Improved enantioselectivity of immobilized chiral bisoxazolines by partial precapping of the siliceous mesocellular foam support with trimethylsilyl groups

Article abstract of DOI:10.1002/adsc.200606027

Siliceous mesocellular foams (MCF) were partially surface-modified with trimethylsilyl (TMS) groups prior to the immobilization of chiral tert-butylbisoxazolines. The resulting MCF-supported bisoxazoline-Cu(I) catalyst showed superior enantioselectivity (

Full text of DOI:10.1002/adsc.200606027

FULL PAPERS
DOI: 10.1002/adsc.200606027
Improved Enantioselectivity of Immobilized Chiral Bisoxazolines
by Partial Precapping of the Siliceous Mesocellular Foam Support
withTrimet hy lsilyl Groups
a
a
a,
Su Seong Lee, Sukandar Hadinoto, and Jackie Y. Ying *
a
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669
Fax : (+65)-6478-9020; e-mail: jyying@ibn.a-star.edu.sg
Received: January 27, 2006; Accepted: May 7, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Siliceous mesocellular foams (MCF) were tion, compared to that supported on MCF without
partially surface-modified with trimethylsilyl (TMS) TMS precapping. This heterogenized catalyst also ex-
groups prior to the immobilization of chiral tert-bu- hibited excellent recyclability.
tylbisoxazolines. The resulting MCF-supported bisox-
azoline-Cu(I) catalyst showed superior enantioselec- Keywords: chiral bisoxazoline; copper; cyclopropa-
tivity (up to 95% ee) in asymmetric cyclopropana- nation; immobilization; mesoporous silica
Introduction
provement is required to achieve heterogenized cata-
lysts with comparable performances as their homoge-
Chiral bisoxazolines (BOX) have been used in a wide neous counterparts.
variety of asymmetric reactions as one of the most ef- In modifying the silica surface with 3-aminopropyl-
[1]
ficient classes of chiral ligands. Conventionally, a trialkoxysilane, a new approach has been developed
low substrate/catalyst ratio is used to obtain high to prevent the strong interaction between the amine
enantioselectivity and reactivity for BOX ligands. groups and the silanol groups of the silica surface
This, combined with the high cost of the ligands, has prior to postcapping of the remaining silanol
[
13]
limited the practical application of chiral BOX cata- groups.
However, a different solution is required to
lysts. These drawbacks could be tackled by immobiliz- immobilize nitrogen-containing ligands, such as BOX.
ing BOX ligands on supports,[ which would facilitate
the catalyst recovery and reuse, and allow for continu- novel and simple approach to reduce the strong inter-
ous packed-bed reactor operations. actions between the ligands and the silica surface in
2]
Herein, we report the successful development of a
Most of the successfully heterogenized BOX li- the covalent immobilization of chiral BOX.
gands were immobilized on insoluble or soluble poly-
[
3–8]
mer supports.
Although the silica support is more
mechanically robust and thermally stable, only very Results and Discussion
few successful silica-supported BOX catalysts have
[
9–12]
been reported;
most silica-supported bisoxazoline Polymer-supported BOX ligands with one linker
catalysts showed poor enantioselectivities in asym- group have demonstrated high reactivity and enantio-
metric cyclopropanation reactions.
selectivity, despite breaking the C symmetry of the
2
[
5–7]
As a nitrogen-based ligand, BOX shows a strong in- BOX ligands.
Bulky substituents on the bridge
teraction with the silica surface. This is evident by the carbon of BOX decreased the regioselectivity in
[
3,12]
low R values of BOX on TLC plates when dichloro- asymmetric cyclopropanation reactions.
