Stepwise fabrication and architecture of heterogeneous 9-thiourea epiquinine catalyst with excellent enantioselectivity in the asymmetric Friedel-Crafts reaction of indoles with imines

Article abstract of DOI:10.1016/j.jcat.2008.09.006

Heterogeneous 9-thiourea epiquinine catalysts are prepared by covalently anchoring 9-(3,5-bis(trifluoromethyl)phenylthiourea)epiquinine on mesoporous silica surfaces via mercapto linker. It is found that 9-thiourea epiquinine moieties are located comparab

Full text of DOI:10.1016/j.jcat.2008.09.006

Journal of Catalysis 260 (2008) 81–85
Contents lists available at ScienceDirect
Journal of Catalysis
www.elsevier.com/locate/jcat
Stepwise fabrication and architecture of heterogeneous 9-thiourea epiquinine
catalyst with excellent enantioselectivity in the asymmetric Friedel–Crafts
reaction of indoles with imines
∗
Peng Yu, Jing He , Lan Yang, Min Pu, Xiaodan Guo
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Heterogeneous 9-thiourea epiquinine catalysts are prepared by covalently anchoring 9-(3,5-bis(trifluoro-
methyl)phenylthiourea)epiquinine on mesoporous silica surfaces via mercapto linker. It is found that
9-thiourea epiquinine moieties are located comparably on the interior and exterior surfaces of SBA-15
while preferentially on the exterior surface of MCM-41. The heterogeneous 9-thiourea epiquinine are
applied as catalysts in the asymmetric Friedel–Crafts reaction of indoles with imines. SBA-15 supported
9-thiourea epiquinine, with more catalytic sites inside the channels, is found more chemselective and
enantioselective than the counterpart supported on MCM-41. The effects of solvents and molecule
dimensions of both substrates on the reaction effectiveness and selectivity are also discussed.
© 2008 Elsevier Inc. All rights reserved.
Received 12 July 2008
Revised 27 August 2008
Accepted 5 September 2008
Available online 8 October 2008
Keywords:
heterogeneous catalysis
Asymmetric C–C formation
9-Thiourea epiquinine
Enantioselectivity
Friedel–Crafts reaction
Mesoporous silica
1. Introduction
tion of cinnamylacetate, for example, was enhanced from 64 to
91% by rhodium(I) complex supported in the nano-sized channels
Cinchona alkaloids have become a matter of current interest
[1,2] because of their applications as homogeneous organocata-
lysts in a variety of asymmetric nucleophilic processes, including
conjugate addition [3–8], Michael addition [9,10], Mannich [11,
12], Friedel–Crafts [13,14], aza-Henry [15], aza-Michael [16], and
Diels–Alder [17] reactions. The separation and subsequent recycle
of homogeneous catalysts are problematic, however, making the
entire process economically nonviable for industrial applications.
The immobilization of homogeneous catalysts to solid supports has
thus become an important strategy to obtain heterogeneous cat-
alysts retaining the active sites of homogeneous analogues while
easily separated and recycled [18]. Some papers [19,20] have re-
ported the heterogenization of chiral organic molecules using or-
ganic polymers and inorganic solids as supports. Mesoporous silica
materials with well-defined uniform mesopores and readily mod-
ified surfaces have been attracting much attention as supports for
chiral catalytic sites [21–26]. Enhanced enantioselectivities in the
epoxidation reaction were observed on MCM-41 supported Mn(III)
salen [21] and Cr(III) Schiff base complexes [22]. The PdII and RhI
complexes immobilized in mesoporous silica were also found to
exhibit superior performance in the enantioselective allylic ami-
nation of cinnamyl acetate [23,24]. The ee of the allylic amina-
of MCM-41. Mesoporous materials have apparently been demon-
strated promising for designing and exploiting industrially robust
heterogeneous catalysts.
