A highly efficient, enantioselective and recyclable mesoporous silica-based Mn(ii) catalyst for asymmetric oxidation of thioanisole

Article abstract of DOI:10.1039/c4ra07256a

The development of environmentally benign reactions is an important goal in synthetic organic chemistry and chemical engineering. However, catalytic enantioselective oxidations using transition-metal complexes are limited when the oxidant is hydrogen peroxide. A hydrazone based asymmetric ligand (H₂L) was used to prepare a heterogeneous catalyst by the immobilization of its manganese complex, [{Mn(H₂O)₂Cl₂}₂(H₂Btar)] (1), on mesoporous support SBA-15 via the post grafting method (H₂L = 2,3-O-4-hydroxybenzhydrazidebenzylidene-d-tartrate). The heterogeneous asymmetric catalyst 1-SBA15 was characterized by elemental analysis; small angle X-ray diffraction (SAX), scanning electron microscopy (SEM), nitrogen sorption measurement, Fourier transform infrared spectroscopy and EPR spectroscopy. The asymmetric oxidation of thioanisole (PhSMe) was achieved with hydrogen peroxide in the presence of 1-SBA15 in excellent conversion and enantioselectivity (>99% ee). Compared to a homogeneous catalyst, the heterogenized catalyst is more stable and can be recycled four times without any significant loss of activity. Immobilization of complex 1 onto SBA-15 increased the selectivity toward sulfone in the oxidation.

Full text of DOI:10.1039/c4ra07256a

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A highly efficient, enantioselective and recyclable
mesoporous silica-based Mn(II) catalyst for
asymmetric oxidation of thioanisole
Cite this: RSC Adv., 2014, 4, 48827
a
a
Sohaila Alavi, Hassan Hosseini-Monfared and Pavlo Aleshkevychb
*
The development of environmentally benign reactions is an important goal in synthetic organic chemistry
and chemical engineering. However, catalytic enantioselective oxidations using transition-metal complexes
are limited when the oxidant is hydrogen peroxide. A hydrazone based asymmetric ligand (H2L) was used to
prepare
a
heterogeneous catalyst by the immobilization of its manganese complex,
[{Mn(H2O)2Cl2}2(H2Btar)] (1), on mesoporous support SBA-15 via the post grafting method (H2L ¼ 2,3-O-
4-hydroxybenzhydrazidebenzylidene-D-tartrate). The heterogeneous asymmetric catalyst 1-SBA15 was
characterized by elemental analysis; small angle X-ray diffraction (SAX), scanning electron microscopy
(SEM), nitrogen sorption measurement, Fourier transform infrared spectroscopy and EPR spectroscopy.
The asymmetric oxidation of thioanisole (PhSMe) was achieved with hydrogen peroxide in the presence
of 1-SBA15 in excellent conversion and enantioselectivity (>99% ee). Compared to a homogeneous
catalyst, the heterogenized catalyst is more stable and can be recycled four times without any significant
loss of activity. Immobilization of complex 1 onto SBA-15 increased the selectivity toward sulfone in the
oxidation.
Received 17th July 2014
Accepted 18th September 2014
DOI: 10.1039/c4ra07256a
www.rsc.org/advances
recycle oen causes problems. One mechanism to overcome
this problem is to immobilize the asymmetric catalyst onto a
Introduction
support and the resulting heterogeneous asymmetric catalyst
can, in principle, be readily re-used.10 Heterogeneous chiral
catalysts which are friendly to the environment have the
inherent advantages of the easier handling, separation and
recovery from the reaction mixture over the homogeneous
catalysts.11 Current research on preparation of heterogeneous
chiral catalysts is very active and needed in pharmaceutical,
food, and agricultural industries.12 The Jacobsen catalyst
(Mn(III) salen complexes) is now widely used as enantioselective
epoxidation catalyst in laboratory and industrial scale.13 Various
approaches have been used for heterogenization of the chiral
Mn(III) salen catalysts (anchoring the catalysts on a solid
support,11,14–20 onto activated carbons,21 dendrimer,22 meso-
porous silica17–19,23 and encapsulation in zeolite using ship-in-a-
bottle methodology24) where the Mn(III) complexes were sup-
ported via axial/apical coordination or through covalent
bonding of the ligand to the support. All of these approaches are
interesting; however, the complex immobilization onto
supports oen requires complicated synthetic manipulations/
or the structure modication of the homogeneous catalyst,
which ultimately results in lower activity and enantioselectivity.