There-
f
methane is used as the eluent. Such a strong interac- fore, in our attempt, chiral BOX ligands were also im-
tion can affect the formation of metal complexes after mobilized onto mesoporous solids by one linker
covalent immobilization of BOX ligands, resulting in group (Scheme 1). A two-step process gave 68%
low enantioselectivity and regioselectivity. Although overall yield for tert-butylbisoxazoline with a methyl
postcapping of the remaining silanol groups on the substituent at the bridge carbon of 1.
silica surface after immobilization of BOX ligands
This BOX ligand was easily modified to obtain a
can increase the enantioselectivities,[10–12] further im- trimethoxysilane linker group, which could react with
1248
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 1248 – 1254

Improved Enantioselectivity of Immobilized Chiral Bisoxazolines
FULL PAPERS
[
14]
was easily anchored onto siliceous MCF
high loading (0.268 mmolg ) (Scheme 2).
with a
À1
The surface of MCF was partially modified with
TMS groups prior to the immobilization of BOX li-
gands in preparing 8. Hexamethyldisilazane (HMDS)
0.4 mmol) was added to the MCF (1.0 g) well-dis-
persed in toluene to produce TMS-modified MCF 5
(
(
À1
0.794 mmolg by elemental analysis). This modifica-
tion not only allowed for the uniform BOX immobili-
zation onto the partially TMS-precapped MCF by
preventing the strong interaction between the ligands
and the support surface, but also provided for ease of
control in the amount of ligands loaded.
To examine the effect of partial TMS-precapping of
MCF, the modified BOX 3 was also directly immobi-
lized onto unmodified MCF 4 to yield 6 (Scheme 2).
In preparing the final MCF-supported BOX catalysts
Scheme 1. a) (S)-tert-leucinol, 908C, 85%; b) p-toluenesul-
fonic chloride, triethylamine, DMAP, CH Cl , room temper-
2
2
ature, 80%.
a silanol group on the silica surface (Scheme 2). Bis- 7 and 9, the remaining silanol groups in 6 and 8, re-
oxazoline 1 was reacted with methyllithium (MeLi) or spectively, were capped with TMS groups by vapor
LiN(SiMe ) to give a lithiated product, which was re- phase grafting of HMDS. Excess HMDS was added
A
C
H
T
R
E
U
N
G
3 2
acted with 3-iodopropyltrimethoxysilane to give tri- to a closed reactor system under high vacuum, and
methoxysilane-modified BOX. The modified BOX 3 then vaporized to react with the free silanol groups of
Scheme 2. a) i) MeLi, THF, À508C, 1 h; ii) 3-iodopropane, THF, À508C, then warmed to room temperature overnight. b) i)
MeLi, THF, À508C, 1 h; ii) 3-iodopropyltrimethoxysilane, THF, À508C, then warmed to room temperature over 2 days. c)
HMDS, toluene, room temperature to 608C. d) toluene, 1208C, 24 h. e) HMDS, vapor phase reaction, 758C.
Adv. Synth. Catal. 2006, 348, 1248 – 1254
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.asc.wiley-vch.de
1249

Su Seong Lee et al.
FULL PAPERS
MCF at ca. 758C. The extra, unreacted HMDS was
A homogeneous ligand 2 was synthesized with a
readily removed under vacuum. This method allowed similar structure to 3. Both 2 and 3 did not have C2
for the easy and efficient TMS-postcapping of the re- symmetry any more due to their different substituents
sidual silanol groups without using any organic at the bridge carbon. The homogeneous catalyst
medium. Photoacoustic Fourier-transform infrared 2:CuOTf showed a lower enantioselectivity (86% ee
(
PA-FT-IR) spectroscopy and elemental analysis con- for the trans isomer and 84% ee for the cis isomer)
firmed that more TMS capping was achieved on the and a slightly higher trans/cis ratio (68/32) than the
support surface by the vapor phase reaction than by heterogeneous catalyst 9:CuOTf. Such increased
the wet chemical route.
enantioselectivity after immobilization was also ob-
[
5,7]
The immobilized BOX ligands formed BOX-Cu(I) served in a polymer-supported catalyst.
complexes on reaction with Cu(I) triflate in CH Cl .