9-(3,5-bis(trifluoromethyl)phenylthiourea)epiquinine (abbrevi-
ated 9-thiourea epiquinine), a cinchona alkaloid-derived chiral bi-
functional thiourea organocatalyst, is energetic in a large number
of asymmetric C–C formation reactions. But its heterogenization
has seldom been reported. This work, therefore, reports the immo-
bilization of 9-(3,5-bis(trifluoromethyl)phenylthiourea)epiquinine
on mesoporous SBA-15 and MCM-41 materials through a covalent
bottom-up approach. The stepwise fabrication and the architecture
of the heterogeneous 9-thiourea epiquinine catalyst is especially
concerned. It has been demonstrated in our previous report [27]
that SBA-15 supported 9-thiourea epiquinine exhibits enhanced
enantioselectivity (up to 99%) in the asymmetric Friedel–Crafts
reaction of imines with indoles, and could be recycled and re-
generated readily as well.
2. Experimental
MCM-41-SH and SBA-15-SH were prepared by the post-synthe-
sis grafting of parent mesoporous silica with 3-mercaptopropyltri-
methoxysilane (MPTMS, 95%, Alfa Aesar) in anhydrous toluene.
9-(3,5-bis(trifluoromethyl)phenyl thiourea)epiquinine, synthesized
according to the previous report [3], AIBN as radical initiator, and
Corresponding author. Fax: +86 10 64425385.
*
E-mail address: jinghe@263.net.cn (J. He).
0021-9517/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2008.09.006

82
P. Yu et al. / Journal of Catalysis 260 (2008) 81–85
MCM-41-SH or SBA-15-SH were refluxed in chloroform under inert
atmosphere to give MCM-41-SQT or SBA-15-SQT.
carbons) and 186 ppm (C=S), all characteristic of the existence
of 9-(3,5-bis(trifluoromethyl)phenylthiourea)epiquinine moiety, are
clearly observed, exactly as expected. The resonances at 114.0
and 141.5 ppm corresponding to C atoms in –CH=CH2 groups
are absent, indicative of the reaction between thiol groups on
MCM-41-SH and SBA-15-SH surface and vinylic functional group in
9-(3,5-bis(trifluoromethyl)phenylthiourea)epiquinine. Correspond-
The solid state NMR spectra were recorded on
a Bruker
Avance 300M solid-state spectrometer at resonance frequencies
of 75.5 MHz for 13C and 59.6 MHz for 29Si. 1H and 13C NMR
in solution were recorded on a Bruker Avance 600 spectrome-
ter. N2 sorption isotherms were measured on a Quantachrome
Autosorb-1 system. Elemental analyses (EA) were performed on
ingly, in the FT-IR spectra (Supplemental material), the absorption
−1
a Bruker CHNS elemental analyzer. FT-IR spectra were taken on
band at 2576 cm
arising from S–H vibration emerges upon the
−1
a Bruker Vector 22 spectrometer at a resolution of 4 cm
using
grafting of 3-mercaptopropylsilyl group and vanishes due to the
the standard KBr method. VCD spectrum was recorded on a Bruker
covalent linkage of 9-thiourea epiquinine moiety. The FT-IR bands
−1
Vector 22 spectrometer equipped with VCD/IRRAS model PMA37
at 1512, 1473, 1401, and 1383 cm
originating from the aryl rings
−1
at resolution of 4 cm
.
in the anchored 9-thiourea epiquinine also appear. Obvious VCD
signals that are not present for the parent SBA-15 and SBA-15-SH
are observed for SBA-15-SQT (Supplemental material).
In a typical asymmetric Friedel–Crafts reaction, indole (99%,
Aldrich, 0.2 mmol) was introduced in one portion to a suspension
of N-benzylidenebenzenesulfonamide (99%, Aldrich, 0.1 mmol) and
catalyst (1 mol% accounted by 9-thiourea epiquinine, vacuumed
prior to use) in 0.3 mL of solvent, then to oscillate for 5 days.
The filtrate was subjected to flash chromatography (silica gel: ethyl
acetate/hexane = 1/3) to afford the desired product and uncon-
verted imines. The same procedure was repeated except the usage
of catalyst, to afford the racemic product in 17% yield. The enan-
tiomer excess (ee) was determined on a Shimadzu LC-10Atvp HPLC
with a Daicel Chiralcel OB-H column (wavelength = 254 nm) using
i-PrOH/hexane (20/80, v/v) as mobile phase.
