Heterogenization of chiral mono oxazoline ligands by grafting onto mesoporous silica MCM-41 and their application in copper-catalyzed asymmetric allylic oxidation of cyclic olefins

Article abstract of DOI:10.1016/j.jcat.2016.05.021

A series of chiral 4-oxazolinylaniline ligands 8 were conveniently synthesized on a gram scale from inexpensive and commercially available 4-aminobenzoic acid in four steps. The obtained organic chiral ligands have been covalently grafted onto ordered mesoporous silicas MCM-41 and the resulting inorganic–organic hybrid materials have been characterized by thermogravimetric analysis (TGA), differential thermal analysis (DTA), powder X-ray diffraction, BET and BJH nitrogen adsorption–desorption methods, energy-dispersive X-ray spectroscopy (EDX), CHN analysis, scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FT-IR). The catalytic and induced asymmetric effects of the chiral copper (I) complexes of these new chiral supported heterogeneous catalysts on the asymmetric allylic oxidation of cycloolefins were investigated under different conditions. Reactions using the catalyst exhibited moderate to good enantioselectivities, up to 80%, and good yields, up to 95% better than the corresponding homogeneous reaction. The catalyst could be recovered easily and reused five times without remarkable loss of reactivity, yield, or enantioselectivity. This is, to the best of our knowledge, the first heterogenization of chiral 4-oxazolinylaniline ligands on an inorganic (silica) surface and their application as a heterogeneous catalyst in the asymmetric Kharash–Sosnovsky reaction.

Full text of DOI:10.1016/j.jcat.2016.05.021

Journal of Catalysis 340 (2016) 344–353
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Heterogenization of chiral mono oxazoline ligands by grafting onto
mesoporous silica MCM-41 and their application in copper-catalyzed
asymmetric allylic oxidation of cyclic olefins
Saadi Samadi a, , Khosrow Jadidi b, , Behnam Khanmohammadi , Niloofar Tavakoli b
⇑
⇑
b
a
Laboratory of Asymmetric Synthesis, Department of Chemistry, Faculty of Science, University of Kurdistan, 66177-15175 Sanandaj, Iran
Department of Chemistry, Shahid Beheshti University, G.C., Tehran 1983963113, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of chiral 4-oxazolinylaniline ligands 8 were conveniently synthesized on a gram scale from inex-
pensive and commercially available 4-aminobenzoic acid in four steps. The obtained organic chiral
ligands have been covalently grafted onto ordered mesoporous silicas MCM-41 and the resulting inor-
ganic–organic hybrid materials have been characterized by thermogravimetric analysis (TGA), differen-
tial thermal analysis (DTA), powder X-ray diffraction, BET and BJH nitrogen adsorption–desorption
methods, energy-dispersive X-ray spectroscopy (EDX), CHN analysis, scanning electron microscopy
Received 6 April 2016
Revised 19 May 2016
Accepted 23 May 2016
Keywords:
(SEM), and Fourier transform infrared spectroscopy (FT-IR). The catalytic and induced asymmetric effects
Chiral recyclable heterogeneous catalysts
Chiral mono oxazoline ligands
Mesoporous MCM-41
of the chiral copper (I) complexes of these new chiral supported heterogeneous catalysts on the asym-
metric allylic oxidation of cycloolefins were investigated under different conditions. Reactions using
the catalyst exhibited moderate to good enantioselectivities, up to 80%, and good yields, up to 95% better
than the corresponding homogeneous reaction. The catalyst could be recovered easily and reused five
times without remarkable loss of reactivity, yield, or enantioselectivity. This is, to the best of our knowl-
edge, the first heterogenization of chiral 4-oxazolinylaniline ligands on an inorganic (silica) surface and
their application as a heterogeneous catalyst in the asymmetric Kharash–Sosnovsky reaction.
Ó 2016 Elsevier Inc. All rights reserved.
Asymmetric allylic oxidation
1
. Introduction
In recent years, many researchers have taken great interest in
Among different immobilization techniques, the formation of
covalent bonds between the support and the chiral ligand is the
most common strategy, which is to be more resistant toward
leaching during catalytic reactions, which may improve catalyst
recycling.
Chiral oxazoline ligands are a class of nitrogen ligands that have
been successfully used in the asymmetric catalysis of a variety of
reactions for the past decade [14–16]. These chiral ligands offer
the advantage that they are in general less structurally complex
than bis- and trisoxazoline ligands and are readily achieved in a
few steps from amino alcohols [17–19]. Although there is wide
application of chiral bisoxazoline ligands in catalytic asymmetric
synthesis, mono oxazoline versions have rarely had the same
impact, but some examples show that mono oxazoline gave better
enantioselectivity than bisoxazoline. However, these versatile
the heterogenizing of homogeneous catalysts, which is mainly
due to the practical advantages of easy separation of catalysts
and products by simple filtration and reuse of expensive chiral cat-
alysts. One of the main drawbacks of heterogeneous catalysts is
their often reduced activity relative to their homogeneous counter-
parts [1–4], although some contrary cases have been reported
[
5,6]. These phenomena have been related to the local density of
the catalysts on the heterogeneous material, to accessibility of
the catalysts on the materials, and to various interactions with
functional groups of the materials [3].
Inorganic solids are of great importance, especially since the
discovery of a new class of mesoporous molecular sieves such as
MCM and SBA containing reactive silanol groups, which offer large
pores and high surface area to the guest ligand–metal complex
with considerable stability under different reaction conditions
catalytic systems suffer from one major drawback:
a high
catalyst-to-substrate ratio is required (generally 1–10 mol%) and
their separation and recycling is therefore a prerequisite for their
development as useful catalysts. Regarding the preparation of mul-
tipurpose catalysts and for the above-mentioned reasons, mono
[
7–13].
⇑
Corresponding authors.
http://dx.doi.org/10.1016/j.jcat.2016.05.021
021-9517/Ó 2016 Elsevier Inc. All rights reserved.
