Enantioselective catalysis over chiral imidazolidin-4-one immobilized on siliceous and polymer-coated mesocellular foams

Article abstract of DOI:10.1002/adsc.200600240

A highly efficient and enantioselective organocatalyst, imidazolidin-4-one, has been successfully immobilized on siliceous MCF and polymer-coated MCF. The resulting heterogenized catalyst demonstrated excellent catalytic performance and recyclability for

Full text of DOI:10.1002/adsc.200600240

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DOI: 10.1002/adsc.200600240
Enantioselective Catalysis over Chiral Imidazolidin-4-one Immo-
bilized on Siliceous and Polymer-Coated Mesocellular Foams
Yugen Zhang,a Lan Zhao,a Su Seong Lee,a and Jackie Y. Yinga,*
a
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669, Singapore
Fax : (+65)-6478-9020; e-mail: jyying@ibn.a-star.edu.sg
Received: May 24, 2006; Accepted: August 18, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: A highly efficient and enantioselective or-
ganocatalyst, imidazolidin-4-one, has been success-
fully immobilized on siliceous MCF and polymer-
coated MCF. The resulting heterogenized catalyst
demonstrated excellent catalytic performance and
recyclability for Friedel–Crafts alkylation and
Diels–Alder cycloaddition. The performance of the
supported catalysts in relation to the surface envi-
ronment of siliceous MCF was examined. It was
found that partially pre-capping the MCF with
TMS groups enabled us to attain well-dispersed cat-
alysts on the siliceous support with optimal perfor-
mance. We have also developed a polymer-coated
MCF, which retained the porous structure of MCF
without the surface silanol groups. High reactivity
and excellent recyclability were achieved by the or-
ganocatalyst immobilized on polymer-coated MCF.
method. This heterogenized catalyst demonstrated ex-
cellent catalytic performance and recyclability for
Friedel–Crafts alkylation and Diels–Alder cycloaddi-
tion. MCF[4,5] is a novel mesoporous material with
unique advantages as a solid support for catalysts.
Templated by oil-in-water microemulsions, MCF has
a high surface area of 500–800 m2/g, and a 3-dimen-
sional pore structure with ultralarge, cell-like pores
(23–42 nm) that are connected by windows of 9–
22 nm. Such a pore structure would minimize any
steric effects associated with the immobilization of
bulky compounds, and facilitate the diffusion of large
substrates.
It has been known that the silanol groups on silica
surface might affect the activity of the immobilized
catalyst.[5,6] The silanol groups could interact with the
active catalytic sites, such as through hydrogen bond-
ing or proton migration in the case of organocatalysts;
they could also catalyze certain side-reactions.[7] In
this work, we examined the performance of supported
catalysts in relation to the surface environment of sili-
ceous MCF. We have also developed a polymer-
coated MCF through the direct radical polymerization
of the modified catalyst, cross-linker and vinyl mono-
Keywords: heterogeneous catalysis; mesoporous
materials; organocatalysis; polymer coating; silica
In the past few years, there has been a tremendous in- mer that had been impregnated onto the MCF sup-
crease in research activities on organic catalysis, espe- port.[8] The polymer-coated MCF retained the MCF
cially chiral organic catalysis.[1] Simple organic mole- pore structure, without the surface silanol groups.
cules could effectively catalyze a variety of fundamen- High reactivity and excellent recyclability were ach-
tally important transformations, leading to highly ieved by the organocatalyst immobilized on polymer-
enantioselective products.[1,2] Unlike organometallic coated MCF.
catalysts, organic catalysts do not involve metals,
Imidazolidin-4-one (1), one of the key chiral orga-
giving them greater applicability in the pharmaceuti- nocatalysts recently developed by MacMillanꢀs group,
cals industry.[1c] However, organocatalysts still have has been used as an efficient catalyst for a variety of
some drawbacks, such as the requirement of high cat- highly enantioselective reactions.[9,10] It also has great
alyst loading, and the difficulty in catalyst separation potential for industrial application.[1c] Therefore, het-
from the product stream. Solid-supported catalysts erogenization of this organocatalyst would be very at-
might offer the solution to these challenges.[3] Herein, tractive. Imidazolidin-4-one supported on polymer
we report the immobilization of an organocatalyst of and silica gel was recently studied for the cycloaddi-
high efficiency and enantioselectivity, imidazolidin-4- tion reactions.[11] However, the reported chemical effi-
one, on siliceous mesocellular foam (MCF) and poly- ciency of these heterogenized catalysts was much
mer-coated MCF by a novel one-step immobilization lower than the homogeneous catalyst.[11c] Herein, by
Adv. Synth. Catal. 2006, 348, 2027 – 2032
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Yugen Zhang et al.