The loading of 9-thiourea epiquinine in SBA-15-SQT is typically
0.16 mmol/g calculated according to the EA results, slightly less
than 0.20 mmol/g, the value estimated according to the SH con-
tent in SBA-15-SH. This means that not all SH groups in SBA-15-SH
effectively took part in the linkage with 9-(3,5-bis(trifluoromethyl)
phenylthiourea)epiquinine. The loading of 9-thiourea epiquinine in
MCM-41-SQT is calculated as 0.20 mmol/g, which is higher than
that in SBA-15-SQT. The observed difference is rationally attributed
to the fact that the content of thiol group in MCM-41-SH is
0.91 mmol/g, higher than 0.74 mmol/g in SBA-15-SH. The αs-plot
analysis has been performed in this work to assess the location
of organic moieties introduced onto the MCM-41 and SBA-15 sur-
faces. A nonporous α-quartz with a BET surface area of 1.21 m2/g
is used as reference material. The analyzed interior and exterior
surface areas are presented in Table 1. Compared with parent SBA-
15, the exterior surface area and the mesoporous surface area of
SBA-15-SH decrease by 23 and 31%. The exterior surface area and
the mesopores surface area of SBA-15-SQT decrease by 16 and 24%
in comparison with that of SBA-15-SH. The decrease percentage
of exterior surface area and mesoporous surface area, caused by
the covalent linkage of 9-thiourea epiquinine to SBA-15-SH, is both
7%. It could thus be deduced that 9-thiourea epiquinine has been
incorporated comparably on the interior and exterior surfaces of
SBA-15-SH. In the case of MCM-41, the grafting of mecaptopropyl
groups decreases the exterior surface area and mesopores surface
area in 9.3 and 17.6%. The following incorporation of 9-thiourea
epiquinine reduces the exterior surface area and the mesopores
surface area in 9.3 and 2.4%. The interior surface area is reduced in
less percentage than exterior surface area in the incorporation of
9-thiourea epiquinine, which is closely associated with the smaller
pore size of MCM-41 materials. The heterogenization of 9-thiourea
epiquinine might give rise to partial blocking of MCM-41 channels.
Or more severe diffusion resistance in the smaller pores makes 9-
thiourea epiquinine anchored on the exterior surface more readily.
The quantity of Si–OH on the exterior surface of MCM-41 is cal-
culated as 0.62 mmol/g based on the 29Si MAS NMR spectrum.
The maximum quantity of –SH groups on the exterior surface is
thus 0.31 mmol/g, supposed that the external Si–OH is entirely
grafted. The estimated content of 9-thiourea epiquinine on the
exterior surface is larger than actually observed in MCM-41-SQT
(0.20 mmol/g).
3. Results and discussion
3.1. Immobilization of 9-thiourea epiquinine on mesoporous silica
The immobilization of 9-(3,5-bis(trifluoromethyl)phenylthio-
urea)epiquinine on mesoporous MCM-41 or SBA-15 material is
achieved using a bottom-up approach with mercapto group as
a linker. The grafting of mecaptopropyl groups and the follow-
ing covalent linkage of 9-(3,5-bis(trifluoromethyl)phenylthiourea)
epiquinine are investigated by solid state 29Si and 13C NMR spec-
troscopy. In the 29Si CP/MAS NMR spectra shown in Fig. 1 (top),
SBA-15-SH presents a marked decrease in the relative intensity
of Q3 linkage at δ = −101 ppm to Q4 at δ = −111 ppm, as a
result of the grafting reaction between the surface silanols and
silane moieties. Simultaneously, the resonances at −48, −58, and
−65 ppm associated with T1, T2 and T3 linkages (Tm = (SiO)m–Si–
(OCH3)3−mR, m = 1–3) are observed, further verifying the anchor
of mecaptopropyl groups to silica walls. In the case of MCM-41,
Tm linkages and similar change in the relative intensity of Q3 to
Q4 are observed, except that no T3 linkage is present for MCM-
41-SH. Due to the following anchoring of 9-thiourea epiquinine,
T3 emerges and T2 intensifies for MCM-41-SQT. For SBA-15-SQT,
T1 resonance gets weakened while both T2 and T3 are enhanced.