0

S. Samadi et al. / Journal of Catalysis 340 (2016) 344–353
345
oxazoline ligands appear to be suitable candidates to heterogenize
20–22].
In 2013 our groups reported for the first time the use of
nanoparticles and nanoporous materials as additives in the pres-
ence of biaryl bisoxazoline in the asymmetric allylic oxidation of
cycloolefins [14–16]. The results showed that these materials have
a participatory role in increasing the efficiency of the reaction. The
intense effect of these additives in this reaction encouraged us to
investigate the heterogenization of chiral 4-oxazolinylaniline
ligands on mesoporous MCM-41.
materials were purchased from Aldrich, Merck, Fluka, and Sigma.
Olefins were distilled from calcium hydride before use. All solvents
for the reactions were of reagent grade and were dried and distilled
immediately before use, as follows: acetonitrile and acetone from
[
2 5
P O , methylene chloride from calcium hydride. Column chro-
matography was performed using silica gel 60 (230 ± 400 mesh)
eluted with ethyl acetate and n-hexane. TLC was performed using
silica gel 60 F256 plates with visualization by UV.
2.2. Preparation of the chiral mono oxazoline ligands 8a–8d
The allylic oxidation of olefins using peresters in the presence of
copper catalysts to give allylic esters is known as the Kharash–Sos-
novsky reaction. This reaction has been the subject of great interest
over the past decade and provides access to chiral allylic alcohols,
which are key intermediates in natural product synthesis [23–29].
In this paper, we report the preparation and characterization of the
chiral heterogeneous catalyst OX-R-MCM-41 as a novel chiral
oxazoline-based solid catalyst and employ copper complexes of
OX-R-MCM-41 in the enantioselective Kharash–Sosnovsky
reaction.
The preparation of the chiral 4-oxazolinylaniline ligands is out-
lined in Scheme 1.
2.2.1. Typical procedure for the synthesis of hydroxy amides 6a–6d
To a solution of 4-((tert-butoxycarbonyl)amino)benzoic acid 2
[
1
(
30] (S2 in the Supplementary Material) (0.71 g, 3.0 mmol) in
0 ml of anhydrous methylene chloride were added 0.62 ml
6 mmol) of oxalyl chloride and then DMF (2 drops) at 0 °C. The
reaction mixture was stirred for 4 h at 0 °C under N . The solvent
2
was removed under reduced pressure to afford the acid chloride
as a light yellow solid (0.77 g, 99%). This solid residue was then dis-
solved in 10 ml of anhydrous methylene chloride, cooled to 0 °C,
and slowly added to a stirred solution of (S)-phenyl glycinol 5a
2
. Experimental
2.1. Materials and characterization methods
(
0.45 g, 3.2 mmol) [31] and triethylamine (0.5 ml, 3.6 mmol) in
Melting points were measured on an Electrothermal 9100 appa-
10 ml anhydrous methylene chloride at 0 °C. The reaction mixture
was allowed to warm to room temperature slowly and stirred
overnight under nitrogen. TLC analysis of the reaction (50:50
EtOAc/n-hexane) confirmed the formation of the new compound
6a. After completion of the reaction, brine (10 ml) was added and
the aqueous layer was extracted with ethyl acetate (3 ꢀ 5 ml)
and dried over magnesium sulfate, filtered, and concentrated. Pro-
duct 6a was separated by flash chromatography (20–50% EtOAc/n-
hexane) as a white solid with 86% total yield. Compounds 6b–6d
were synthesized by a similar method. The total yields for 6b, 6c,
and 6d were 90%, 85%, and 92%, respectively [14].
ratus and are uncorrected. Optical rotations were measured with a
1
13
Perkin–Elmer 341 polarimeter at 589 nm. H and C NMR spectra
were recorded on a BRUKER DRX-300 AVANCE spectrometer at
3
3 6
00.13 and 75.47 MHz, 62.5 MHz in CDCl and DMSO-d using
TMS (d = 0.0 ppm) as internal standard. IR spectra were recorded
on a Bomen FT-IR-MB-series instrument. TGA–DTA analysis was
carried out from 10 to 800 °C at a heating rate of 10 °C/min in air
using a STA 503 M system from Bähr GmbH, Germany. X-ray
diffraction patterns were obtained on a STOE diffractometer with
Cu Ka radiation. Electron microscopy was performed on a TESCAN
WEGAM scanning electron microscope (SEM). The enantiomeric
excess (ee) of the products was determined by HPLC analysis using
an EC 250/4.6 Nucleocel Alpha S column. All reactions were per-
formed under dry, oxygen-free nitrogen. All reagents and starting
(S)-tert-butyl (4-((2-hydroxy-1-phenylethyl)carbamoyl)phenyl)ca
1
rbamate (6a): Mp: 194–197 °C, H NMR (300 MHz, CDCl
3
): d
H
(ppm) = 1.54 (9H, s), 4.00–4.04 (2H, m), 5.25–5.30 (1H, m), 6.68
(1H, s, NH), 6.78–6.80 (1H, d, J = 6.5 Hz, NH), 7.30–7.40 (5H, m,
Scheme 1. Synthesis of chiral 4-oxazolinyl aniline ligands 8a–8d.

3
46
S. Samadi et al. / Journal of Catalysis 340 (2016) 344–353
Ar), 7.43–7.46 (2H, d, J = 8.5 Hz), 7.76–7.79 (2H, d, J = 8.5 Hz); 13C
NMR (75 MHz, DMSO-d ): d (ppm) = 28.5, 56.3, 64.9, 80.0, 117.4,
27.3, 127.4, 128.5,128.7, 128.9, 141.9, 142.7, 143.8, 153.1, 166.2.
4.54–4.64 (1H m), 6.68 (1H s, NH), 7.25–7.34 (5H m, Ar), 7.42–
6
C
7.45 (2H, d, J = 8.3 Hz), 7.89–7.92 (2H d, J = 8.4 Hz); IR (KBr,
ꢁ1
1
cm ): 1516, 1638, 1724, 2920, 2979, 3360; MS m/z (%): 353 (M,
ꢁ
1
IR (KBr, cm ): 1516, 1630, 1704, 2632, 2977, 3355.
