Enantioselective addition of diethylzinc to aldehydes using immobilized chiral BINOL-Ti complex on ordered mesoporous silicas

Article abstract of DOI:10.1016/j.tetasy.2006.05.017

A chiral (S)-BINOL ligand has been covalently grafted on ordered mesoporous silicas-MCM-41 and SBA-15 and the resulting inorganic-organic hybrid materials used as chiral auxiliaries in Ti-promoted enantioselective addition of diethyl zinc to aldehydes und

Full text of DOI:10.1016/j.tetasy.2006.05.017

Tetrahedron: Asymmetry 17 (2006) 1506–1513
Enantioselective addition of diethylzinc to aldehydes
using immobilized chiral BINOL–Ti complex
on ordered mesoporous silicas
Kavita Pathak, Achyut P. Bhatt, Sayed H. R. Abdi,* Rukhsana I. Kureshy,
Noor-ul H. Khan, Irshad Ahmad and Raksh V. Jasra
Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute (CSMCRI), Bhavnagar 364 002, Gujarat, India
Received 23 March 2006; accepted 17 May 2006
Abstract—A chiral (S)-BINOL ligand has been covalently grafted on ordered mesoporous silicas—MCM-41 and SBA-15 and the result-
ing inorganic–organic hybrid materials used as chiral auxiliaries in Ti-promoted enantioselective addition of diethyl zinc to aldehydes
under heterogeneous conditions. These heterogeneous catalysts showed promising activity and enantioselectivity. The catalyst having
a larger pore diameter with capping of free silanol moiety with trimethylsilyl (TMS) group was found to be more active with enantio-
selectivities up to 81% being achieved. The catalyst worked well up to three cycles with retention of enantioselectivity after washing with
10% HCl in methanol.
ꢀ 2006 Elsevier Ltd. All rights reserved.
1. Introduction
ionic tag.16 Its immobilization on inorganic support has
been scarcely explored.17,18 Inorganic supports have many
advantages over most polymers because of their superior
mechanical and thermal stability.19 Recently, ordered meso-
porous solids such as MCM-41 and SBA-15, with their
well-defined uniform mesopores and facile surface modifi-
cation, were shown to be potential materials for the hetero-
genization of valuable chiral homogeneous catalyst.20–25
Various ligand systems such as b-amino alcohols based
N-alkylnorephedrine, ephedrine, proline, and dendrimers
immobilized on inorganic supports, have been investigated
for the enantioselective alkylation of aldehydes.26,27 In con-
tinuation of our earlier works on the heterogenization of
chiral BINOL on siliceous support for La catalyzed enan-
tioselective nitroaldol reaction,28 we herein report, the
immobilization of chiral BINOL ligand covalently bonded
to mesoporous silicas such as MCM-41 and SBA-15. The
active heterogeneous catalyst for enantioselective addition
of diethylzinc to various aldehydes was generated in situ
by the interaction of bonded BINOL ligand with Ti(OiPr)4.
The study deals with (a) strategy to support chiral BINOL
on mesoporous silicas, (b) catalytic activity of these sup-
ported catalysts, (c) minimizing undesired catalytic activity
on the silica surface by capping of free hydroxyl groups of
silica surface with (TMS) group,29 and (d) regeneration of
the expensive chiral catalyst.
Optically active secondary alcohols are important inter-
mediates in the synthesis of many naturally occurring
compounds, biologically active intermediates, and materials,
such as liquid crystals. Enantioselective addition of diethyl-
zinc to aldehydes is one of the most fundamental methods
that afford optically active secondary alcohols through a
C–C bond formation reaction.1,2 Chiral 1,10-bi-2-naphthol
(BINOL) is one of the most useful ligands and has made a
considerable contribution to asymmetric synthesis under
homogeneous conditions.3–5 It has also contributed to the
catalytic asymmetric addition of diethylzinc to aldehydes
utilizing a BINOL–Ti complex as an efficient catalyst under
homogeneous systems,6–8 although the heterogeneous
asymmetric catalytic system has an inherent advantage in
its easy recovery, product separation from the reaction
mixture and reuse of the expensive chiral catalyst,9–11 that
can narrow the gap between homogeneous and hetero-
geneous catalysis. In this direction, some attempts have been
made for heterogenization of chiral BINOL on organic
polymers by grafting it onto polymer back bone,12–14
cross-linking co-polymerization,15 and BINOL with an
*
Corresponding author. Fax: +91 278 2566970; e-mail: shrabdi@
csmcri.org
0957-4166/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.05.017

K. Pathak et al. / Tetrahedron: Asymmetry 17 (2006) 1506–1513
1507
2. Results and discussion
15 in toluene to afford 7/70. Demethylation of compounds
7 and 70 afforded 8a and 8b, respectively. The silica matrix
bears many hydroxyl groups that may react with Ti metal
ions in order to create non-chiral catalyst sites on silica
support. Therefore, in order to understand the role of
hydroxyl groups belonging to silica’s, compounds 7 and
70 were capped with a TMS group and then demethylated
to give solid ligands 9a and 9b.