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Scheme 1. Synthesis of catalyst precursors.
using the novel siliceous MCF support and the unique amide nitrogen. The phenolic group in 2 and 3, and
immobilization and surface modification schemes, het- the hydroxy group in 6 were functionalized by allylsi-
erogenized catalysts of comparable chemical and ste- lane[12] through an O-alkylation process to give 4, 5
reochemical efficiency as the unsupported imidazoli- and 7, respectively, in good yields. Finally, 4, 5, 7 were
din-4-one were achieved.
anchored onto the surface to generate the supported
To immobilize the organocatalyst onto MCF, the catalysts 10, 11 and 12, respectively (see Scheme 2).
aryl residue and the amide nitrogen atom of 1 were By using 3–5% p-toluenesulfonic acid (p-TsOH) as
modified for building different linkages (Scheme 1). catalyst, the original allylsilane immobilization
The characteristics of the linkages, such as flexibility method developed by Hayashiꢀs group[12] was modi-
and length, could be critical to the reactivity of the fied and extended to various types of silica sup-
heterogenized catalysts. Starting from (S)-tyrosine ports.[13] Styrene-modified catalyst monomer 8 or 9
methyl ester hydrochloride, the imidazolidinones 2 was synthesized by O-alkylation of the phenolic
and 3 were easily obtained in 80–82% yield. On the group of 2 or 3 with p-chloromethylstyrene.
other hand, (S)-phenylalanine reacted with ethanol-
In an effort to examine the relationship between
amine and then acetone to generate imidazolidinone the catalyst performance and the MCF surface envi-
6, with the ethylene hydroxy group anchored on the ronment, heterogenized catalysts A, B and C were de-
Scheme 2. MCF surface modification and catalyst immobilization processes.
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Enantioselective Catalysis over Chiral Imidazolidin-4-one
COMMUNICATIONS
Scheme 3. Synthetic Scheme for catalysts supported on polymer-coated MCF.
veloped with the use of different MCF surface modifi-
cation schemes (see Scheme 2). In all cases, spherical
MCF microparticles of 2–10 mm were used as the sup-
port;[14] they have a BET surface area of 504 m2/g, a
cell-like pore size of 22 nm, and a window size of
13 nm. A was obtained from a one-step immobiliza-
tion onto the MCF support with uncapped silanols. B
was derived by post-capping of MCF silanols with
hexamethyldisilazane (HMDS) after the catalyst im-
mobilization. C was attained by immobilizing the cat-
alyst onto an MCF support whose silanols have been
partially pre-capped with trimethylsilyl (TMS) groups
Scheme 4. Friedel–Crafts alkylation and Diels–Alder cyclo-
addition reactions.
(0.6–0.8 mmol/g) using
HMDS.
a controlled amount of
The catalysts supported on polymer-coated MCF,
13 and 14, were prepared by modifying the method of
Ryoo and co-workers.[8] As illustrated in Scheme 3,
MCF was first fully pre-capped with TMS groups by
reaction with excess HMDS at 808C. Next, styrene-
modified catalyst monomer 8 or 9, styrene, divinyl-
benzene (DVB) and a,a’-azoisobutyronitrile (AIBN)
were wet-impregnated onto MCF. The monomers on
the pore walls of MCF were directly polymerized by
heating to form a uniform polymer-supported catalyst
layer on the MCF framework. The catalyst loading,
polymer composition and swelling property of this
catalyst system could be easily controlled by changing
the ratio of different vinyl monomers.