S)-tert-butyl (4-((1-hydroxy-3-phenylpropan-2-yl)carbamoyl)
phenyl)carbamate (6b): Mp: 167–170 °C,
CDCl ): d (ppm) = 1.53 (9H, s), 2.98–3.00 (2H, brd, J = 6.8 Hz),
100), 297 (73), 261 (42), 205 (62), 161 (33), 91 (27), 57 (61).
(
(S)-tert-butyl (4-(4-isopropyl-4,5-dihydrooxazol-2-yl)phenyl)car-
1
1
H
NMR (300 MHz,
bamate (7c): H NMR (300 MHz, CDCl
3
): d
H
(ppm) = 0.83–0.86
3
H
(3H, d, J = 6.7 Hz), 0.93–0.95 (3H, d, J = 6.6 Hz), 1.41 (9H, s), 1.78–
3
3
.66–3.72 (1H, dd, J = 10.9, 5.2 Hz), 3.77–3.82 (1H, dd, J = 10.9,
.0 Hz), 4.34 (1H, m), 6.38–6.40 (2H, brd, J = 7.1 Hz, NH), 6.74
1.80 (1H, m), 4.04–4.08 (1H, m), 4.31 (1H, m), 4.81 (1H, brs),
7.39–7.82 (5H, m); IR (KBr, cm ): 1528, 1638, 1704, 2925, 2974,
3363.
ꢁ1
(
1H, s, NH), 7.25–7.32 (5H, m, Ar), 7.37–7.40 (2H, d, J = 8.4 Hz),
7
=
1
1
.59–7.62 (2H, d, J = 8.4 Hz); 13C NMR (75 MHz, CDCl
28.3, 37.0, 53.4, 64.4, 81.1, 117.7, 126.8, 128.1, 128.3, 128.8,
3
): d
C
(ppm)
(S)-tert-butyl (4-(4-isobutyl-4,5-dihydrooxazol-2-yl)phenyl)car-
1
bamate (7d): H NMR (300 MHz, CDCl
3
): d
H
(ppm) = 0.93–0.97
ꢁ1
29.3, 137.6, 141.6, 152.3, 167.6; IR (KBr, cm ): 1513, 1627,
(6H, m), 1.25 (9H, s), 1.32–1.39 (1H, m), 1.64–1.81 (2H, m), 3.89–
3.94 (1H, m), 4.20–4.31 (1H, m), 4.40–4.46 (1H, m), 6.60–6.62
698, 2634, 2979 3355; MS m/z (%): 370 (M, 6), 279 (36), 220
13
(
57), 164 (93), 120 (72), 91 (57), 57 (100).
S)-tert-butyl(4-((1-hydroxy-3-methylbutan-2-yl)carbamoyl)
(2H, brd, J = 8.7 Hz), 7.70–7.72 (2H, brd, J = 8.6 Hz).
C NMR
(
(75 MHz, CDCl
3
): d
C
(ppm) = 22.7, 23.0, 25.5, 29.4, 45.7,53.4, 64.8,
1
ꢁ1
phenyl)carbamate (6c): Mp: 157–160 °C,
CDCl ): d (ppm) = 1.02–1.06 (6H, t, J = 6.1 Hz), 1.54 (9H, s), 2.00–
.06 (1H, m), 3.81 (2H, brs), 3.94 (1H, brs), 6.29–6.31 (1H, brd,
J = 7.2 Hz, NH), 6.7 (1H, s, NH), 7.43–7.46 (2H, d, J = 7.9 Hz), 7.72–
H
NMR (300 MHz,
72.8, 114.1, 117.5, 129.8, 149.4, 163.5; IR (KBr, cm ): 1516,
1633, 2917, 2957, 3426.
3
H
2
.75 (2H, d, J = 8 Hz). 1 C NMR (75 MHz, CDCl
3
(ppm) = 19.1,
7
1
1
3
): d
C
2.2.3. Typical procedure for the synthesis of 4-oxazolinylaniline ligands
8a–8d
9.7, 28.3, 29.3, 57.6, 64.1, 81.2, 117.8, 128.4, 128.6, 141.6, 152.2,
ꢁ1
67.9; IR (KBr, cm ): 1513, 1619, 1699, 2656, 2983, 3343.
S)-tert-butyl(4-((1-hydroxy-4-methylpentan-2-yl)carbamoyl)
A portion of 2.8 mmol (1 g) of (S)-tert-butyl (4-(4-phenyl-4,5-
dihydrooxazol-2-yl)phenyl) carbamate 7a was placed in a round-
bottom flask (25 ml) containing 6:1 dichloromethane:trifluoroace
tic acid (9 ml). The reaction solution was stirred for 3 h at room
temperature. After completion of the reaction (as determined by
TLC analysis), 10 ml of water was added. The organic layer was
washed with 10 ml of aqueous sodium hydrogen carbonate (5%)
and then dried over Na SO to yield a concentrated light yellow
(
1
phenyl)carbamate (6d): Mp: 150–152 °C,
CDCl ): d (ppm) = 0.95 (3H, brs), 0.97 (3H, brs), 1.2–1.31 (1H,
m), 1.53 (9H, s), 1.63–1.72 (2H, m), 3.60–3.65 (1H, dd, J = 10.6,
H NMR (300 MHz,
3
H
5
6
.6 Hz), 3.74–3.79 (1H, dd, J = 10.6, 3.0 Hz), 4.244.25 (1H, m),
.34 (1H, d, J = 8.0 Hz, NH), 6.87 (1H, s, NH), 7.39–7.42 (2H, d,
13
J = 8.3 Hz), 7.68–7.71 (1H, d, J = 8.3). C NMR (75 MHz, CDCl
3
): d
C
2
4
(
ppm) = 22.3, 23.1, 25.0, 28.3, 40.3, 50.4, 66.2, 81.1, 117.8, 128.1,
oil. Purification using flash chromatography (30–40% EtOAc/n-
hexane) afforded a light yellow product 8a in 85% yield. Com-
pounds 8b–8d were synthesized by a similar method. The yields
ꢁ
1
1
2
28.3, 141.6, 152.4, 167.7; IR (KBr, cm ): 1516, 1635, 1710,
860, 2957, 3445.
for other products were 84%, 93%, and 86%, respectively [30].