2.1. Modification and immobilization of chiral BINOL
In order to retain the flexibility of the free BINOL and to
develop the catalyst system akin to the structure that was
used under homogeneous condition, we synthesized 8a,
9a and 8b, 9b according to the steps shown in Scheme 1.
Thus monoesterification of (S)-1,10-bi-2-naphthol was
achieved leading to the formation of compound 2 in excel-
lent selectivity. Bromination of 2 yielded exclusively mono-
brominated product with the bromine attached on the 6th
position of the non-esterified naphthyl moiety 3. The
brominated product upon saponification afforded (S)-6-
monobromo-1,10-bi-2-naphthol 4. Protection of the hydr-
oxyl groups of 4 with CH3I under basic conditions yielded
(S)-6-bromo-2,20-dimethoxy-1,10-binaphthyl 5. To achieve
covalent grafting to mesoporous silica, compound 5 was
treated with Mg/I2 and 3-chloropropyltrimethoxysilane to
obtain O-methylated BINOL 6 with a silanol arm. Com-
pound 6 was then refluxed with calcined MCM-41/SBA-
The characterization of the mesoporous silica-supported
ligands was accomplished by various physico-chemical
techniques. The grafted amount of chiral auxiliary was found
to be 16–18 mg/100 mg of solid support calculated from
elemental analysis and thermal gravimetric data. FT-IR
spectra of immobilized ligands 8a and 9a showed the char-
acteristic bands of CH2 for aliphatic C–H stretching vibra-
tions at 3000–2900 cmꢀ1 and bands at ꢁ1465 and
1454 cmꢀ1 due to the C@C stretching vibration of the
attached BINOL group implying that the modified BINOL
was covalently grafted to the mesoporous silicas MCM-
41. Furthermore, the FT-IR spectra for compound 9a
Br
Br
b
c
98%
a
OH
O
OH
O
OH
OH
OH
OH
C(CH3)3
98%
C(CH3)3
96%
O
O
4
1
2
3
87%
d
Br
OCH3
OCH3
5
e
O
H3CO
H3CO Si
H3CO
O
Si
OCH3
OCH3
OCH3
OCH3
OCH3
OH
OH
f
OH
7, 7'
6
g
h
O
O
O
O
Si
OCH3
Si
OCH3
OH
OH
OH
OH
OH
OTMS
OTMS
OH
OH
OH
8a, 8b
9a, 9b
8a, 9a: MCM-41 supported ligand
8b, 9b: SBA-15 supported ligand
Scheme 1. The synthetic route for anchoring of BINOL with functional groups onto silica surface. Reagents and conditions: (a) pivaloyl chloride, Et3N,
CH3CN, 0 ꢁC; (b) Br2, CH3CN, 0 ꢁC, 3 h; (c) KOH, H2O, THF, rt, 24 h; (d) CH3I, K2CO3, acetone, 18 h; (e) (i) Mg/I2, THF, reflux, 9 h; (ii)
chloropropyltrimethoxysilane, toluene, reflux, 12 h; (f) calcined MCM-41/SBA-15, toluene, 48 h; (g) BBr3, CH2Cl2, ꢀ78 ꢁC; (h) (i) HMDS, reflux, 12 h; (ii)
BBr3, CH2Cl2, ꢀ78 ꢁC.