The species 6:CuOTf, a BOX-Cu(I) catalyst immo-
2
2
The resulting heterogenized catalysts were examined bilized on unmodified MCF, showed a significantly
for the asymmetric cyclopropanation of styrene lower trans/cis ratio (59/41) and a lower enantioselec-
(
Table 1). The species 9:CuOTf, whose MCF support tivity (82% ee for the trans isomer and 79% ee for
was modified with TMS both before and after BOX the cis isomer). This inferior performance could be at-
immobilization, provided excellent enantioselectivity tributed to the presence of surface silanol groups on
and reactivity. This heterogenized BOX-Cu(I) catalyst the MCF support, which were not capped with TMS.
offered 95% enantiomeric excess (ee) for the trans When 6 was subjected to TMS postcapping by vapor
isomer, and 92% ee for the cis isomer. Species phase reaction, 7 was obtained. The species 7:CuOTf
9
:CuOTf was successfully recycled five times without showed a higher trans/cis ratio (64/36) and enantiose-
loss of enantioselectivity and reactivity. Even with a lectivity (90% ee for the trans isomer and 87% ee for
very small amount of this heterogenized catalyst (0.2 the cis isomer), compared to 6:CuOTf. However, its
mol%, TON of 500), high reactivity and enantioselec- catalytic properties still could not match those of
tivity (94% ee for the trans isomer and 91% ee for 9:CuOTf. Unlike 9:CuOTf, 7:CuOTf was not subject-
the cis isomer) were attained. A slightly higher trans/ ed to precapping, which served towards pretreating
cis ratio (67/33) and the same enantioselectivity were the surface of MCF with TMS prior to the introduc-
achieved by 9:CuOTf when the reaction medium was tion of the BOX ligands. The TMS precapping might
changed from CH Cl to toluene.
have provided for a better dispersion of BOX ligands
on the silica support, reducing their interactions with
2
2
[a]
Table 1. Asymmetric cyclopropanation of styrene.
[b]
[b]
[d]
[d]
Catalyst
Run no.
Catalyst [mol%]
Yield [%]
trans/cis ratio
% ee trans
% ee cis
2
6
7
9
:CuOTf
:CuOTf
:CuOTf
:CuOTf
1
1
1
1
2
3
4
5
1
1
1
2
2
2
2
2
2
2
2
80
70
73
68/32
59/41
64/36
65/35
65/35
65/35
64/36
64/36
66/35
67/33
65/35
86
82
90
95
95
95
94
95
94
95
94
84
79
87
92
91
91
90
89
91
92
92
[
c]
80 (78 )
76
78
76
[c]
77 (75 )
9
9
9
:CuOTf[
:CuOTf
0.2
2
2
78
72
86
e]
[f]
2
:Cu
A
C
H
T
R
E
U
N
G
(OTf)
[
a]
Conditions: styrene/EDA=1.2, CH Cl , Ar, 258C. EDA was added slowly over 2 h, and the reaction medium was further
2
2
stirred for another hour.
Determined by means of a GC calibration factor between n-dodecane and the product.
Yield of isolated product.
Determined by GC (Chiraldex B-PM, 50 m ” 0.25 mm I.D.).
In toluene.
Reduced by phenylhydrazine before use.
[
[
[
[
[
b]
c]
d]
e]
f]
TM
1250
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 1248 – 1254

Improved Enantioselectivity of Immobilized Chiral Bisoxazolines
FULL PAPERS
each other and with the surface silanol groups on the propene was cyclopropanated by 9:CuOTf under the
MCF a priori. Both 9:CuOTf and 7:CuOTf were sub- similar reaction conditions as those reported for the
jected to postcapping by vapor phase reaction follow- homogeneous catalyst [0.1 mol% catalyst, TON=
ing the ligand immobilization; this would serve to- 1000, 2-methylpropene/EDA=7, addition of ethyl di-
[
15]
wards minimizing subsequent interactions between azoacetate (EDA) for 5 h].