1
2
.2.2. Typical procedure for the synthesis of compounds 7a–7d
A flame-dried round-bottom flask with a stirrer bar was
(S)-4-(4-phenyl-4,5-dihydrooxazol-2-yl)aniline (8a):
(300 MHz, CDCl ): d
H NMR
3
H
(ppm) = 4.21–4.26 (1H, t, J = 8.0 Hz), 4.73–
charged with white hydroxy amid 6a (3.5 mmol, 1.1 g, 1 equiv),
-(dimethylamino) pyridine (0.02 g, 0.18, 0.1 equiv) and CH Cl
10 ml) under N . Triethylamine (1.1, 7.7 mmol, 2.2 equiv) was
4.79 (1H, t, J = 9.1 Hz), 5.32–5.38 (1H, m), 6.68–6.71 (2H, d,
J = 8.0 Hz), 7.28–7.36 (5H, m, Ar), 7.84–7087 (2H, d, J = 8.1 Hz);
C NMR (75 MHz, DMSO-d ): d (ppm) = 69.9, 74.7, 113.8, 126.8,
6 C
4
(
2
2
1
3
2
added to mixture of reaction. The flask was placed in ice, and a
128.7, 130.2, 131.7, 142.8, 149.8, 151.3, 166.5; MS m/z (%): 239
solution of p-toluenesulfonyl chloride (0.67 g, 3.5 mmol, 2 equiv)
(M + 1, 100), 208 (75), 180 (20), 120 (63), 89 (30), 65 (23);
D
in 4 ml of CH
stirred at room temperature for 12 h. The reaction mixture was
diluted with 10 ml of CH Cl , and upon washing with 10 ml of sat-
urated aqueous NH Cl, two layers were separated, and the aqueous
layer was extracted again with CH Cl
(2 ꢀ 5 ml). The combined
organic extracts were washed with 10 ml of saturated aqueous
NaHCO , the organic layer was dried over Na SO , and a concen-
2
Cl
2
was added. The light yellow clear solution was
[a]
20 = +40.3° (c 1.0, CH
2
Cl
2
).
(S)-4-(4-benzyl-4,5-dihydrooxazol-2-yl)aniline (8b): 1H NMR
(300 MHz, CDCl ): d (ppm) = 2.94–3.02 (1H, m), 3.22–3027 (1H,
m), 4.60–4.82 (3H, m), 6.60–6.63 (2H, d, J = 8.5 Hz), 7.19–7.34
2
2
3
H
4
1
3
2
2
(5H, m), 7.75–7.81 (2H, d, J = 8.8 Hz); C NMR (75 MHz, CDCl
3
):
d
C
(ppm) = 34.8, 70.3, 73.1, 109.9, 123.3, 124.8, 125.2, 128.7,
3
2
4
129.8, 131.8, 141.3, 151.3, 166.1; MS m/z (%): 253 (12.5, M + 1),
trated, light yellow oil was achieved and purified by column chro-
matography (25–75% EtOAc/n-hexane) to afford a pure light yellow
product 7a in 85% yield. Compounds 7b–7d were synthesized by a
similar method. The total yields were 85%, 80%, and 95%, respec-
tively [14–16].
216 (11.25), 179 (79.7), 161 (93.8), 133 (37.5), 120 (100), 106
D
(31.3), 91 (76.5), 65 (51.3); [
a
]
20 = +10.7° (c 1.0, CH
2
Cl
2
).
(S)-4-(4-isopropyl-4,5-dihydrooxazol-2-yl)aniline (8c): 1H NMR
(300 MHz, CDCl ): d (ppm) = 0.97–0.99 (3H, d, J = 6.7 Hz), 1.03–
3
H
1.1 (3H, d, J = 6.7 Hz), 2.05–2.11 (1H, m), 4.37–4.42 (1H, m),
(
S)-tert-butyl (4-(4-phenyl-4,5-dihydrooxazol-2-yl)phenyl)carba-
4.47–4.53 (1H, m), 4.68–4.75 (1H, m), 6.67–6.70 (2H, d,
J = 8.5 Hz), 7.93–7.95 (2H, d, J = 8.5 Hz). C NMR (75 MHz, CDCl ):
3
d (ppm) = 12.8, 13.8, 25.4, 67.8, 110.0, 127.8, 128.1, 143.4, 148.9
C
MS m/z (%): 204 (23.1, M), 179 (9.6), 161 (100), 133 (55.8), 120
1
13
mate (7a): H NMR (300 MHz, CDCl
3
): d
.24–4.30 (1H, t, J = 8.2 Hz), 4.76–4.82 (1H, t, J = 9.2 Hz), 5.34–
.40 (1H, dd, J = 8.4, 9.7 Hz), 6.83 (1H, s, NH), 7.328–7.36 (5H, m,
H
(ppm) = 1.54 (9H, s),
4
5
1
3
Ar), 7.44–7.47 (2H, d, J = 8.6 Hz), 7.96–7.99 (2H, d, J = 8.6 Hz);
NMR (75 MHz, DMSO-d
C
(86.5), 106 (57.7), 92 (28.8), 85.2 (19.2), 77 (40.4), 72 (75).
D
6
): d
C
(ppm) = 28.5, 56.3, 65.0, 79.9, 117.5,
[a]
20 = +27.6° (c 1.0, CH
2
Cl
2
).