1508
K. Pathak et al. / Tetrahedron: Asymmetry 17 (2006) 1506–1513
(a)
(b)
2959
1617
1588
1498
3401
817
1466
1144
1173
1388
1465
2929
1630
2974
794
3431
%T
458
457
(c)
1630
758
1076
2963
847
(d)
1086
1631
801
1079
3432
456
4000.0
3000
2000
1500
1000
400.0
cm-1
Figure 1. Representative IR spectra of ligand 4 (a), ligand 8a (b), ligand 9a (TMS capped) (c), and calcined MCM-41 (d).
showed a remarkable reduction in the intensity of –OH
band at 3432 cmꢀ1 due to the capping of –Si–OH with
TMS (Fig. 1(c)).
the conservation of the same type of isotherms indicates
that the structure of the inorganic surface remain intact
after modification. However, a decrease in BET surface
area (SBET), total pore volume, and BJH average pore
diameter were observed, which can be attributed to the
presence of a chiral BINOL auxiliary in the mesopores that
partially blocks the adsorption of nitrogen molecules
(Table 1). TEM images for large pore sized mesoporous
silica SBA-15 showed two dimensional hexagonal symme-
try that remained unaffected on immobilization of chiral
BINOL and also after reusing the supported catalyst
(Fig. 3). Powder X-ray diffraction of the supported chiral
auxiliary confirmed structural hexagonal space group
(p6mm) of the inorganic support that remain preserved
after immobilization even after successive reuse experiment
(Fig. 4).
13C CP MAS NMR spectra of immobilized modified
BINOL on SBA-15 peaks in the range d = 8–35 ppm due
to the propyl group of the silanol arm and d = 109–
155 ppm aromatic carbons due to the naphthyl groups of
BINOL, respectively, additionally support our view of suc-
cessful anchoring of BINOL on mesoporous silicas12
(Fig. 2).
The textural characteristics of this supported chiral ligand
were obtained by an N2 sorption study. Typical adsorption
and desorption isotherm (type IV) were retained after
immobilization of the organic functions on the surface,
A
ppm
0
150
170
160
130 120 110 100 90
70
50 40 30 20 10
140
80
60
Figure 2. 13C MAS NMR spectra of ligand 8b immobilized on SBA-15.

K. Pathak et al. / Tetrahedron: Asymmetry 17 (2006) 1506–1513
1509
Table 1. Textural characterization of mesoporous silicas and immobili-
zation of BINOL during various synthetic steps
A
B
Compound
BET surface
area (m2/g)
Total pore
volume (cm3/g)
BJH pore
diameter (A)
˚
MCM-41
8a
9a
998
864
842
797
493
443
0.9875
0.6142
0.5142
1.1327
0.6768
0.5667
35
29
26
68
63
59
( a)
( b)
(a')
(b')
SBA-15
8b
9b
( c)
( d)
(c')
(d')
2.2. Enantioselective addition of diethylzinc to aldehydes
Enantioselective addition of diethylzinc to various alde-
hydes such as benzaldehyde, 3-methoxy benzaldehyde,
and 4-fluoro benzalaldehyde was carried out using an
immobilized chiral modified BINOL–Ti complex, gener-
ated in situ to give the respective secondary alcohols (Table
2). To compare the catalytic efficacy of the immobilized
catalyst, we used ligand 4 as the catalyst precursor under
homogeneous conditions by keeping other reaction para-
meters constant for the enantioselective addition of diethyl-
zinc to various aldehydes to give excellent conversion
(95–99%), selectivity (95–100%), and enantioselectivity
(88–89%) for the secondary alcohols (entries 1, 6, and 11)
in 7 h. However, when the same reaction was conducted
with MCM-41 immobilized ligand 8a under heterogeneous
conditions, conversions of 78–88% with selectivities of 80–
89% and enantioselectivities of 37–45% were obtained for
the respective secondary alcohols (entries 2, 7, and 12) in
24 h. The longer reaction times under heterogeneous condi-
tions were possibly due to the slow diffusion of reactants to
the catalytic sites in the mesopores of silica.
1
2
3
4
5
6
2
3
4
5
6
Angle (2Theta)
Angle (2Theta)
Figure 4. (A) XRPD pattern of calcined MCM-41 (a), ligand 8a (b),
ligand 9a (TMS capped) (c), and reused catalyst 9a (d). (B) XRPD pattern
of calcined SBA-15 (a0), ligand 8b (b0), ligand 9b (TMS capped) (c0), and
reused catalyst 9b (d0).