As a result, the intensity of Q3 displays a further decrease rela-
tive to Q4 on both SBA-15-SQT and MCM-41-SQT. The secondary
grafting of methoxysilane moiety (T1 to T2 or T3, and T2 to T3)
visibly occurs in the incorporation of 9-thiourea epiquinine. The
relative area ratio of Q4 to Q3, quantitatively estimated from the
solid state 29Si MAS NMR spectra, is 1.6, 1.8 and 2.6 for SBA-
15, SBA-15-SH and SBA-15-SQT, and 1.5, 1.9 and 5.3 for MCM-41,
MCM-41-SH and MCM-41-SQT, respectively. In the 13C CP/MAS
NMR spectra (Fig. 1), SBA-15-SH and MCM-41-SH clearly display
three resonances at 10, 27, and 48 ppm, which are absent for
parent MCM-41 and SBA-15. The resonances at 10 and 27 ppm
are assigned to the –CH2CH2–CH2SH and –CH2–SH of the teth-
ered mecaptopropyl groups. The resonance at 48 ppm originates
from the residual methoxy groups due to the incomplete con-
densation of MPTMS [28]. For SBA-15-SQT and MCM-41-SQT, new
resonances at 31, 33 (Si–(CH2)3–S–CH2–CH2–), 55 (O–CH3), 26, 39,
41, 53, 59 (quinidine), 101, 121, 125, 128, 131, 143, 158 (aromatic
The grafting of mecaptopropyl groups and the following incor-
poration of 9-thiourea epiquinine make no adverse impacts on the
long-range ordered structure. All of the materials exhibit well-
defined type IV isotherms with sharp steeps and H1-type hys-
teresis loops (Supplemental material), which is characteristic of
well-ordered mesoporous channels. The surface area, pore volume,
and pore size at maximum distribution is gradually reduced with
stepwise introduction of organic groups, as shown in Table 1. The
increasing wall thickness coincides with the stepwise linkage of
organic moieties on the surfaces. The mercapto-functionalized in-

P. Yu et al. / Journal of Catalysis 260 (2008) 81–85
83
Fig. 1. 29Si (top) and 13 C (middle) CP/MAS NMR spectra, pore size distribution (bottom) calculated using Barrett–Joyner–Halenda (BJH) method based on the desorption
branch.
termediates and the resulting heterogeneous catalysts all possess
the pore size distributions as narrow as the parent mesoporous
materials, as can be seen in Fig. 1 (bottom).
Crafts reaction of indoles with imines. The reaction of indole
(1a) with N-benzylidenebenzenesulfonamide (2a) in various sol-
vents was first performed using SBA-15-SQT as catalyst (Ta-
ble 2). The reactions at ambient temperature in EtOAc (entry 6)
and toluene (entry 2) afford the corresponding N-(indol-3-yl-
phenylmethyl)benzenesulfonamide (3aa) in nearly 100 and 96% ee.
The reaction in THF (entry 5) turns out racemic product. The other
solvents lead to moderate enantioselectivity and the products with
3.2. Catalytic property of heterogeneous 9-thiourea epiquinine catalysts
The catalytic activities and enantioselectivities of MCM-41-
SQT and SBA-15-SQT are evaluated by the asymmetric Friedel–

84
P. Yu et al. / Journal of Catalysis 260 (2008) 81–85
Table 1
Both indoles and imines have been screened as the substrates
for the asymmetric Friedel–Crafts reaction. As shown in Table 2,
6-OCH3 (entry 9) and 5-CH3 (entry 10) substituted indoles pro-
ceed with the addition reaction successfully. The observed chem-
ical selectivity and ee slightly decrease with increased dimension
of indole molecules. It could rationally be explained by the fact
that smaller molecules diffuse into the support channels more
easily. The reaction taking place inside the channels turns out
better chemical selectivity and enantioselectivity. Altering the N-
protecting group in imine from –Bs (entry 7) to –Ts (entry 11)
reduces the yield, chemical selectivity, and ee. When the aryl
group (entry 11) is replaced by an alkyl group (entry 12), en-
hanced chemical selectivity, yield and ee are observed. The effects
of imine structures on the Friedel–Crafts reaction could also be
well interpreted by the reasons proposed in the discussion of in-
dole effects. The reaction inside the nano-sized channels involving
smaller imine molecules, –Bs (relative to –Ts) or alkyl (relative to
aryl) substituted for example, favors both chemical selectivity and
enantioselectivity. It has been found in our previous research [27]
that electron-attracting –Cl or –NO2 substituted benzylidene moi-
ety inhibits the occurrence of addition reaction. No products could
be detected in the reaction mixture.