1
27.4, 128.5, 128.7, 139.1, 142.0, 142.7, 153.1, 166.1; IR (KBr,
(S)-4-(4-isobutyl-4,5-dihydrooxazol-2-yl)aniline (8d): 1H NMR
(300 MHz, CDCl ): d (ppm) = 0.96–0.99 (6H, m), 1.48–1.58 (1H,
ꢁ
1
cm ): 1516, 1630, 1710, 2920, 2977, 3357
S)-tert-butyl (4-(4-benzyl-4,5-dihydrooxazol-2-yl)phenyl)carba-
3
H
(
m), 1.77–1.91 (2H, m), 4.48–4.63 (2H, m), 4.93–5.10 (1H, m),
6.65–6.67 (2H, d, J = 8.5 Hz), 7.90–7.92 (2H, d, J = 8.5 Hz). MS m/z
1
mate (7b): H NMR (300 MHz, CDCl
3
): d
.71–2.79 (1H dd, J = 13.5, 9.0 Hz), 3.23–3.30 (1H, dd, J = 13.7,
.7 Hz), 4.13–4.18 (1H, t, J = 7.6 Hz), 4.32–4.38 (1H, t, J = 8.8 Hz),
H
(ppm) = 1.54 (9H, s),
2
4
(%): 218 (M, 2.5), 203 (1), 161 (25), 120 (44), 85 (64), 71 (97), 57
D
(100). [a]
20 = +14.3° (c 1.0, CH
2
Cl
2
).

S. Samadi et al. / Journal of Catalysis 340 (2016) 344–353
347
2.3. Preparation of the chiral heterogeneous catalysts 9
3. Results and discussion
The preparation of the chiral heterogeneous catalysts 9 is out-
3.1. Synthesis of chiral 4-oxazolinylaniline ligands 8a–8d
lined in Scheme 2.
The MCM-41 and Cl-MCM-41 mesoporous silicas were synthe-
sized according to previously reported procedures [32,33] (S43 in
the Supplementary Material).
A flame-dried 100 ml Schlenk flask equipped with a stirrer bar
was charged with 2 g of modified support Cl-MCM-41 and 20 ml
of anhydrous toluene under nitrogen. A stirred solution of (S)-4-
Synthesis of chiral mono oxazoline ligands 8 can be performed
with excellent yields and enantiomeric excess from the inexpen-
sive starting material 4-aminobenzoic acid 1 (Scheme 1). The syn-
thesis starts with production of Boc-protected intermediate 2 by
the reaction of di-tert-butyl dicarbonate and triethylamine in aque-
ous dioxane in high yield [30]. Next, 4-((tert-butoxycarbonyl)
amino) benzoic acid 2 was converted to acid chloride 3 by oxalyl
chloride reagent in the presence of a catalytic amount of DMF at
0 °C. The hydroxy amides 6a–6d were obtained after treatment
of acid chloride 3 with four individual S-amino alcohols 5a–5d
[31] at 0 °C. To achieve the best procedure for the synthesis of
the desired ligand 8, we required an efficient method for cycliza-
tion of the hydroxyamides’ precursor 6a–6d. For this purpose, we
tested several methods, and in the best procedure, compounds
(
4-phenyl-4,5-dihydrooxazol-2-yl)aniline 8a (5 mmol, 0.95 g) and
triethylamine (1 ml) in anhydrous toluene (20 ml) was added
slowly to the reaction mixture. The solution was allowed to warm
and reflux with stirring overnight under N
porous (Ox-Ph-MCM-41) was collected by filtration and washed
thoroughly with CH Cl /CH OH. Then it was Soxhlet extracted with
CH Cl /CH OH (1:1) for 24 h to remove any untethered species,
and the light gray solid was dried at room temperature [32,33].
2
. The modified non-
2
2
3
2
2
3
7
a–7d were prepared at high yield by stirring the mixture of the
hydroxyamides 6a–6d overnight in the presence of p-TsCl, DAMP,
and Et at room temperature. In the final step, chiral
-oxazolinyl aniline ligands 8a–8d were achieved by removing
2.4. Enantioselective allylic oxidation of cycloolefins
3
N
4
Enantioselective allylic oxidation of cycloolefins is outlined in
the protective group from 7a to 7d in the presence of trifluo-
roacetic acid in methylene chloride (14% TFA/DCM) (Scheme 1)
Scheme 3.
The chiral heterogonous catalyst 9a was dried under vacuum at
0 °C for 4 h prior to use. To a flame-dried round-bottom flask
25 ml), the required amounts of dried chiral heterogeneous cata-
lyst 9a (100 mg), copper salt (0.55 mmol, 20 mg), and anhydrous
CH CN (4 ml) were added and stirred for 3 h at ambient tempera-
ture under N . To this solution was added phenylhydrazine
l, 0.06 mmol) with stirring. The color of the solution changed
[
14,30].
4
(
3.2. Synthesis of the chiral heterogeneous catalyst OX-R-MCM-41 9a–
9
d
3
2
For preparation of the chiral heterogeneous catalyst, MCM-41
(6 l
was prepared and characterized according to the literature. The
surface hydroxyl groups of MCM-41 were first reacted with the
methoxy groups of 3-chloropropyltriethoxysilane. In this grafting
reaction, two main methoxy groups of the silylating agent were
replaced with silanol groups of the surface [33–36]. The chloro-
propyl groups were then reacted with amino groups of chiral
from blue green to red. Next cycloolefin (5 mmol) was added.
The reaction mixture was cooled to –10 °C and then tert-butyl
p-nitrobenzoperoxoate (0.203 g, 0.85 mmol) (S63 in the Supple-
mentary Material) was added dropwise to the reaction solution
under nitrogen. The mixture was kept at ꢁ10 °C until TLC showed
to complete disappearance of perester. The chiral heterogeneous
catalyst 9a was collected by filtration and then thoroughly washed
with methylene chloride (3 ꢀ 8 ml) and air-dried. Chiral catalysts
were reused under the same conditions. After filtration the dichlor-
4
-oxazolinyl aniline ligands 8a–8d to yield new supported hetero-
geneous catalysts OX-R-MCM-41 9 (Scheme 2).
omethane phase was washed with saturated aqueous NH
10 ml). The extracted organic layers were combined and washed
with saturated aqueous NaHCO (10 ml), dried over MgSO , and
concentrated. The crude residue obtained was purified by flash
chromatography (2–10% EtOAc/n-hexanes) to yield a white solid
4
Cl
3.3. Characterization of the chiral catalyst OX-R-MCM-41 9a–9d
(
3
4
Due to the insolubility of OX-R-MCM-41 in all common organic
solvents, its structural investigation was limited to its physico-
chemical properties, thermogravimetric analysis (TGA), differential
thermal analysis (DTA), BET and BJH nitrogen adsorption–desorp-
tion methods, scanning electron microscopy (SEM), powder X-ray
1
1–15 with up to 95% yield (S63–S8 in the Supplementary
Material) [14].