8a and 9a. Highest enantioselectivity (81%) was achieved
with TMS capped SBA-15 supported BINOL ligand 9b
(entry 5) with conversion and selectivity almost similar to
the homogeneous catalytic system. The catalytic activity
of the TMS-capped silica without a chiral ligand showed
only 10% conversion of the product under similar reaction
conditions (entry 16). This residual activity of the silica
possibly causes a reduction of the enantioselectivity.26d
After the first use of immobilized catalysts, compounds 9a
and 9b were filtered before quenching the reaction mixture
by NH4Cl solution. Their recovered catalyst was washed
with toluene and dried under vacuum at 110 ꢁC for 4–5 h
for reuse. In the second run, with catalyst 9a, the conver-
sion decreased markedly (from 90% to 60%), similarly with
catalyst 9b (from 98% to 67%), (Table 3, catalytic runs 1
and 2) probably due to the blockage of catalytic sites with
the reactants. Therefore, the recovered catalyst was washed
sequentially with 10% HCl in MeOH, H2O, and finally with
acetone under centrifugation.30 This treatment resulted in
the restoration of activity and selectivity of the immobi-
lized catalyst. The hexagonal porosity of the dried material
was intact as confirmed by XRPD analysis (Fig. 4d and d0)
and TEM image (Fig. 3C). The catalytic system worked
well for two more repeat catalytic experiments with some
loss in activity for the enantioselective addition of diethyl
SBA-15 immobilized ligand 8b resulted in conversions of
up to 94% with high selectivity of 92% and good enantio-
selectivity of 69% in the product 1-phenyl 1-propanol using
benzaldehyde as a substrate (entry 4). These improvements
over MCM-41 based catalysts were attributed to the bigger
pore size of the SBA-15 support facilitating the diffusion
of reactants. Significant improvement in conversion
(82–98%), selectivity (87–99%), and enantioselectivity
(51–81%) (entries 3, 5, 8, 10, 13, and 15) were observed when
TMS-capped silicas 9a and 9b were used as catalyst precur-
sors for the addition of diethyl zinc with various aldehydes
under heterogeneous reaction conditions. The overall per-
formance was better for larger pore sized SBA-15 sup-
ported catalysts 8b and 9b than MCM-41 based catalyst
Figure 3. Representative TEM images of calcined SBA-15 (A), ligand 9b (TMS capped) (B), and reused catalyst 9b (C).

1510
K. Pathak et al. / Tetrahedron: Asymmetry 17 (2006) 1506–1513
Table 2. Enantioselective addition of diethylzinc to aromatic aldehydes using immobilized (S)-BINOL–Ti complexesa
O
H
HO
L*+Ti(OiPr)4
PhCH3, 0 ºC
Et2Zn
*
+
Ar
Ar
H
Entry
ArCHO
L*
Conversionc (%)
Selectivityd (%)
eee (%)
Configne
1
2
3
4
5
4
99
82
90
94
98
100
88
92
92
99
89
45
62
69
81
S
S
S
S
S
8a
9a
8b
9b
C6H5CHO
6
7
8
9
4
95
78
82
76
86
95
80
87
80
90
88
37
51
58
70
S
S
S
S
S
8a
9a
8b
9b
m-MeOC6H4CHO
10
11
12
13
14
15
4
97
88
90
90
96
98
89
91
93
98
89
41
57
63
78
S
S
S
S
S
8a
9a
8b
9b
p-FC6H4CHO
16b
C6H5CHO
TMS capped SBA-15
10
—
—
—
a Reactions were carried out with 8 mol % of 4 at 0 ꢁC for 7 h (entries 1, 6, and 11) under homogeneous condition and 8a, 8b, 9a, and 9b for 24 h under
heterogeneous reaction condition using 1.5 mmol Ti(OiPr)4, 3.0 mmol Et2Zn, and 1.0 mmol substrate in 2 mL toluene.
b Reaction performed with TMS capped surface SBA-15 without ligand for 24 h.
c Determined by 1H NMR spectroscopy.
d % Selectivity: 100([R] + [S])/([R] + [S] + [PhCH2OH]).
e Determined by HPLC using Daicel Chiralcel OD column.