Catalyst 9:CuOTf of-
the ligands and the MCF surface. However, if there fered high enantioselectivity (92% ee) and high yield
were preexisting strong interactions between the li- (90%) over 2 runs, achieving a total TON of 2000.
gands and the MCF surface, such interactions would The best insoluble polymer-supported BOX showed
not be removed by the postcapping. These interac- only 53% yield with a much lower conversion of
[
7]
tions might prevent proper complexing between the 66%, even after 44 h under these conditions. Cata-
ligands and Cu, thereby affecting the enantioselectivi- lyst 9:CuOTf also showed excellent chemoselectivity
ty and reactivity of the catalyst. This might explain (95%).
the inferior performance of 7:CuOTf compared to
Due to the poor solubility of CuOTf and Cu
A
C
H
T
R
E
U
N
G
(OTf)
2
9
:CuOTf. These results illustrated that TMS precap- in CH Cl , long reaction times are required to com-
2
2
ping and postcapping of the silica support were both plex CuOTf and Cu
important towards optimizing the enantioselectivity lized BOX. To reduce the complex formation time,
and reactivity of the heterogenized BOX catalysts. Cu(OTf) was completely dissolved in tetrahydrofur-
A
H
R
U
G
AHCTREUNG
2
The heterogeneous catalyst 9:CuOTf was also ap- an (THF), and then added to the MCF-immobilized
plied to the asymmetric cyclopropanation of 1,1-di- BOX ligands 9 dispersed in THF. The resulting cata-
phenylethylene (Table 2) and 2-methylpropene lyst was washed thoroughly with THF, and dried
(
Table 3). For 1,1-diphenylethylene, high enantioselec- under vacuum. The elemental analysis showed that
tivity (92% ee) was obtained over two runs. 2-Methyl- most BOX ligands (>98%) formed complexes with
the copper added. In the case of highly cross-linked
polymer-supported BOX, only ca. 60% of the sup-
Table 2. Asymmetric cyclopropanation of diphenylethylene
by 9:CuOTf.
ported ligands formed complexes with copper.[ This
7]
suggested that the TMS-modified mesoporous silica
with an ultrahigh surface area was more effective,
providing greater catalyst accessibility with the mono-
layer immobilization of catalyst on the support sur-
face. The complexes were reduced by phenylhydra-
[b]
[c]
Run no.
Catalyst [mol%]
Yield [%]
% ee
zine before the catalytic reaction, and then used in
the asymmetric cyclopropanation of styrene. Al-
though the ee value was slightly lower than that of
heterogenized 9:CuOTf prepared directly in CH Cl ,
1
2
2
2
73
74
92
92
2
2
[
a]
Conditions: 1,1-diphenylethylene/EDA=2, CH Cl , Ar,
superb recyclability was achieved. Catalyst 9:CuOTf
prepared in THF maintained high enantioselectivity
2
2
258C. EDA was added slowly over 2 h, and the reaction
medium was further stirred for another hour.
Yield of isolated product.
Determined by HPLC (Chiralcel OD-H, hexane:2-prop-
anol=99.4:0.6).
(
92% ee for the trans isomer and 90% ee for the cis
isomer) and trans/cis ratio (65/35) over 12 runs
Figure 1). When the reaction time was cut by half (to
.5 h) for the 11th run, the catalyst provided the same
enantioselectivity with only slightly lower yield
[
[
b]
c]
(
1
Table 3. Asymmetric cyclopropanation of 2-methylpropene (75%).
[a]
by 9:CuOTf.
Conclusions
Chiral BOX ligands were effectively immobilized
onto a mesoporous silica support, which was partially
modified with TMS groups before ligand introduction.
Following the ligand immobilization, the support was
postcapped with TMS groups. This two-step modifica-
tion of siliceous MCF support led to superior catalyst
enantioselectivity. The resulting MCF-immobilized
BOX-Cu(I) catalyst provided 95% ee for the trans
isomer and 92% ee for the cis isomer, and 80% yield
for the asymmetric cyclopropanation of styrene. It
was also successfully recycled 12 times without losing
[
b]
[c]
Run no.
