Trans-1,2-Diaminocyclohexane mesoporous silica for asymmetric catalysis: Enhancement of chirality through confinement space by the plug effect

Article abstract of DOI:10.1039/c2cc18095b

trans-1,2-Diaminocyclohexane functionalized mesoporous silica was applied as an ideal catalyst for asymmetric Michael addition of various nitroalkane derivatives. Short channels and plugs in the pore structure offered chiral enhancement in Michael addition. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc18095b

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trans-1,2-Diaminocyclohexane mesoporous silica for asymmetric
catalysis: enhancement of chirality through confinement space
by the plug effectw
Eun-Young Jeong, Cheang-Rae Lim, Hailian Jin and Sang-Eon Park*
Received 27th December 2011, Accepted 27th January 2012
DOI: 10.1039/c2cc18095b
trans-1,2-Diaminocyclohexane functionalized mesoporous silica
was applied as an ideal catalyst for asymmetric Michael addition
of various nitroalkane derivatives. Short channels and plugs in the
pore structure offered chiral enhancement in Michael addition.
important bottlenecks of mesoporous materials for the appli-
cation in catalysis, adsorption and as carriers of drugs, etc.10–14
Our group has also reported many kinds of plug effects.15
Previous studies have reported that L-proline functionalized
mesoporous silica prepared by using L-proline and sodium
metasilicate having short channels and plugs in the pore
structure provided chiral enhancement.16 Moreover, the high
activity of a metal free catalyst should increase the plausibility
of finding novel catalysts for this important C–C bond-forming
reaction.
The use of chiral ligands and their transition metal complexes
have been shown to improve catalyst stability and product
regio- and enantio-selectivity in some reactions for the pharma-
ceutical systems.1–3 However, homogeneous chiral ligands and
their metal complexes have problems such as expensive cost
for separation of chiral catalysts from reaction mixtures and
inefficient recycling. Another drawback is that of product
contamination by metal leaching; this is especially improper
for the production of pharmaceuticals. It has recently been
explained that use of heterogeneous chiral ligands as a catalyst
is a good way to resolve such problems and can improve
catalyst stability and enantioselectivity.4,5
Herein, we explain results of the highly enantioselective
addition of a,b-unsaturated ketones to nitroalkenes catalyzed
at ambient temperature by a structurally simple, heterogenized
trans-1,2-diaminocyclohexane complex catalyst along with a
protonic acid.
We describe the direct synthesis of plugged trans-1,2-diamino-
cyclohexane functionalized mesoporous silica in the same pot, i.e.,
in one-step. The procedure for synthesis of trans-1,2-diamino-
cyclohexane functionalized mesoporous silica (DMS) is demon-
strated in Scheme 1 (see the Experimental section in ESIw).
trans-1,2-Diaminocyclohexane functionalized mesoporous
silicas were prepared by trans-1,2-diaminocyclohexane precursor
and sodium metasilicate in P123 templating and HCl both by a
microwave method (MW) or a hydrothermal method (HT).
trans-1,2-Diaminocyclohexane to SiO2 molar ratios in the
initial synthesis mixture were 5%, 7.5% and 10%. The XRD
patterns of all samples are shown in Fig. S1 (see ESIw). The
XRD patterns of DMS-5-HT, DMS-7.5-HT and DMS-10-HT
displayed three well-resolved peaks at 2y = 0.9–1.181 and
2y = 1.4–1.81 indexed as (100), (110), and (200) of the 2D
hexagonal mesostructure.
trans-1,2-Diaminocyclohexane has been used as one of the
typical chiral moieties. Sundararajan et al. reported the use of
1,2-diaminocyclohexane derivatives as chiral reagents in
asymmetric conjugate additions.6 Kobayashi and co-workers
had considered a variety of aluminium-based ligands derived
from (R,R)-N,N0-arylsulfonyl-1,2-diaminocyclohexane derivatives
as efficient catalysts for the cyclopropanation of cinnamyl
alcohol.7 Can Li et al. also reported trans-(1R,2R)-diamino-
cyclohexane functionalized mesoporous ethane-silicas.8
Based on our research interest in asymmetric catalysis,9 we
found that asymmetric metal free Michael addition proceeded
with high enantioselectivity by short channels and plugs in the
pore structure of our catalyst. These results stimulated our
interest in finding whether short channels and plugs in the pore
structure would be able to enhance chiral activity in the
asymmetric addition of unmodified ketones to nitro-alkanes.