Both MCM-41-SQT and SBA-15-SQT catalysts efficiently promote
the enantioselective reaction of indole with N-benzylidenebenzene-
sulfonamide, as shown in Table 3 (entries 2 and 3), producing
higher chiral induction (96 and 99% ee) than their homogeneous
counterpart (entry 1, 93% ee). The simple mixture of SBA-15-SH
and 9-(3,5-bis(trifluoromethyl)phenylthiourea)epiquinine as cata-
lyst turns out similar reactive effectiveness and chiral induction
(entry 4) to the homogeneous system (entry 1), indicative of
the close relationship between the enhancement of enantiose-
lectivity and the covalent attachment of 9-thiourea epiquinine to
the support surfaces. Similar behavior has been reported earlier
on supported chiral Mn(III) and Cr(III) complexes [21,22,30]. The
MCM-41-SQT catalyst is more active while less chemically selec-
tive and asymmetrically inductive than SBA-15-SQT, proposed to
arise from the difference in the location of catalytic sites. The cat-
alytic sites located mainly on the exterior surface of MCM-41 are
The mesoporous surface areas and exterior surface areas analyzed by αs-plot
method, and the textural properties determined from N2 adsorption experiments.
Sample
Sex
Smeso
SBET
Pore
Pore
Wall
thickness
(nm)
(m2/g)a
(m2/g)b
(m2/g)c
volume
diameter
(cm2/g)d
(nm)e
SBA-15
59
45
38
107
97
88
736
504
383
902
743
725
778
544
395
1014
822
622
1.3
0.9
0.7
1.9
0.9
0.8
6.6
6.5
5.7
2.5
2.2
1.7
4.7
6.1
6.7
2.2
2.2
2.8
SBA-15-SH
SBA-15-SQT
MCM-41
MCM-41-SH
MCM-41-SQT
a
Exterior surface area by αs-plot analysis.
Mesoporous surface area by αs-plot analysis.
Specific surface area calculated using Brunauer–Emmett–Teller method based on
b
c
the adsorption branch of N2 sorption isotherms.
d
Single point pore volume estimated from the adsorbed amount at p/p0 = 0.99.
e
The maximum in pore size distribution calculated using Barrett–Joyner–Halenda
(BJH) method based on the desorption branch of N2 sorption isotherms.
reversed configuration. MeOH, toluene, MeCN, and EtOAc as sol-
vents give a chemical selectivity above 60% in each case. But the
reaction in MeOH (entry 1), toluene (entry 2), or MeCN (entry 3)
produces the desired product (3aa) in a yield of below 50% be-
cause of the unsatisfactory reactivity (low conversion). THF or
CH2Cl2 as solvent turns out deficient activity and as well chem-
ical selectivity. Although EtOAc was generally considered a poor
solvent in hydrogen-bonding controlled organocatalysis, good yield
and enantioselectivity are produced using EtOAc as solvent in our
case. Wittkopp and Schreiner [29] have previously reported sim-
ilar observations in homogeneous thiourea-catalyzed Diels–Alder
reactions. They found that the strong catalytic effectiveness was
also present in highly coordinating polar solvents, such as water.
Moderate increase in the reaction temperature in EtOAc retains the
enantioselectivity while enhances the yield (entry 7). Further ele-
◦
vating the reaction temperature to 50 C (entry 8), however, causes
a marked decrease in the yield, arising from the decrease in the
chemical selectivity. The following experiments are thus performed
◦
at 40 C using EtOAc as solvent.
Table 2
Asymmetric Friedel–Crafts reactions of indoles with imines on SBA-15 supported 9-thiourea epiquinine:
Entry
Solvent
T
( C)
1
2
3
Conv.