Catalyst [mol%]
Yield [%]
% ee
1
2
0.1
0.1
90
90
92
92
[
a]
Conditions: 2-methylpropene/EDA=7, CH Cl , Ar. EDA
2
2
was added slowly over 5 h at 08C; the reaction medium
was then warmed to room temperature over 14 h.
Yield of isolated product.
[
[
b]
c]
TM
Determined by GC (Chiraldex G-TA, 50 m ” 0.25 mm
I.D.).
Adv. Synth. Catal. 2006, 348, 1248 – 1254
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1251

Su Seong Lee et al.
FULL PAPERS
tracted with CH Cl (3”30 mL). The combined organic ex-
2
2
tracts were dried over Na SO , filtered, and concentrated
2
4
under vacuum. The concentrated liquors were purified by
flash chromatography to give product 1; yield: 2.03 g
2
D
3
1
(
(
7.26 mmol, 80%); [a] : À93.3 (c 1.05, CHCl ). H NMR
3
400 MHz, CDCl ): d=4.17 (m, 2H), 4.08 (m, 2H), 3.85 (m,
3
2
H), 3.54 (q, J=7.3 Hz, 1H), 1.48 (d, J=7.3 Hz, 3H), 0.881
1
3
(
s, 9H), 0.878 (s, 9H); C NMR (100 MHz, CDCl ): d=
3
1
65.5, 165.3, 75.5, 68.9, 34.0, 33.8, 25.7, 25.6, 15.2; anal.
calcd. for C H N O : C 68.53, H 10.07, N 9.99; found: C
16
28
2
2
6
8.14, H 10.22, N 9.84; HR-MS: m/z=281.2231, calcd. for
+
[
M+H] : 281.2229
Synthesis of Homogeneous BOX 2
Figure 1. % Yield (&),% trans isomer (*),% ee for trans
isomer (n), and% ee for cis isomer (,) for the asymmetric
cyclopropanation of styrene over 9:CuOTf prepared in
THF. Conditions: styrene/EDA=1.2, CH Cl , Ar, 258C. For
the first 10 runs, EDA was added slowly over 2 h, and the
reaction medium was further stirred for another hour. For
runs 11 and 12, EDA was added slowly over 1 h, and the re-
action medium was further stirred for another 30 min.
MeLi (0.516 mL, 1.6m in Et O, 0.825 mmol) was added into
2
the solution of BOX 1 (0.21 g, 0.75 mmol) in 10 mL of THF
at À508C. After stirring for 30 min, 3-iodopropane
2
2
(
0.110 mL, 1.13 mmol) was added, and the solution was
warmed to room temperature. After stirring overnight,
aqueous NH Cl was added to the reaction mixture, and the
4
aqueous layer was extracted with ethyl acetate. The com-
bined organic extracts were washed with brine, and then
any enantioselectivity and reactivity. This heterogen-
ized chiral BOX catalyst is expected to show high
enantioselectivity for various asymmetric reactions.
dried over MgSO , filtered, and rotary-evaporated to give
4
the crude product. The crude product was purified by flash
column chromatography to obtain product 2; yield: 170 mg
2
D
3
1
The partial TMS precapping of MCF represents an (70%); [a] : À43.7 (c 1.05, CHCl ). H NMR (400 MHz,
3
effective means of preparing a suitable silica support CDCl3): d=4.6 (m, 4H), 3.83 (m, 2H), 1.96 (m, 1H), 1.81
(
m, 1H), 1.46 (s, 3H), 1.30 (m, 2H), 0.91 (t, J=7.2 Hz, 3H),
for immobilizing a wide variety of chiral BOX ligands
and nitrogen-containing ligands. This approach can be
applied to achieve well-dispersed chiral ligands with-
out strong interactions with the silica surface, so that
excellent enantioselectivity, reactivity and recyclabili-
ty can be attained.
1
3
0
7
1
9
3
.86 (s, 18H); C NMR (100 MHz, CDCl ): d=168.2, 168.0,
5.5, 75.3, 68.72, 68.69, 42.4, 38.7, 33.9, 33.8, 25.8, 25.7, 21.4,
7.7, 14.5; anal. calcd. for C H N O : C 70.76, H 8.69, N
.92; found: C 70.35, H 8.52, N 9.75; HR-MS: m/z=
23.2705, calcd. for [M+H] :=323.2699.