Table 3. Recycling data for addition of diethylzinc to benzaldehyde as
the regenerated catalyst was used for further catalytic runs
with the retention of enantioselectivity.
representative substrate using immobilized ligand with Ti(OiPr)4 as
catalysta
Catalytic run
L + Ti(OiPr)4
Conversion (%)
ee (%)
4. Experimental
1
2
3*
4*
9a (9b)
9a (9b)
9a (9b)
9a (9b)
90 (98)
60 (67)
85 (94)
77 (89)
62 (81)
58 (76)
60 (80)
59 (78)
4.1. General
1H and 13C NMR spectra were recorded using a Bruker,
F113V (200 and 50 MHz) FT-NMR spectrometer. The
IR spectra were recorded on a Perkin–Elmer Spectrum
GX spectrophotometer in KBr/Nujol mull. Microanalysis
of the complex was carried out on a CHNS analyzer, Per-
kin–Elmer model 2400. Inductive coupled plasma spec-
trometer (Perkin–Elmer Instrument, Optical Emission
spectrometer, Optima 2000 DV) was used for Ti estima-
tion, X-ray powder diffraction patterns of the samples
were recorded on Philips X00 pert MPD diffractometer using
a Using 8 mol % ligand at 0 ꢁC, reaction time—24 h.
* After washing with 10% HCl in MeOH, H2O, and acetone.
zinc to benzaldehyde (Table 3, catalytic runs 3* and 4*).
This approach simplifies the recovery of chiral auxiliaries,
and can be transposed and reused.
˚
CuKa (= 1.5405 A) radiation with 2h step size of 0.02ꢁ and
3. Conclusion
step time of 5 s of curved CuKa monochromator under
identical conditions. Specific rotation was measured by
polarimeter model Digipol 781 Rudolph instruments,
USA. Thermal measurements of the samples were carried
out on a Mettler Toledo (TGA/SDTA 851ꢁ) instrument.
N2 gas adsorption–desorption isotherms were performed
using Micromeritics ASAP-2010 at 77 K. The pore dia-
meter of the samples was determined from the desorption
branch of the N2 adsorption isotherm employing the Bar-
ret, Joyner, and Halenda (BJH) method. TEM analysis
was accomplished by transmission electron microscope
Philips Tecnai 20. The conversion and ee of secondary
In conclusion, mesoporous silica-supported BINOL has
been synthesized, which can be used to generate Ti–BINOL
complexes. These silica-supported chiral catalysts were
used for the enantioselective addition of diethylzinc to
aldehydes with moderate conversions for the secondary
alcohols under heterogeneous reaction condition. The
TMS capped catalyst 9b with a larger pore size gave excel-
lent conversions and higher enantioselectivities (up to 81%)
in the product 1-phenyl 1-propanol. The reuse of expensive
chiral BINOL based catalyst was effectively worked out by
washing the used catalyst with 10% HCl in methanol while

K. Pathak et al. / Tetrahedron: Asymmetry 17 (2006) 1506–1513
1511
25
alcohols were determined by HPLC (Shimadzu SCL-
10AVP) using Chiralcel OD column. All reagents were
obtained from commercial sources and used without puri-
fication. All the solvents used in the present study were
purified by the known methods.31
solid. (Yield; 2.8 g, 98%) ½aꢂD ¼ þ6:35 (c 0.55, THF); IR
(KBr) 3476, 3401, 1587, 1496, 1144, 816, 749, 421 cmꢀ1
;
1H NMR (CDCl3): d 4.94 (s, 1H), 5.04 (s, 1H), 6.99 (d,
J = 8.0 Hz, 1H), 7.05 (d, J = 7.2 Hz, 1H), 7.25–7.37 (m,
5H), 7.48 (t, J = 5.8 Hz, 1H), 7.82–7.85 (m, 2H), 7.93 (d,
J = 8.6 Hz, 1H), 8.0 (s, 1H); 13C NMR d 111.05, 112.11,
118.25, 118.91, 119.85, 124.20, 124.65, 126.3, 127.98,
128.65, 130.10, 130.61, 131.11, 132.40, 132.65, 133.10,
133.65, 133.9, 153.0, 153.5. Anal. Calcd for C20H13O2Br:
C, 65.76; H, 3.56. Found: C, 65.60; H, 3.45.
