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
This journal is The Royal Society of Chemistry 2012