3
1
9
34
2
2
+
Synthesis of MCF (4)
Experimental Section
Spherical MCF particles were synthesized by modifying the
conventional MCF synthesis method.[ Triblock copolymer
P123 (4 g) was dissolved in an acidic solution (10 mL of HCl
14]
Synthesis of BOX Ligand 1 with One Methyl Group
at the Carbon Bridge
and 65 mL of H O). Trimethylbenzene (TMB) (3.4 mL) was
2
then added, and the resulting solution was heated to 37–
408C with vigorous stirring. After 2 h of stirring, 9.2 mL of
tetraethoxysilane (TEOS) were added and stirred for 5 min.
The solution was transferred to an autoclave, and aged at
408C for 20 h under static conditions. To increase the pore
Dimethyl methylmalonate (1.78 g, 12.2 mmol) and (S)-tert-
leucinol (3.0 g, 25.6 mmol) were heated to 908C under an
argon flow, which removed the methanol generated during
the reaction. After 12 h, white solids were obtained. To
remove the remaining (S)-tert-leucinol, high vacuum was ap-
plied at 608C. Dihydroxymethylmalonodiamide (3.27 g,
size, NH F (46 mg) in 5 mL of water was added to the solu-
4
tion prior to aging at 1008C. It was then aged at 1008C for
24 h. The resulting precipitate was filtered, washed with
water and ethanol, and dried. The white powder obtained
was calcined in air at 5508C for 6 h. Surface area=
10.37 mmol) was obtained with a high yield (85%).
A 100-mL Schlenk flask was charged with dihydroxyme-
thylmalonodiamide (2.87 g, 9.08 mmol), 4-(dimethylamino)-
pyridine (DMAP, 0.12 g, 0.99 mmol), and 40 mL of CH Cl .
2
À1
2
2
561 m g ; window pore size=16 nm; cell pore size=
28 nm; average particle size=5 mm.
Triethylamine (6 mL, 43.5 mmol) was then added. A solu-
tion of p-toluenesulfonyl chloride (3.77 g, 19.8 mmol) in
1
0 mL of CH Cl was added. The resulting bright yellow so-
2 2
lution was stirred at room temperature for 27 h. It was dilut-
ed with 20 mL of CH Cl , and washed with saturated Partial Modification of MCF (5)
2
2
NH Cl. The aqueous layer was back-extracted with CH Cl
4
2
2
(
3”30 mL). The combined organic extracts were washed
MCF (3.0 g) was degassed at 1208C overnight under
vacuum before reaction. HMDS (253 mL, 1.2 mmol) was
with saturated NaHCO . The aqueous layer was back-ex-
3
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Adv. Synth. Catal. 2006, 348, 1248 – 1254

Improved Enantioselectivity of Immobilized Chiral Bisoxazolines
FULL PAPERS
added to a suspension of MCF in toluene (20 mL). The mix- Cyclopropanation of Styrene by MCF-Supported
ture was stirred at room temperature under argon for 5 h,
and then heated at 608C overnight. The resulting suspension
BOX Catalysts
was filtered, washed with methanol, and then dried under
A
H
R
U
G
A
H
R
U
G
2
vacuum. Elemental analysis: found (wt%): C 2.86, H 0.94;
was added to the MCF-immobilized BOX (0.022 mmol) in
CH Cl (3 mL). The mixture was stirred at room tempera-
À1
loading of TMS groups: 0.794 mmolg
.