O
N
O
OH
OH
O
O
O
OH
O
O
H2
N
O
O
O
OH
O
O
(S)
R
Si
Si
Cl
Cl
Si
Si
NH
N
1
(EtO)
3
Si
Cl
(S) R
4
-
OH
OH
OH
OH
OH
1
Toluene, Et
Reflux, 24 h
3
N,
M
Toluene, Reflux, 24 h
Then Soxhlet with
-
4
C
M
M
C
Then Soxhlet with
CH Cl : MeOH (1:1)
2
CH Cl
2
: MeOH (1:1)
M
2
2
NH
N (S) R
O
O
O
OX-R-MCM-41
MCM-41
Cl-MCM-41
R= a: Ph, b: Bn, c:
i
Pr, d:
i
Bu
Chiral Heterogeneous Catalyst (9a-d)
Scheme 2. Synthesis of chiral heterogeneous catalysts OX-R-MCM-41 9a–9d.
O
2
CPhNO
2
-p
2 2
O CPhNO -p
O
t-BuOO
+
(S)
Catalyst 9 ( 0.1 gr)
CuPF (CH CN) (5mol%)
CH CN (4ml), t (°C)
(S)
n
n
6
3
4
11: n=1
2: n=2
13: n=3
4: n=4
n=1
n=2
n=3
n=4
3
1
15
PhNHNH (6µL)
2
NO
10
2
1
1
,5-COD
Scheme 3. Allylic oxidation of cycloolefins in the presence of a chiral catalyst (OX-R-MCM-41).

3
48
S. Samadi et al. / Journal of Catalysis 340 (2016) 344–353
diffraction, energy-dispersive X-ray spectroscopy (EDX), CHN anal-
ysis, and Fourier transform infrared (FT-IR) spectral data.
silanols. These data confirmed the covalent attachment of oxa-
zolinyl aniline to MCM-41 (Fig. 2). TAG analysis of Cl-MCM-41
shows that the chlorine loading in the grafted material was
1
5/64% or 1.95 mmol chlorine/g of SiO
the oxazoline loading in chiral catalyst OX-R-MCM-41 was in the
range of 0.19–0.45 mmol oxazoline/g SiO (Table 1).
2
. TGA analysis shows that
3
.3.1. Spectroscopic characterization and thermogravimetric analysis
The FT-IR spectra of Cl-MCM-41 confirmed the presence of
2
organosilane and chlorine groups on the surfaces of the meso-
porous silicas with characteristic peaks (3444, 2928, and
6
ture of MCM-41 was preserved after modification. In the FT-IR
spectrum of the chiral catalyst OX-R-MCM-41, the characteristic
band at 1000–1250 cm
SiAOASi structure. The spectra showed a broad band around
3
presence of asymmetric vibration in the range 1656–1708 cm
for C@N confirmed the presence of the oxazoline group (Table 1).
ꢁ1
99 cm ) and clearly demonstrated that the mesoporous struc-
3.3.2. Textural properties of the chiral catalyst OX-R-MCM-41 9a–9d
The XRD patterns of the MCM-41 and the chiral catalyst OX-R-
MCM-41 are presented in Fig. 3. The XRD patterns of MCM-41 show
four reflections (a very intense peak (100) and three additional
high-order peaks (110), (200), and (210) with lower intensities).
This result is characteristic of hexagonal pore structures. The
(100) reflection of OX-R-MCM-41 with decreased intensity was
retained after functionalization, while the (110), (200), and
(210) reflections became weak and diffuse, which could be due to
contrast matching between the silica framework and organic moi-
eties that are located inside the mesoporous channels of MCM-41.
ꢁ1
is due to the SiAO stretching in the
ꢁ
1
200–3680 cm , which is due to adsorbed water molecules. The
ꢁ1
OX-R-MCM-41 showed the characteristic asymmetric vibration of
the CH
2
2
groups of the propyl chain of the silylating agent at around
ꢁ1
ꢁ1
958 cm . The absence of a very strong peak at 1544 cm for NH
2
bending [33–36], indicates the successful grafting of chiral 4-
oxazolinyl aniline ligands 8 onto the surface of MCM-41 (Fig. 1).
The FT-IR spectra of the chiral catalyst OX-Ph-MCM-41 9a are pre-
sented in Fig. 1.
The organic content was investigated using TGA measurements
of the samples Cl-MCM-41 and OX-R-MCM-41. As shown in Fig. 2,
the weight loss of the samples in the temperature range 150–
5
00 °C was 10.6 wt% and 15.64 wt% for Cl-MCM-41 and OX-R-
MCM-41, respectively. All samples showed a weight loss of
–2 wt% at temperatures lower than 150 °C, corresponding to des-
1
orption of physically adsorbed water. The DTA analysis data of
samples showed a two-step weight loss at 370 and 500 °C due to
the decomposition of propylaniline oxazoline from the surface of
Table 1
Density of grafted oxazoline, based on wt% N and weight loss in thermogravimetric
traces.
Entry Catalyst Wt%
Oxazoline grafted groups (base on
v
ꢀ C@N
(FT-IR)
change CHN analyst) mmol oxazoline/g SiO
2
1
2
3
4
9a
9b
9c
9d
10.7
4.8
3.9
0.45
0.19
0.19
0.28
1708
1678
1694
1682
Fig. 2. Thermo gravimetric analysis (TGA) of OX-R-MCM-41, Cl-MCM-41 and MCM-
6.2
41.