The MCF-supported chiral amine catalysts A–C
were first studied for the asymmetric Friedel–Crafts
alkylation[15] (see Scheme 4). Prior to this study, there
were reports of polymer- and silica-supported imida-
zolidinone-based catalysts,[11] but these catalysts were
not examined for Friedel–Crafts alkylation. Actually,
there has been very little literature on heterogeneous
asymmetric Friedel–Crafts alkylation in general.[16]
Table 1 shows that the MCF-supported catalysts were
almost similar in reactivity to the unsupported imida-
zolidinone. However, their enantioselectivity and re-
cyclability strongly depended on the MCF surface
modification. Catalyst 10A gave a good yield at 258C,
but a lower enantiomeric excess (ee) (67%) than the
unsupported catalyst (82%).[9g] This was possibly due
to the interaction between the amine groups of the
Table 1. Comparison of catalysts derived from 2 in the syn-
thesis of 15.
Entry Catalyst Run Loading TFA Time Yield ee
#
(mol%)
[h]
[%][a] [%][b]
1
2
3
4
5
6
7
8
2
2
1
1
1
1
2
20
10
10
10
10
1
1
1
1
1
0
0.2
1
0.2
1
0.2
0.2
0.2
1
1
0.2
3
8
6
6
5
16
8
6
8
6
8
8
8
72
8
8
78
76
71
72
70
68
69
71
70
72
70
69
69
21
72
71
81
82
65
67
52
60
59
68
64
74
70
69
68
92
71
71
10A[c]
10A[d]
2[e] 10
2[e] 10
10B
10C
1
2
1
2
3
4
1
1
2
10
10
10
10
10
10
10
10
10
9
10
11
12
13
14
15
16
10C[f]
13
[a]
Yields based upon isolation of the corresponding alcohol
after NaBH4 reduction.
[b]
Product ratios determined by chiral gas chromatography
(GC) (Chiraldex G-TA).
[c]
[d]
[e]
[f]
The catalyst loading was 0.20 mmolgÀ1 of MCF.
The catalyst loading was 0.37 mmolgÀ1 of MCF.
Second run of a different batch.
Reaction was run at À308C.
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Yugen Zhang et al.
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Table 2. Comparison of catalysts derived from 3 and 7 in the
synthesis of 15.
catalyst with the silanol groups of the uncapped silica
surface changing the environment of the catalyst
active site. It was found that the catalyst loadings on
MCF (0.1–0.4 mmol/g) did not significantly affect the
properties of the supported catalysts. The enantiose-
lectivity of 10A was reduced considerably upon reuse.
To retain the activity of the supported catalyst, ca.
20% additional trifluoroacetic acid (TFA) should be
introduced, to counter the effect of TFA leaching
likely during catalyst washes.
Catalyst 10B was post-capped with TMS groups
after the immobilization of 4. Compared to the un-
capped catalyst 10A, catalyst 10B showed slightly
better enantioselectivity and recyclability (Table 1, en-
tries 8–9). Catalyst 10C, which was partially pre-
capped with TMS prior to the immobilization of 4,
demonstrated superior enantioselectivity and reusabil-
Entry Catalyst Run Loading TFA Time Yield ee
#
(mol%)
[h]
[%][a] [%][b]
1
2
3
4
5
6
7
3
11C
1
1
2
1
2
1
1
20
10
10
10
10
20
10
1
1
0.2
1
0.2
1
3
6
8
8
8
4
8
77
72
69
70
70
78
60
80
71
65
68
68
90
40
14
7
12C
1
[a]
Yields based upon isolation of the corresponding alcohol
after NaBH4 reduction.
Determined by chiral GC (Chiraldex G-TA).