Plug formation inside pores is of great interest in the view point
of stability improvement as well as the confinement effect
in catalytic application of mesoporous materials, which are
Lab. of Nano-Green Catalysis and Nano Center for Fine Chemical
Fusion Technology, Dept of Chemistry, Inha University, In-Cheon,
Korea. E-mail: separk@inha.ac.kr; Fax: +82-872-8670;
Tel: +82-860-7675
w Electronic supplementary information (ESI) available: Experimental,
1H NMR, XRD, nitrogen isotherms, TG-DTA and TEM images. See
DOI: 10.1039/c2cc18095b
Scheme 1 The preparation route of trans-1,2-diaminocyclohexane
functionalized mesoporous silica.
c
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Chem. Commun., 2012, 48, 3079–3081 3079

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the chiral moiety. These results confirm the incorporation of
chiral moiety ligands in DMS-7.5-n.
On the other hand the 29Si MAS NMR spectra show two
peaks with chemical shifts at ꢀ60, ꢀ66 and ꢀ90, ꢀ100,
ꢀ110 ppm, which could be attributed to the RSi(OSiR)n
(Tn) and (RSiO)nSi(OR)4ꢀn (Qn) species. The Q3/Q4 ratio is
reduced when synthesized by a thermal method, indicating
increased condensation of the silicate structure owing to long
synthesis time (Table 1; ESIw, Fig. S3). The TG-DTA measure-
ments of DMS-n-HT and DMS-n-MW (ESIw, Fig. S4) have
been carried out to compare the stability of the catalyst and the
amount of chiral ligand in the catalyst, which is an important
issue associated with a heterogeneous catalytic system.
The TEM images of DMS-n-HT and DMS-n-MW show
that the cylindrical mesoporous channels and the channel
directions of the 2D-hexagonal structures were parallel to
the vertical direction (Fig. S5, see ESIw). Furthermore, the
different wall thicknesses and the split and/or disconnected
pores were ‘plugged’ with amorphous silica which was in good
agreement with a two-step desorption branch of the adsorption
and desorption isotherms. This amorphous silica is produced by
the aggregation of sodium metasilicate by the trans-1,2-diamino-
cyclohexane moiety.
Fig. 1 Nitrogen adsorption–desorption isotherms of DMS-n [n =
precursor to SiO2 molar ratios in the initial synthesis mixture] synthesized
by a thermal method (a) and DMS-n [n = precursor to SiO2 molar ratios
in the initial synthesis mixture] synthesized by a microwave method (b).
For DMS-n-MW samples, two weak broad peaks at
2y = 1.4–1.81 were observed, which indicate disordered
mesostructures (see ESIw, Fig. S1).
N2 adsorption–desorption isotherms of all samples are
shown in Fig. 1. The DMS-postsynthesis sample shows type-IV
adsorption–desorption isotherms with a very large H2 type18
hysteresis loop in the 0.5 to 0.8 P/P0 range. These isotherms
suggest that the relative order in mesopore size and the
characteristic hysteresis loop account for the large-tubular
pores of general SBA-15 (Fig. S2, see ESIw).
The DMS-n-HT and DMS-n-MW samples have a type IV
isotherm and H1 hysteresis loop. It is noticed that the samples
exhibit steep capillary condensations at P/P0 = 0.76–0.80 and
a narrow hysteresis loop, which indicate a uniform pore size.
Particularly, these isotherms have two-step desorption
branches due to the pore blocking. Desorption branches run
through the P/P0 of 0.65 to 0.5 due to the partially blocked
mesopores induced by amorphous silica nanoparticles. After
that, cavitation of condensed nitrogen gas takes place inside
pores or on the walls of blocked pores.
The reaction between chalcones and R-NO2 was used as an
asymmetric 1,4-addition reaction to study the effect of the
plugged surface on stereoselectivity (Fig. 2). The catalytic
studies were started with DMS-Post and DMS-7.5-MW for
comparing the plugged surface effect.
Fig. 2b gives the results of the asymmetric 1,4-addition
reaction catalyzed by DMS-7.5-MW, along with results from
DMS-Post for comparison. The first observation concerns
enantioselectivity. As described, enantioselectivity of up to
91% with 1e was obtained using DMS-7.5-MW. This finding
is interesting, because the confinement effect caused by the
existence of blocked mesopore volume was able to increase the
enantioselectivity.