4.2. (S)-2-Hydroxy-20-pivaloyloxy-1,10-binaphthyl 2
Compound 2 was synthesized according to a reported
procedure.32 (S)-2,20-Dihydroxy-1,10-binaphthyl 1 (4.0 g,
14.0 mmol) and (C2H5)3N (5.9 mL, 42.0 mmol) were added
to pivaloyl chloride (1.68 g, 14.0 mmol) in CH3CN (45 mL)
at 0 ꢁC. The mixture was allowed to warm to rt and stirred
for 6 h. The crude mixture was dissolved in diethyl ether
and washed with aq 1 M HCl (20 mL), saturated aq NaH-
CO3 (20 mL), and brine. The organic phase was dried over
anhydrous Na2SO4 and filtered. Evaporation of the solvent
afforded the desired product that was purified by column
4.5. (S)-6-Bromo-2,20-dimethoxy-1,10-binaphthyl 5
To a solution of 4 (2.5 g, 6.84 mmol) in anhydrous acetone
(80 mL) were added anhydrous K2CO3 (2.83 g, 20.5 mmol)
and methyl iodide (2.91 g, 20.5 mmol) at room temperature
and the mixture refluxed for 18 h under dry conditions. The
solvent was completely removed under vacuum and the res-
idue dissolved in CH2Cl2 (80 mL) and H2O (70 mL). The
aqueous layer was further extracted with CH2Cl2
(3 · 20 mL). The combined organic layer was dried over
anhydrous Na2SO4. After the removal of the solvent, the
chromatography [silica gel, n-hexane/EtOAc (6:1)] to give
25
2 (yield; 4.96 g, 96%). ½aꢂD ¼ ꢀ54:5 (c 0.5, THF); IR
1
(KBr) 3414, 2969, 1720, 1509, 1280, 1154, 813 cmꢀ1; H
NMR (CDCl3): d 0.75 (s, 9H), 5.08 (s, 1H), 7.01 (d,
J = 8.4 Hz, 1H), 7.15–7.34 (m, 6H), 7.46 (t, J = 5.8 Hz,
1H), 7.78 (d, J = 7.6 Hz, 1H), 7.83 (d, J = 8.8 Hz, 1H),
7.93 (d, J = 8.0 Hz, 1H), 8.02 (d, J = 8.8 Hz, 1H); 13C
NMR d 27.31, 39.59, 115.10, 119.0, 122.69, 123.94,
124.37, 125.4, 126.50, 127.10, 127.54, 128.28, 128.76,
129.18, 129.89, 131.13, 131.55, 133.05, 134.37, 134.51,
149.19, 152.65, 178.66. Anal. Calcd for C25H22O3: C,
81.08; H, 5.95. Found: C, 80.91; H, 5.90.
pale yellow product was washed with methanol to give 5
25
as white solid (yield; 2.34 g, 87%). ½aꢂD ¼ þ48:1 (c 0.5,
CHCl3); IR (KBr) 2933, 1586, 1492, 1265, 1251, 807,
749 cmꢀ1 1H NMR (CDCl3) d 3.75 (s, 6H), 6.94 (d,
;
J = 9.2 Hz, 1H), 7.11 (d, J = 9.2 Hz, 1H), 7.18–7.27 (m,
3H), 7.47 (d, J = 9 Hz, 2H), 7.84 (d, J = 6.6 Hz, 1H),
7.89 (d, J = 5.7 Hz, 1H), 7.95 (d, J = 10.8 Hz, 1H), 8.01
(d, J = 2.8 Hz, 1H); 13C NMR (50 MHz, CDCl3) d 57.3,
110.1, 111.3, 117.1, 118.2, 123.8, 124.0, 125.7, 127.1,
128.3, 129.5, 129.8, 130.2, 130.6, 130.8, 131.3, 132.3,
152.1, 152.7, Anal. Calcd C22H17BrO2: C, 67.17; H, 4.33.
Found: C, 67.02; H, 4.30.