[b]
ity compared to catalysts 10A and 10B (see Table 1, modification and catalyst linker group. The study il-
entries 10–13). This was likely because the organo- lustrated the impact of strong interactions between
catalysts 4 were more well-dispersed throughout the the silica surface and the catalyst, which could be
porous support and better separated from each other, minimized with the appropriate surface modification
when they were loaded onto partially pre-capped scheme. In catalysts 13 and 14, the MCF surface sila-
MCF support. An ee value as high as 92% was nols were fully pre-capped with TMS and coated with
achieved with catalyst 10C when the reaction was run polymer. Catalysts 13 and 14 showed similar enantio-
at À308C (Table 1, entry 14). However, a low yield selectivities as the best MCF-supported catalysts 10C
(21%) was noted even after 72 h, possibly due to the and 11C (see Table 1 and Table 2). However, catalysts
high viscosity of the THF-H2O solvent mixture at the 13 and 14 demonstrated superior recyclabilities
low temperature.
(Table 1, entries 15 and 16; Table 2, entries 4 and 5).
The Diels–Alder cycloaddition reaction (Scheme 4)
Besides manipulating the surface chemistry of the
MCF support, we also attempted to modify the link- was also studied to compare the MCF-supported cata-
age chemistry of the organocatalyst. For example, the lysts to the other heterogenized catalysts. Table 3
linker group was attached to the amide group (instead shows that the MCF-supported catalysts 10C and 12C
of the phenyl group) of 1 in the case of 7 (in contrast and the polymer-coated MCF-supported catalyst 13
to 4 and 5). Catalyst 7 was found to exhibit a higher (entries 4, 5, 7–10) provided slightly lower enantiose-
enantioselectivity (90%) (Table 2, entry 6) in the ho- lectivities and similar diastereoselectivities, compared
mogeneous Friedel–Crafts reaction than the original to the polymer- and silica gel-supported catalysts (en-
MacMillanꢀs catalyst[9g] (catalyst 2) (81%) (Table 1, tries 11–13). However, catalysts 10C, 12C and 13 gave
entry 1). However, its heterogenized counterpart, cat- the high yields achieved by the homogeneous cata-
alyst 12C, showed lower activity and significantly re- lysts (2 and 7) with just slightly longer reaction times.
duced enantioselectivity. This showed that the alkyl In contrast, the polymer- and silica gel-supported cat-
spacer group attached to the amide nitrogen has some alysts only produced moderate or poor yields even
positive impact on the catalytic properties in the ho- with higher catalyst loading or much longer reaction
mogeneous system, but the linker had adverse effect time. These findings illustrated the benefits of MCFꢀs
on the organocatalystꢀs enantioselectivity when it was three-dimensional interconnected pore structure, ul-
involved in the catalyst immobilization onto the MCF tralarge pore size, and high surface area. Organocata-
support. In contrast, although lower in enantioselec- lysts were known in many cases to be robust to air
tivity than their original homogeneous catalysts (2 and moisture, but sensitive to the presence of acids or
and 3, respectively), the heterogenized catalysts 10C bases.[13,17] Thus, providing a well-controlled catalyst
and 11C still demonstrated good enantioselectivities. environment via a direct one-step catalyst immobiliza-
This suggested that attaching the homogeneous cata- tion scheme and an effective support surface modifi-
lysts through a linker group at the phenyl group of 1 cation was critical to the successful development of
would minimize any adverse impact of heterogenizing heterogenized organocatalysts. The issue of support
the organocatalysts for the Friedel–Crafts alkylation surface modification could be by-passed with the de-
reaction.
From the above results, we could conclude that the
velopment of polymer-coated MCF.
In conclusion, MCF-supported imidazolidin-4-one
performance of the MCF-supported imidazolidinone catalysts were developed by a direct one-step immobi-
catalysts was greatly dependent on the MCF surface lization process. The MCF surface was carefully modi-
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Table 3. Comparison of catalysts derived from 2 and 7, and polymer-supported and silica gel-supported catalysts in the syn-
theses of 16a and 16b.
Entry
Catalyst
Run #
Loading (mol%)
TFA
Time [h]
Yield [%][a]
exo:endo Ratio[b]
ee [%][c]
1
2
3
4
5
6
7
8
2
10A
1
1
2
1
2
1
1
2
1
2
1
1
1
5
5
5
5
5
5
5
5
5
5
10
20
3.3
1
1
0.2
1
0.2
1
1
0.2
1
0.2
HBF4
HCl
HCl
24
30
44
30
44
24
30
44
35
44
-
99
93
96
93
96
99
95
96
94
95
68
70
33
1.3:1
1.3:1
1.3:1
1.3:1
1.3:1
1.3:1
1.3:1
1.3:1
1.3:1
1.3:1
1.3:1
1.2:1
1:1.1
93/93
73/74
63/67
83/87
76/84
91/90
83/85
74/80
83/86
83/86
88/-
10C
7
12C
9
13
10
11
12
13
PEG[d]
JandaJel[e]
Silica gel[e]
24
64
99/99
90/90
[a]
Yields based upon isolated product or GC.