Fig. 1. FT-IR spectra of MCM-41, Cl-MCM-41, OX-Ph-MCM-41.

S. Samadi et al. / Journal of Catalysis 340 (2016) 344–353
349
Fig. 3. XRD patterns of the parent MCM-41, Cl-MCM-41 and OX-R-MCM-41.
Table 2
Textural properties of different supports.
Sample
d
100
a
0
BET surface area
(m /g)
BET total pore volume
(cm /g)
Average pore diameter
(nm)
BJH pore volume
(cm /g)
WTH
a
b
2
3
3
(Å)c
(nm)
(nm)
MCM-41
Cl-MCM-41
OX-Ph-MCM-41
4.1
4.7
4.9
4.8
5.4
5.7
1275
596
210
1.25
0.449
0.195
3.87
3.02
3.71
0.93
0.42
0.155
8.9
23.8
17.2
a
b
c
k = 2d100 sinh (k = 1.54060 Cu).
Unit cell parameter: a
0
= 2d100 3.
/p
Wall thicknesses were calculated as a
0
– pore size.
In other words, the attachment of the organic functional groups
onto the surface of the mesoporous channels tends to reduce the
scattering power (or scattering contrast) of the silicate wall.
2
The N adsorption–desorption isotherms of the functionalized
mesoporous silica are shown in Fig. 4a. The isotherm of OX-R-
MCM-41 (9a–9d) is classified as type IV, characteristic of meso-
porous materials [33–37]. A linear increase of adsorbed volume
at low pressure followed by a noticeable increase in the adsorbed
2
N adsorption–desorption is a common method of characteriz-
ing mesoporous materials. This method provides information such
as the specific surface area, average pore diameter, and pore vol-
ume. BET surface area, pore size, and pore volume for MCM-41
and the chiral catalyst OX-Ph-MCM-41 are presented in Table 2.
MCM-41 has more surface area than OX-Ph-MCM-41. In other
words, the surface area of MCM-41 decreased significantly with
an increase in 4-oxazolinyl aniline content, which may be attribu-
ted to pore blocking due to overloading with oxazoline. The pore
volume of MCM-41 showed the same trend as that of surface area,
while the average pore diameter of OX-Ph-MCM-41 is slightly
higher than that of Cl-MCM-41 (Table 2).
volume of N
observed. The adsorption at low relative pressure P/P
the monolayer adsorption of N onto the walls of the mesopores
and it does not represent the presence of any micropores. The
inflection about P/P = 0.1 in the adsorption isotherm of OX-Ph-
MCM-41 suggests the filling of the mesopore system. N adsorp-
tion in the mesopore region is saturated at about P/P = 0.3. Larger
pores are filled at higher P/P
2
and finally by a slow increase at high pressure is
0
occurs from
2
0
2
0
0
.
Morphological changes were investigated by scanning electron
microscopy (SEM) of MCM-41, Cl-MCM-41 (S44-S46 in the Sup-
plementary Material), and OX-Ph-MCM-41 (Fig. 4b). The EDX
spectrum shows the elemental composition of OX-Ph-MCM-4
(Fig. 4c).
Combining the results from XRD and N
the thickness of the silicate wall of OX-Ph-MCM-41 is 17.2 Å
Table 2).
2
adsorption–desorption,
(

3
50
S. Samadi et al. / Journal of Catalysis 340 (2016) 344–353
Fig. 4. (a) N
1.
2
adsorption–desorption isotherm of chiral OX-Ph-MCM-41 at 77 K. b). Scanning electron micrographs of OX-Ph-MCM-41. c). EDX spectrum of the OX-Ph-MCM-
4
Table 3
Effects of heterogeneous catalysts 9a–d on reaction at different temperatures.
Table 4
Effects of catalyst loading on reaction.
Entry
Catalyst
Temperature (°C)
Time (h)
Yield (%)
ee (%)
1
2
3
4
9a
9a
9a
9a
9a
9b
9c
9d
8a
25
10
0
ꢁ10
ꢁ10
ꢁ10
ꢁ10
ꢁ10
ꢁ10
3.5
24
82
195
456
185
195
186
150
95
93
85
82
60
82
70
75
30
38
52
54
70
65
60
33
33
20
Entry
CuPF
6
(mol%)
Catalyst (g)
Time (h)
Yield (%)
ee (%)
1
2
3
3
5
6
5
5
5
5
10
2.5
0.1
195
340
185
220
175
>400
82
63
82
40
65
25
70
53
35
25
45
37
0.05
0.15
0.025
0.1
a
5
6
7
8
9
0.1
a
In the absence of phenylhydrazine.
from 25 to ꢁ10 °C leads to a favorable ee% and greatly decreases
the reaction yield and reactivity (entries 1–4). Further reduction
of the reaction temperature results in a decrease in reactivity and
yield without any significant increase in enantioselectivity.
The effect of phenylhydrazine was also investigated. It was
observed that in the absence of phenylhydrazine, enantioselectiv-
ity and yield only slightly decrease but reaction rate drops consid-
erably (entry 5). In fact, phenylhydrazine acts as a reducing agent
to regenerate Cu(I) in the catalytic cycle [14,27].
The phenyl- or benzyl-substituted oxazolines in catalysts 9a
and 9b resulted in considerably higher enantioselectivities than
the other two catalysts 9c and 9d carrying alkyl substitutions
(entries 4 and 6 vs. 7 and 8). The highest enantioselectivity was
achieved by employing catalyst 9a in ꢁ10 °C (entry 4, Table 3).
For comparison, in the reaction carried out under the homoge-
neous catalytic influence of chiral 4-oxazolinyl aniline ligands 8a,
low yield (30%) and enantiomeric excess (20%) of the desired pro-
duct were obtained after 150 h (Table 3, entry 9). Therefore, the
remarkable efficiency of OX-R-MCM-41 can be explained by the
existence of multiple catalytic centers (Table 3, entries 1–8).