4.3. (S)-6-Bromo-2-hydroxy-20-pivaloyloxy-1,10-binaphthyl 3
Compound 3 was synthesized according to a known proce-
dure.32 To a solution of 2 (4 g, 10.8 mmol) in CH3CN
(50 mL) was added bromine (1.10 mL, 21.5 mmol) at
0 ꢁC. The reaction mixture was stirred at 0 ꢁC for 3 h,
quenched with aqueous Na2SO3, and extracted with diethyl
ether. The organic phase was washed sequentially with sat-
urated aq NaHCO3, aqueous 1 M HCl, brine and dried
over anhydrous Na2SO4. After removal of the solvent, 3
4.6. (S)-6-(1-Propyltrimethoxy silane)-2,20-dimethoxy-1,10-
binaphthyl 6
Compound 6 was synthesized according to a literature
procedure.28 Magnesium turnings (0.92 g, 38 mmol), 65 mL
of dried and degassed THF and a crystal of iodine were
allowed to react with 5 (2 g, 5.8 mmol) in 15 mL of THF
under a dry argon atmosphere for 30 min at ambient
temperature, followed by its gentle refluxing for 9 h. The
resulting mass was cooled to room temperature and a solu-
tion of 3-chloropropyltrimethoxysilane (1.15 g, 1 equiv, in
25 mL of dry THF) then added dropwise over a period
of 40 min. The reaction mixture was then refluxed for
12 h and the solvent distilled out completely under an inert
atmosphere. Dry toluene (30 mL) was added to the result-
ing residue that was stirred for 2 h and filtered under inert
atmosphere to afford 6 in solution. Compound 6 is highly
moisture sensitive and hence an aliquot from the above
solution was taken for spectroscopic characterization,
while the rest of the solution was used directly for subse-
was obtained as white solid (yield, 3.92 g, 98%).
25
½aꢂD ¼ þ6:0 (c 0.52, THF); IR (KBr) 3404, 2969, 1720,
1494, 1152, 815, 489, 422 cmꢀ1 1H NMR (CDCl3): d
;
0.76 (s, 9H), 5.13 (s, 1H), 6.87 (d, J = 8.8 Hz, 1H), 7.20–
7.35 (m, 5H), 7.48 (td, J1 = 7.2 Hz, J2 = 1.2 Hz, 1H), 7.75
(d, J = 8.8 Hz, 1H), 7.92 (m, 2H), 8.04 (d, J = 8.8 Hz,
1H); 13C NMR d 27.32, 39.59, 115.36, 118.15, 119.0,
120.26, 122.66, 126.22, 127.14, 128.41, 129.24, 130.17,
130.72, 131.81, 133.0, 133.78, 134.07, 134.90, 135.1,
149.16, 153.0, 178.61. Anal. Calcd for C25H21O3Br: C,
66.82; H, 4.67. Found: C, 66.12; H, 4.50.
4.4. (S)-6-Bromo-2,20-dihydroxy-1,10-binaphthyl 4
1
A mixture of 3 (3.5 g, 7.78 mmol), KOH (1.3 g, 23 mmol),
THF (25 mL), and water (10 mL) was stirred for 16 h at
ambient temperature under N2. The reaction mixture was
diluted with EtOAc and the organic phase washed with
aqueous 1 M HCl (25 mL), saturated aq NaHCO3, and
brine in that order. The organic phase was then dried over
Na2SO4 and concentrated in vacuum to give 4 as a yellow
quent synthesis. H NMR (CDCl3) d 0.92 (broad t, J =
7 Hz, 2H), 0.75–0.85 (m, 2H), 1.84 (broad t, J = 7 Hz,
2H), 3.56 (s, 9H), 3.75 (s, 6H), 6.95 (d, J = 9 Hz, 1H),
7.02 (d, J = 9 Hz, 1H), 7.17–7.35 (m, 3H), 7.42 (d, J =
3.8 Hz, 1H), 7.48 (d, J = 3.5 Hz, 1H), 7.84 (d, J = 8.5 Hz,
1H), 7.88 (d, J = 9 Hz, 1H), 7.95 (d, J = 9 Hz, 1H), 8.0
(d, J = 2 Hz, 1H); 13C NMR (50 MHz, CDCl3) d 7.4, 8.7,

1512
K. Pathak et al. / Tetrahedron: Asymmetry 17 (2006) 1506–1513
26.9, 47.8, 50.9, 51.3, 114.9, 115.8, 124.0, 125.6, 125.9,
126.7, 126.8, 127.0, 127.5, 127.8, 128.5, 128.6, 129.1,
129.4, 130.0, 130.2, 130.4, 130.5, 149.9, 150.1. Anal. Calcd
C28H32O5Si: C, 70.58; H, 6.72. Found: C, 70.12; H, 6.44.