In general, the heterogeneous Mn(III) salen catalysts led to the
lower activities and/or enantioselectivities. Although manga-
nese complexes of many chiral ligands (in particular, the
Jacobsen catalyst) with variety of donor atoms are known today
and used comprehensively for asymmetric catalytic reactions,
Due to the great importance of the chiral compounds for the
total synthesis of natural products, pharmaceuticals and agri-
cultural agents,1 the synthesis of enantiopure epoxide and
sulfoxide building blocks has attracted special attention.
Enantiopure epoxides are highly valuable chiral molecules,
useful for the synthesis of various biologically active molecules.2
Chiral sulfoxides are an important class of compounds as chiral
auxiliaries in asymmetric carbon–carbon bond forming reac-
tions,3,4 as bioactive ingredients in the pharmaceutical industry5
and constitute chiral synthons in organic synthesis for the
preparation of biologically active compounds.6 Therefore,
design of simple and efficient chiral catalysts for an enantio-
selective epoxidation/or sulfoxidation7,8 is one of the core
research activities in asymmetric catalysis8 and requires the
preparation of readily accessible chiral metal complexes.9
Enantioselective transformations have been extensively
studied using homogeneous asymmetric catalysts for many
years. However, these catalysts do not applied signicantly in
the industrial synthesis of ne chemicals. A central reason is
that homogeneous asymmetric catalyst design requires rela-
tively bulky ligands and catalyst re-use through recovery and
aDepartment of Chemistry, University of Zanjan 45195-313, Zanjan, Islamic Republic
of Iran. E-mail: monfared@znu.ac.ir
bInstitute of Physics, Polish Academy of Sciences (PAN), Al. Lotnikow 32/46, PL-02-668
Warsaw, Poland. E-mail: pavloa@ifpan.edu.pl
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there exists no report on the use of a chiral heterogeneous
Mn(II)–tartrate catalyst. Very recently we have reported25 man-
ganese(II) complexes containing chiral and achiral dicarboxylate
ligands. In this context we are attracted to the chiral Mn(II)–
tartrate catalyst synthesized by immobilization on mesoporous
silica.
The incorporation of chiral catalyst on mesoporous silica
materials for asymmetric catalysis has attracted a great deal of
interest due to its tunable pore dimension, well-dened pore
arrangement, relatively large specic surface area and pore
volume.26 The large specic surface area and pore volume can
enhance the loading amounts of the metal catalyst and increase
its catalytic efficiency, while the tunable pore dimension and
well-dened pore arrangement can avoid the aggregation of
catalytic active species and maintain excellent stereocontrol
performance. More importantly, these mesoporous silica-
supported chiral catalysts do not swell or dissolve in common
organic solvents, they are easy and reliable to be reused via
simple precipitation or nanoltration methods. Furthermore,
they also exhibit superior thermal and mechanical stability in
catalytic process, showing a potential application in industry.
Herein, we report the results of the catalytic potential of a
Mn(II) complex by an easy to synthesis and cheap benzhy-
drazide-D-tartrate ligand immobilized onto the mesoporous
silicate SBA-15, [{Mn(H2O)2Cl2}2(HBtar)]/SBA15 (H2Btar ¼ 2,3-
(ꢀ)-O-4-hydroxybenzhydrazidebenzylidene-D-tartrate), in asym-
metric oxidation of thioanisole and alkenes with H2O2 as the
oxidant. Katsuki and Sharpless reported in 1980, a highly
enantioselective epoxidation of allylic alcohols using
a
titanium/tartrate/t-butyl hydroperoxide system.27 Kagan28 and
Modena29 described enantioselective sulfoxidation with alkyl
hydroperoxides using titanium tartrate catalysts. The selective
oxidation of suldes to sulfoxides has attracted much attention
since 1980s and there are excellent reviews on this topic.30–35
Results and discussion
Scheme 1 (A) The ligand H2Btar synthesis and (B) covalent modifica-
tion of SBA-15 silica surface by silanization reaction with 3-(chloro-
propyl)-triethoxysilane and conversion to organofunctionalized SBA-
15 silica with H2Btar followed by the formation of supported
[{Mn(H2O)2Cl2}2(HBtar)] complex (1-SBA15): (a) toluene, reflux, 6 h; (b)
H2Btar, ethanol, reflux, 3 h; (c) MnCl2$4H2O, ethanol, reflux, 3 h.