3
.4. Catalytic and induced asymmetric effects of the chiral copper (I)
complexes of OX-R-MCM-41 in the enantioselective Kharash–
Sosnovsky reaction
3.4.1. Optimization of the model reaction
In order to evaluate the potential of the new heterogeneous cat-
alyst 9 in the catalytic asymmetric reaction, its application in the
copper-catalyzed allylic oxidation of cycloolefins was investigated.
In a typical experimental procedure, the reactions were carried out
using cyclohexene (5 mmol) as the substrate and tert-butyl 4-
nitrobenzoperoxoate (0.85 mmol) 10 as an oxidant in the presence
of various heterogeneous catalysts 9a–9d (0.1 g) and [Cu (CH
PF (5 mol%) at different temperatures in acetonitrile. The reactions
3 4
CN) ]
6
were monitored by TLC for consumption of the perester and
stopped at the given time [14,24]. The obtained results are summa-
rized in Table 3.
Temperature dependence of yields and enantiomeric excess of
products was observed. A decrease in the reaction temperature

S. Samadi et al. / Journal of Catalysis 340 (2016) 344–353
351
Table 5
Synthesis of allylic esters 11–15 by allylic oxidation of various cycloolefins over OX-R-MCM-41 catalyst.
O
t-BuOO
catalyst 9 (0.1gr)
CuPF (5mol%)
O
2
CPhNO
(S)
2
-p
2 2
O CPhNO -p
n
6
(
S)
+
n
n=1
n=3
n=4
3
CH CN, T(°C)
NO
2
11: n=1
PhNHNH2
1
1
3: n=3
4: n=4
15
1
,5-COD
Catalyst
11
13
14
15
Time (h)
Yield (%)
ee (%)
Time (h)
Yield (%)
ee (%)
Time (h)
Yield (%)
ee (%)
Time (h)
Yield (%)
ee (%)
9
9
9
9
a
b
c
150
185
250
235
70
62
65
60
53
48
22
15
175
215
225
225
75
55
50
55
50
57
28
20
250
255
280
300
59
59
55
45
53
50
18
10
140
180
210
225
95
85
75
80
80
68
32
23
d
Fig. 5. Correlation between recyclability of OX-Ph-MCM-41 and time, yield, and ee%.
The effects of catalyst loading were also investigated. The
results described in Table 4 indicate that the enantioselectivity,
reactivity, and yield are influenced by the amount of this catalyst.
The best result was achieved when the amount of OX-Ph-MCM-41
was 0.1 g of catalyst containing 0.045 mmol of oxazoline per
active even in fifth runs, with a gradual decrease in activity, while
moderate to good enantioselectivity is still observed. The XRD,
SEM, and IR results clearly demonstrate that the mesoporous struc-
ture of catalyst 9a was preserved after 10 times recycling. The
recovery of the catalyst was very easy. The catalyst was simply
filtered from the resulting mixture, dried at 40 °C for 4 h under vac-
uum, and reused in subsequent runs. No fresh catalyst was added.
5
5
mmol of substrate (cyclohexene) and the amount of CuPF
% mmol (entry 1, Table 4) [14,24].
6
was
3
.4.2. Evaluation of the scope of the reaction
To expand the generality of this novel catalytic method, various
3
.6. Proposed mechanism for the model reaction in the presence of the
chiral heterogeneous catalysts 9a–9d
cycloolefins were tested under the optimized conditions and the
results are presented in Table 5. In the case of 1,5-cycloocadiene
in the presence of catalyst 9a, the reaction was carried out at a
higher rate and yield and attained the highest enantiomeric excess
A mechanistic rationalization for asymmetric allylic oxidation
of cyclohexene in the present of the chiral heterogeneous catalysts
9
oxygen bond in tert-butyl 4-nitrobenzoperoxoate to form Cu(II)
benzoate and tert-butoxy radicals. The tert-butoxy radical abstracts
a prochiral hydrogen atom from the allylic position of the olefin to
give tert-butyl alcohol and allylic radicals. The allylic radicals add
to the Cu(II) benzoate to generate Cu(III) benzoate, which can rear-
range via a six-membered cyclic transition state to give allylic ben-
zoate and regenerated Cu(I) catalyst [38]. Fortunately, the Cu(III)
a–9d is shown in Scheme 4. The Cu(I) species cleaves the oxygen–
(
80%) in comparison with other cycloolefins.
3.5. Recycling of the catalyst
The reusability of the chiral heterogeneous catalyst 9a (OX-Ph-
MCM-41) was also examined. As shown in Fig. 5, the catalyst was
recycled for subsequent runs. It was found that the catalyst is still

3
52
S. Samadi et al. / Journal of Catalysis 340 (2016) 344–353
-
MCM- 41
MCM- 41
PF
6
MCM- 41
PF6
-
O
O
O
O
H
O
O
Si
O
O
O
O
O
H
O
O
Si
O
O
O
O
O
H
O
O
O
O
Si
Si
O
Si
Si
O
CuPF
6
(CH
3
CN)
4
.
2
O N
HO
N
N
N
N
N
+
N
H
H
H
H
H
H
O
N
R
O
O
O
O
N
O
N
R
N
R
N
CuI
Cu(II)
R
R
R
O
.
O
+
t-BuOH
(R= a: Ph, b: Bn, c: i-Pr, d: i-Bu)
2
O N
.
PF6-
MCM- 41
MCM- 41
-
O
O
O
O
H
O
O
O
PF
6
Si
Si
O
O
O
O
H
O
O
Si
O
Si
O
N
N
H
H
O
+
N
N
H
H
2
O N
S
O
N
O
N
(
III)
Cu
O
N
R
O
N
R
R
R
CuI
O
O
2
O N
Si face
Scheme 4. Proposed pathway for asymmetric allylic oxidation of cyclohexene in the present of chiral heterogeneous catalysts 9a–9d.
intermediate has been detected by rapid injection NMR (RI-NMR)
39].
ciated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.jcat.2016.05.021.
[
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1
1
[
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