(%)a
Sel.
Yield
(%)c
ee
◦
(%)b
(%)c
1
2
3
4
5
6
7
8
9
MeOH
Toluene
MeCN
CH2Cl2
THF
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
30
30
30
30
30
30
40
50
40
40
40
40
1a: R1 = H
1a: R1 = H
1a: R1 = H
1a: R1 = H
1a: R1 = H
1a: R1 = H
1a: R1 = H
1a: R1 = H
1b: R1 = 6-OMe
1c: R1 = 5-Me
1a: R1 = H
1a: R1 = H
2a: R2 = H, P = Bs
2a: R2 = H, P = Bs
2a: R2 = H, P = Bs
2a: R2 = H, P = Bs
2a: R2 = H, P = Bs
2a: R2 = H, P = Bs
2a: R2 = H, P = Bs
2a: R2 = H, P = Bs
2a: P = Bs, R2 = H
2a: P = Bs, R2 = H
2b: P = Ts, R2 = H
2c:
3aa
3aa
3aa
3aa
3aa
3aa
3aa
3aa
3ba
3ca
3ab
3ae
54
30
46
28
42
77
83
71
76
77
80
86
81
79
65
25
24
92
93
42
91
86
85
93
44
24
30
7
10
71
77
30
69
66
68
80
56
96
84
87
Null
99
99
98
99
93
90
96
10
11
12
a
Conversion calculated as the molar percentage of the reacted in the total imines.
Chemselectivity determined as the molar percentage of the imines converted into the desired product in all the reacted imines.
Isolated desired products.
b
c

P. Yu et al. / Journal of Catalysis 260 (2008) 81–85
85
Table 3
Asymmetric Friedel–Crafts reactions of indole with imine:
Entry
Catalyst
Conv. (%)b
Sel. (%)c
Yield (% )d
ee (%)d
1
2
3
4
Homogeneous counterparta
SBA-15-SQT
MCM-41-SQT
92
83
87
84
93
90
80
77
77
78
74
93
99
96
92
Physical mixture of SBA-15-SH and homogeneous counterpart
92
a
9-(3,5-bis(trifluoromethyl)phenylthiourea)epiquinine.
Conversion calculated as the molar percentage of the reacted in the total imines.
Chemselectivity determined as the molar percentage of the imines converted into the desired product in all the reacted imines.
Isolated desired products.
b
c
d
more accessible. But the catalytic sites in the mesopores, in the
case of SBA-15-SQT, favor the chemical selectivity and enantiose-
lectivity.
[3] B. Vakulya, S. Varga, A. Csámpai, T. Soós, Org. Lett. 7 (2005) 1967.
[4] J. Wang, H. Li, L. Zu, W. Jiang, H. Xie, W. Duan, W. Wang, J. Am. Chem. Soc. 128
(2006) 12652.
[5] G. Bartoli, M. Bosco, A. Carlone, A. Cavalli, M. Locatelli, A. Mazzanti, P. Ricci, L.
Sambri, P. Melchiorre, Angew. Chem. Int. Ed. 45 (2006) 4966.
[6] B. Wang, F. Wu, F. Wang, X. Liu, L. Deng, J. Am. Chem. Soc. 129 (2007) 768.
[7] M.M. Biddle, M. Lin, K.A. Scheidt, J. Am. Chem. Soc. 129 (2007) 3830.
[8] P. Elsner, L. Bernardi, G.D. Salla, J. Overgaard, K.A. Jørgensen, J. Am. Chem.
Soc. 130 (2008) 4897.
[9] J.-X. Ye, D.J. Dixon, P.S. Hynes, Chem. Commun. (2005) 4481.
[10] S.H. McCooey, S.J. Connon, Angew. Chem. Int. Ed. 44 (2005) 6367.
[11] A.L. Tillman, J. Ye, D.J. Dixon, Chem. Commun. (2006) 1191.
[12] T. Liu, H. Cui, J. Long, B.-J. Li, Y. Wu, L. Ding, Y.-C. Chen, J. Am. Chem. Soc. 129
(2007) 1878.