Enantiomerically pure (ꢀ)-diethyl-D-tartrate was synthesized by
treatment of D-(ꢀ)-tartaric acid with ethanol and then its dihy-
droxyl groups protected by benzaldehyde in the presence of
p-toluenesulfonic acid to produce diethyl-2,3-benzylidene-D-
tartrate in high yield. Ligand H2Btar was obtained by the
condensation reaction of diethyl-2,3-benzylidene-D-tartrate with
4-hydroxybenzhydrazide in good yield and purity (Scheme 1).
Ligand H2Btar is potentially bischelating and could coordinate
to two different metal centers by its symmetric two bidentate
units.
The infrared spectra of the SBA-15, HBtar/SBA-15 and 1-
SBA15 in the range 400–4000 cmꢀ1 are shown in Fig. 1. The
n(OH) stretching vibrations observed in the 3600–3400 cmꢀ1
region are attributed to the hydrogen-bonded silanol groups.36
The band at 1088 cmꢀ1 is assigned to the asymmetric nas(Si–O–Si)
vibrations and the band at 798 cmꢀ1 is assigned to symmetric
vibrations of (Si–O) SBA-15, while the band at 960 cmꢀ1 is
attributed to n(Si–OH) vibrations. The bands due to 1-SBA15 are
weak and masked by the Si–OH bands due to the low concen-
tration of the former in 1-SBA15.
The powder XRD patterns of pristine SBA-15 show a very
intense peak assigned to reection at (100) and two additional
peaks with low intensities at (110) and (200) reections, which
can be indexed for a hexagonal unit cell (Fig. 2). The strongest
peak for d100 ¼ 9.4 nm appears in the region 2q ¼ 0.979ꢁ. It is
observed that the intensities of peaks (110) and (200) decrease
marginally with a little shi toward lower 2q values aer the
attachment of complex [{Mn(H2O)2Cl2}2(H2Btar)] to function-
alized SBA-15; suggesting that the structure of mesoporous SBA-
15 does not collapse by immobilization.37 Some loss in the
intensities of the peaks was observed aer the attachment of
complex revealing that silylation has indeed occurred inside the
mesopores of SBA-15.38
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relate to capillary condensation steps, characteristic of the
39–41
ꢀ
mesoporous materials (20–500 A),
with completely revers-
ible nature, uniformly sized mesopores. It is well known that the
introduction of homogenous catalysts or metals on porous
supports shows a decrease in its specic surface area and pore
volume (Fig.3). The surface area of the used catalyst 1-SBA15 is
almost in the same order of magnitude with that of the fresh
catalyst.
SEM images of SBA-15 and 1-SBA15 show the primary
particles of SBA-15 present a curved cylinders form that
remained unaffected on immobilization of [{Mn(H2O)2Cl2}2
(H2Btar)] (1) complex thus conrming the retention of the
physical structure of the siliceous material (Fig. 4). The curved
cylinder-particles have a diameter of about 500–700 nm and
length up to 1–2 mm, which is typical morphology for SBA-15
mesoporous materials.42,43 Furthermore, the complex immobi-
lized SBA-15 (1-SBA15) is stable and preserved its morphology at
the end of the catalytic oxidation reaction (see the next section),
Fig. 4e and f.
The electronic spectra of [{Mn(H2O)2Cl2}2(H2Btar)] (1) and 1-
SBA15 are shown in Fig. 4. Solid UV-Vis spectrum support the
successful immobilization of the complex as the characteristic
p / p* and n / p* bands at 244 and 279 nm of homogeneous
complex 1 that they are present as broad bands for the immo-
bilized complex (Fig. 5). For complex 1, no d–d bands are
detected and this nding along with pale yellow color of the
complex suggest the existence of manganese as Mn(II).
Fig. 1 FT-IR spectra of (a) SBA-15, (b) HBtar/SBA-15 and (c) 1-SBA15.
EPR studies
Fig. 6 shows the EPR spectra recorded for the powders of
complex 1 and 1-SBA15 at liquid helium temperature. Since they
were powders, the paramagnetic centers are here randomly
orientated with respect to the magnetic eld and the spectrum
represents the envelope of all resonance lines summed over all
possible orientations. As it is seen from Fig. 6, the spectrum of
1-SBA15 exhibits intensive sextet centered at
g
¼
2.0.
