) Catal Sci
Technol 5:724
44. Jaroentomeechai T, Yingsukkamol PK, Phurat C, Somsook E,
Osotchan T, Ervithayasuporn V (2012) Inorg Chem 51:12266
38. Qi X, Tian J, Li Y, Chen L, Yan X (2015) Catal Commun 71:70
39. Liu J, Li P, Zhang Y, Ren K, Wang L, Wang G (2010) Chirality
22:432
1 3