Fig. 2c presents our results with (1a–1e) in the asymmetric
1,4-addition reaction catalyzed by DMS-7.5-HT and DMS-7.5
MW in toluene. The yield and enantioselectivity in reactions of
1a–1e are shown in Table S1–S4, ESI.w
In Fig. 2c, DMS-7.5-MW samples gave higher enantio-
selectivity than DMS-7.5-HT. These results could be explained
by the confinement effect caused by the role of blocked
mesopores and small pore size. It is well indicated that the
Additionally, physicochemical properties such as the surface
area, pore diameter and volume of blocked mesopores are
shown in Table 1. The 13C CP MAS NMR spectra (ESIw, Fig. S3)
of DMS-7.5-HT and DMS-7.5-MW samples exhibit (a), (b),
(c) region signals corresponding to CH2 groups in cyclohexane
and the carbon atom of an aliphatic propyl group attached to
Si. (d), (e) region signals could be assigned to NCH/NCH2 in
Table 1 Physicochemical parameters of the mesoporous materials functionalized with trans-diaminocyclohexane
Vme,opene/ Vme,blockedf/ Pore sizeg/
nm
BET surface
Samplej Method Liganda area/m2 gꢀ1 Vtb/cm3 gꢀ1 Vmic/cm3 gꢀ1 Vmed/cm3 gꢀ1 cm3 gꢀ1 cm3 gꢀ1
Tm/(Tm + Qn)h Q3/Q4
i
DMS–5 HT
DMS–7.5 HT
DMS–10 HT
DMS–5 MW
DMS–7.5 MW
DMS–10 MW
0.43
0.59
0.68
0.21
0.50
0.56
591
579
550
676
660
650
0.80
0.75
0.70
0.63
0.73
0.60
0.074
0.075
0.08
0.10
0.13
0.11
0.73
0.68
0.62
0.53
0.60
0.49
0.52
0.45
0.37
0.20
0.25
0.23
0.21
0.23
0.25
0.33
0.35
0.26
6.6
6.4
6.3
5.4
5.6
5.6
—
10
—
—
7.7
—
—
1.33
—
—
2.24
—
a
b
The amount of ligand in materials was calculated by TG-DTA analysis (see ESI), (mmol gꢀ1). Vt = total pore volume (micropores and
c
d
e
f
mesopores). Vmi = micropore volume. Vme = mesopore volume. Vme,open = volume of open mesopores. Vme,blocked = volume of blocked
g
mesopores. The pore size was calculated by a BJH method (see ESI, Fig. S2). Proportion of organosilicon atoms incorporated into the final
h
material. Q3/Q4 ratio is reduced, indicating increased condensation of the silicate structure.17 j Activation of individual samples at 150 1C during
12 hours before nitrogen adsorption–desorption measurements.
i
c
3080 Chem. Commun., 2012, 48, 3079–3081
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example of organocatalysis promoted by trans-1,2-diamino-
cyclohexane functionalized mesoporous silica and the confinement
effect caused by blocked mesopore volume for addition reaction
of trans-chalcone as a,b-unsaturated ketones.
The authors are grateful for the support of ‘‘National Research
foundation of Korea’’ (NRF) grant funded by the Korea govern-
ment (MEST) (No. 2011-0001321) and Inha University.
Notes and references
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Fig. 2 General structure of all reactants used in this work (a) ee-value
(%) of asymmetric 1,4-addition reaction of 1a, 1e catalyzed by DMS-Post
and DMS-7.5-MW (b), volume of blocked mesopores, pore size
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formation of plugs in the pore structure could enhance the
enantioselectivity by providing a confinement effect.
And the highest yield was reached in the case of DMS-10-HT
owing to higher loading of active sites on mesoporous silica
(Table S1, ESIw). This shows that the high amounts of trans-
1,2-diaminocyclohexane as an active site have a great influence
on liquid-phase heterogeneous catalytic reactions. In addition,
the catalysts also had high durability which was demonstrated
by a recyclability test as shown in Table S1 (ESIw). The reused
catalyst also exhibited good catalytic activity with small loss in
yield and enantioselectivity.
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In conclusion, we have found that directly functionalized
mesoporous materials DMS-n-HT and DMS-n-MW can be
used as an extremely efficient and reusable catalyst. This is the
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3079–3081 3081