2
2
ture for 3 days. In the case of Cu
0.023 mmol) was added to reduce the copper. After the ad-
dition of styrene (153 mL, 1.32 mmol), a solution of EDA
A
H
R
U
G
(
Immobilization of BOX onto Unmodified MCF (6)
(
1.1 mmol, diluted with 2 mL of CH Cl ) was added over
2 2
2
h using a syringe pump. The mixture was stirred for 1 h
MeLi (0.516 mL, 1.6m in Et O, 0.825 mmol) was added into
2
and then centrifuged. The solution portion was collected,
and the trans/cis ratio and yield were determined by gas
chromatography (GC). The enantiomeric excess was deter-
mined by GC using a Cyclodex-B column (50 m ” 0.25 mm
I.D.). The precipitate was washed with CH Cl (5 mL), and
the solution of BOX 1 (0.21 g, 0.75 mmol) in 10 mL of THF
at À508C. After stirring for 30 min, 3-iodopropyltrimethoxy-
silane (0.148 mL, 0.75 mmol) was added, and the solution
was warmed to room temperature. After stirring for 2 days
at room temperature, THF was evaporated and toluene was
added. The toluene solution was added to MCF 4 (2.0 g),
which has been dried at 1508C overnight. The resulting sus-
pension was then heated to 1208C with stirring for 1 day, fil-
tered, and washed thoroughly with toluene, CH Cl , and
2
2
then centrifuged three times. The recovered catalyst was
reused directly for the next experiment.
2
2
methanol. PA-FT-IR: n=2957, 1663, 1089, 842, 811, Acknowledgements
À1
4
60 cm ; elemental analysis: found (wt%): C 7.63, H 1.45,
À1
N 0.78; loading of BOX: 0.279 mmolg
.
This work was supported by the Institute of Bioengineering
and Nanotechnology (Agency for Science, Technology and
Research, Singapore).
TMS Modification of BOX-Immobilized MCF 6 (7)
References
MCF-supported BOX 6 (1.0 g) was degassed at 808C over-
night. Excess HMDS (0.75 mL) was added to the solid
under vacuum. The flask was cooled down using liquid N2
under vacuum. It was sealed and then warmed to room tem-
perature. After that, the flask was placed in the oven at
[
1] a) A. K. Gosh, P. Mathivanan, J. Capiello, Tetrahedron:
Asymmetry 1998, 9, 1; b) A. Pfaltz, Synlett 1999, 835;
c) J. S. Johnson, D. A. Evans, Acc. Chem. Res. 2000, 32,
3
25; d) F. Fache, E. Schulz, M. L. Tommasino, M. Lem-
758C for 5 h. After reaction, excess HMDS was removed
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À1
[2] D. Rechavi, M. Lemaire, Chem. Rev. 2002, 102, 3467.
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4
60 cm ; elemental analysis: found (wt%): C 10.91, H 2.05,
À1
N 0.71; loading of BOX: 0.254 mmolg
.
[
4] M. I. Burguete, J. M. Fraile, J. I. García, E. García-Ver-
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Immobilization of BOX onto Partially TMS-Modified
MCF 5 (8)
[
[
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ally TMS-capped MCF 5 (2.0 g, 0.794 mmol TMS/g of MCF)
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À1
1663, 1089, 842, 811, 460 cm
.
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Further TMS Modification of BOX-Immobilized
MCF 8 (9)
[
9] R. J. Clarke, I. J. Shannon, Chem. Commun. 2001, 1936.
[
[
10] D. Rechavi, M. Lemaire, Org. Lett. 2001, 3, 2493.
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183, 239.
The same procedure as for 7 was applied except that 8 was
used instead of 6. PAS-FT-IR: n=2957, 1663, 1089, 842,
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À1 13
C CP/MAS NMR (100 MHz): d=170.7,
68.0, 76.0, 67.9, 57.7, 50.1, 42.6, 33.2, 24.7, 18.0, 12.5,
8
1
0
11, 460 cm
;
2005, 3577.
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2
9
.5; Si CP/MAS NMR (75 MHz): d=14.3, À50.0, À57.0,
À65.8, À100.28, À107.58; elemental analysis: found
(
wt%):
C
11.45,
.
H
2.25,
N
0.73; loading of BOX:
À1
0
.268 mmolg
Adv. Synth. Catal. 2006, 348, 1248 – 1254
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 1248 – 1254