(Scheme 1). IR (KBr) cmꢀ1: 2963, 2361, 1627, 1236,
1085, 840, 457. Anal. Found: C, 9.92; H, 2.35.
4.11. General procedure for the enantioselective addition of
diethylzinc to aromatic aldehydes
4.7. Synthesis of ordered mesoporous silicas MCM-41/SBA-
15 (a,b)
Immobilized ligands 8a and 8b and 9a and 9b (8 mol %)
were dried under vacuum for 6 h at 110 ꢁC after they were
taken in 2 mL of dry toluene and stirred with Ti(OiPr)4
(1.5 mmol) for 2 h at room temperature under an argon
atmosphere. To the above suspension, a solution of Et2Zn
(1 M solution in hexane, 3.0 mmol) was added, cooled to
0 ꢁC, appropriate aromatic aldehydes were added
(1.0 mmol) and the resulting mixture was stirred for 24 h
at 0 ꢁC. The progress of the catalytic reaction was moni-
tored on HPLC. After completion of the reaction, the
immobilized catalyst was filtered off from the reaction mix-
ture, washed with toluene, dried under vacuum, and kept
for reuse experiments. The filtrate and combined washings
were quenched with saturated NH4Cl solution (10 mL),
washed with water, and dried over anhydrous Na2SO4.
This was then filtered and concentrated to provide a color-
less oil, which was analyzed on HPLC chiralcel OD column
to determine the enantiomeric purity.
Ordered mesoporous silicas MCM-41 was synthesized by
using cetyltrimethyl ammonium bromide (CTAB) as sur-
factant and sodium silicate as silica source in a molar ratio
1SiO2–0.33Na2O–0.5CTAB–74H2O under a hydrothermal
crystallization method, while SBA-15 was synthesized by
using triblock copolymer P123 as a structure directing
agent and TEOS as silica source.20 The calcined mesopor-
ous silicas were characterized by powder XRD, IR, N2
sorption study, SEM, and TEM analysis.
4.8. Immobilization of modified BINOL on mesoporous
silicas 7 and 70
Calcined MCM-41/SBA-15 (1.8 g) was added to the above-
synthesized solution of 6 and the suspension allowed to stir
at reflux temperature under an argon atmosphere for 48 h.
After cooling, the powder was collected by filtration,
washed successively with dry toluene, and then dried under
vacuum. Dried material was subjected to soxhlet-extraction
with dichloromethane for 24 h. Finally the sample was
dried under vacuum at 45–50 ꢁC. Yield; 1.85 g, IR (KBr)
cmꢀ1: 3439, 2959, 2857, 2358, 1635, 1439, 1251, 1084,
964, 807, 796, 689, 558, 461. Anal. Found: C, 5.0; H,
0.88.
Acknowledgments
Authors are grateful to DST and CSIR Net Work Project
on Catalysis, K. Pathak, A. Bhatt, and I. Ahmad to CSIR
(SRF) for financial support. We are also thankful to
Dr. P. K. Ghosh, the Director of the Institute, for provid-
ing necessary instrumentation facilities.
4.9. Removal of the protecting group 8a and 8b
2,20-Dimethoxy-1,10-bi-naphthalene supported silicas 7
and 70 (2.0 g) were taken in dried CH2Cl2 (20 mL) and
cooled to ꢀ78 ꢁC. BBr3 (3 mL, 3.0 mmol, 1 M solution in
CH2Cl2) was added to the cooled suspension dropwise with
continuous stirring for 2 h, after which the reaction mixture
was brought to room temperature, stirred for an additional
2 h and an aqueous saturated solution of NaHCO3 was
slowly added to it. The resulting solid was filtered off,
washed successively with water, acetone, and CH2Cl2,
and finally dried at 55 ꢁC under vacuum for 10 h. Yield;
1.93 g, IR (KBr) cmꢀ1: 3418, 2959, 2358, 1636, 1383,
1231, 1076, 963, 800, 579, 454. Anal. Found: C, 6.68; H,
0.51.
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