Determined by H NMR.
[b]
[c]
[d]
[e]
1
Determined by chiral GC (Chiraldex b-PH).
Refs.[11a,c]
Ref.[11b]
Friedel–Crafts Alkylation (Table 1, Entry 10)
fied to modulate the microenvironment of the anch-
ored organocatalysts. The heterogenized catalysts
showed high enantioselectivity and activity in both
asymmetric Friedel–Crafts alkylation and Diels–Alder
cycloaddition reactions. It was important to partially
pre-capped the MCF surface with TMS groups to
attain separate and well-dispersed catalysts on the sili-
ceous support. Polymer-coated MCF support was also
created with immobilized imidazolidin-4-one. This
system was free of reactive silanol groups, and provid-
ed superior recyclability.
MCF-supported catalyst 10C (0.5 g, 0.07 mmol) was treated
with THF (8 mL), H2O (0.5 mL) and aqueous TFA solution
(0.5M, 140 mL, 0.07 mmol). The mixture was stirred for
10 min, and then N-methylpyrrole (310 mL, 3.5 mmol) was
added. Next, trans-cinnamaldehyde (88 mL, 0.7 mmol) was
added dropwise to the reaction vial. The suspension was stir-
red at room temperature, and the reaction was monitored
by thin layer chromatography (TLC). The suspension was
centrifuged, and the solution was decanted. This procedure
was repeated at least three times using THF as the washing
solvent. The solid catalyst was used directly for the next run.
The combined solution was prepared following literature
procedure[9g] to give a pure alcoholic product in 72% yield
1
(108 mg, 0.5 mmol). The H NMR data were in agreement
Experimental Section
with those reported.[9g] Enantioselectivity was determined by
gas liquid chromatography (GLC) analysis of the corre-
sponding aldehyde (Chiraldex G-TA column, 30 m”0.25 mm
I.D., 58CminÀ1 ramp from 708C to 1708C, 23 psi); R
isomer: tr =26.6 min, and S isomer: tr =27.4 min.[9g]
See Supporting Information for other experimental de-
tails.
Synthesis of Catalyst 10C
MCF was dried under vacuum at 1808C for 16 h. Dry tolu-
ene (30 mL) and HMDS (293 mg, 1.8 mmol) were added to
the dried MCF (5 g), and the resulting suspension was stir-
red at 608C for 16 h. The mixture was filtered, and the solid
was washed successively with methanol, acetone and di-
chloromethane. After drying under vacuum overnight, the
partially TMS-capped MCF (5.26 g) was collected and char-
acterized by IR and elemental analysis (C, H, N). The load-
ing of TMS groups was 0.67 mmol/g based on elemental
analysis and weight gain analysis. The modified MCF
(5.26 g) was dried under vacuum at 858C overnight.
Acknowledgements
This work was supported by the Institute ofBioengineering
and Nanotechnology (Biomedical Research Council, Agency
for Science, Technology and Research, Singapore).
Toluene (30 mL), catalyst precursor 4 (972 mg, 2.5 mmol)
and p-TsOH (8.5 mg, 0.05 mmol) were then added to the
modified MCF sequentially. The suspension was stirred at
1008C for 24 h. The mixture was then filtered, and the solid
was washed thoroughly with methanol, acetone and di- References
chloromethane. After drying under vacuum, catalyst 10C
(5.50 g) was collected and characterized by IR and elemen-
tal analysis. The loading of 4 was 0.14 mmol/g based on ele-
mental analysis and weight gain analysis.
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Yugen Zhang et al.
COMMUNICATIONS
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achieved only with silica samples with acidic surfaces,
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