[13] B. Török, M. Abid, G. London, J. Esquibel, M. Török, S.C. Mhadgut, P. Yan, G.K.S.
Prakash, Angew. Chem. Int. Ed. 44 (2005) 3086.
[14] Y.Q. Wang, J. Song, R. Hong, H. Li, L. Deng, J. Am. Chem. Soc. 128 (2006) 8156.
[15] L. Bernardi, F. Fini, R.P. Herrera, A. Riccia, V. Sgarzani, Tetrahedron 62 (2006)
375.
4. Conclusions
In summary, the 9-thiourea epiquinine supported on MCM-41
and SBA-15 exhibits the reactivity, chemical selectivity and chi-
ral induction as excellent as the homogeneous counterpart in the
asymmetric Friedel–Crafts reaction of indoles with imines. With
the chiral sites anchored comparably in the nano-sized channels
and exterior surface, SBA-15-SQT exhibits higher chemical selec-
tivity and asymmetrical induction but lower conversion slightly
than MCM-41-SQT, in which the catalytic sites are located prefer-
entially on the exterior surface. The substrate screening finds that
the molecule dimensions of both indoles and imines make impact
on the chemical selectivity and enantioselectivity.
[16] D. Pettersen, F. Piana, L. Bernardi, F. Fini, M. Fochi, V. Sgarzani, A. Ricci, Tetra-
hedron Lett. 48 (2007) 7805.
[17] Y. Wang, H.-M. Li, Y.-Q. Wang, Y. Liu, B.M. Foxman, L. Deng, J. Am. Chem.
Soc. 129 (2007) 6364.
[18] F. Cozzi, Adv. Synth. Catal. 348 (2006) 1367.
Acknowledgments
[19] C.E. Song, S. Lee, Chem. Rev. 102 (2002) 3495.
The authors are grateful to the financial support from NSFC,
Program for Changjiang Scholars and Innovative Research Team in
University (IRT0406), and “111” Project (B07004).
[20] M. Benaglia, A. Puglisi, F. Cozzi, Chem. Rev. 103 (2003) 3401.
[21] S. Xiang, Y. Zhang, Q. Xin, C. Li, Chem. Commun. (2002) 2696.
[22] X.-G. Zhou, X.-Q. Yu, J.-S. Haung, S.-G. Li, L.-S. Li, C.-M. Che, Chem. Commun.
(1999) 1789.
[23] B.F.G. Johnson, S.A. Raynor, D.S. Shephard, T. Mashmeyer, J.M. Thomas, G.
Sankar, S. Bromley, R. Oldroyd, L. Gladdenc, M.D. Mantlec, Chem. Commun.
(1999) 1167.
[24] M.D. Jones, R. Raja, J.M. Thomas, B.F.G. Johnson, D.W. Lewis, J. Rouzaud, K.D.M.
Harris, Angew. Chem. Int. Ed. 42 (2003) 4326.
Supplementary material
Supplementary material of this article may be found on Sci-
enceDirect in the online version.
[25] H.D. Zhang, Y.M. Zhang, C. Li, J. Catal. 238 (2006) 369.
[26] R.I. Kureshy, I. Ahmad, N.H. Khan, S.H.R. Abdi, K. Pathak, R.V. Jasra, J. Catal. 238
(2006) 134.
[27] P. Yu, J. He, C.-X. Guo, Chem. Commun. (2008) 2355.
[28] V. Dufaud, M.E. Davis, J. Am. Chem. Soc. 125 (2003) 9403.
[29] A. Wittkopp, P.R. Schreiner, Chem. Eur. J. 9 (2003) 407.
[30] R.I. Kureshy, I. Ahmad, N.H. Khan, S.H.R. Abdi, S. Singh, P.H. Pandia, R.V. Jasra,
J. Catal. 235 (2005) 28.
Please visit doi: 10.1016/j.jcat.2008.09.006.
References
[1] S.J. Connon, Chem. Eur. J. 12 (2006) 5418.
[2] J. López-Cantarero, M.B. Cid, T.B. Poulsen, M. Bella, J.L.G. Ruano, K.A. Jørgensen,
J. Org. Chem. 72 (2007) 7062.