Fig. 2 Small angle XRD patterns of (a) SBA-15 and (b) 1-SBA15.
The process for preparing 1-SBA15 was monitored by N2
adsorption–desorption analysis and the structural parameters
for these materials are presented in Table 1. N2 adsorption–
desorption isotherm of 1-SBA15 sample shows Type IV
isotherm, According to IUPAC classication, type IV isotherms
Table 1 Textural properties of SBA-15, SBA15-Cl and 1-SBA15
Surface area
Pore volume
Pore diameter
(nm)
Catalyst
(m2 gꢀ1
)
(cm3 gꢀ1
)
SBA15
SBA15-Cl
Fresh 1-SBA15
Used 1-SBA15
730.38
535.68
533.71
543.05
0.8761
0.7108
0.7030
0.6312
4.798
5.307
5.268
4.649
Fig. 3 Nitrogen adsorption–desorption isotherm of SBA-15, SBA15-
Cl, fresh 1-SBA15 and used 1-SBA15 after catalysis.
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Fig. 4 SEM images of (a and b) SBA-15 and (c and d) 1-SBA15 and (e and f) recovered 1-SBA15 after catalysis.
Fig.
5 UV-Vis spectra of [{Mn(H2O)2Cl2}2(H2Btar)] (1) in CH3OH Fig. 6 The EPR spectra for powders of complexes [{Mn(H2O)2Cl2}2
(creme color) and 1-SBA15 recorded after dispersion in Nujol.
`
(H2Btar)] (1) and 1-SBA15, recorded at T ¼ 5 K.
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Additionally, a much less intensive resonance can be distin- resonance absorption with the rest of Mn ions are in 3+ elec-
guished in low magnetic eld region (Hres ꢂ 1500 Oe). Such tronic state which is “silent” in the X-band EPR spectroscopy.
spectrum is proved to be associated with Mn2+ ions (3d5 elec-
tronic conguration) by many reports in the literature.44–46 It
was shown for example in the review of Griscom44 and reference
Catalytic activity
The oxidation of thioanisole with H2O2 catalyzed by 1-SBA15
was carried out in acetonitrile for 2–60 min by using 0.3 mol%
of the catalyst, 1.0 mmol of substrate and 2.0 mmol of H2O2.
The reactions were monitored by GC until completion. The
results are summarized in Table 2. Besides sulfoxide, sulfone
was also formed. The formation of sulfones is observed in
various reactions as by-products.31 Efforts were rst directed to
optimize the solvent of the reaction in the catalysis with thio-
anisole as substrate. As is obvious from entries 1–4 in Table 2, in
comparison with acetonitrile, both the substrate conversion
and sulfoxide selectivity decrease when the reaction was carried
out in chloroform, ethanol and methanol. Therefore, acetoni-
trile was used as solvent for all further studies.
therein that in case of small axial distortion as to compare with
Zeeman splitting (|D| < gmBH), the spectrum will giving rise to
the predominant absorption at g-factor close to 2. In this case
the observed sextet is the hyperne splitting structure due to
nuclear spin I ¼ 5/2 of 55Mn ion within doublet characterized by
electronic quantum numbers ms ¼ +1/2, ꢀ1/2.
The spectrum of complex 1 is much intensive than that of 1-
SBA15 and shows poorly resolved traces of ne structure located
below (Hres ¼ 400, 2000 Oe) and above (H ¼ 6300 Oe) the main
resonance line at H ¼ 3100 Oe. This spectrum is also associated
with Mn2+ ions. The difference between spectra of 1 and 1-
SBA15 could be explained by different local environment of Mn
in these complexes. Considering that the number of Mn ions
per gram is the same in both 1-SBA15 and 1 samples the much
weaker resonance in 1-SBA15 could be explained by fact that
only part of Mn ions is in 2+ electronic state, contributing to the
The reaction temperature shows a signicant effect on the
sulfoxide yield. A temperature increase from 25 ꢁC led to
decrease in the yield of sulfoxide (Table 2, entries 5–7).
Hydrogen peroxide has to be used in a controlled manner to
a
Table 2 Asymmetric oxidation of prochiral thioanisole catalyzed by 1/H2O2
H2O2/sulde
molar ratio
No.
Catalyst/solvent
Conv.b (%)
Sulfoxide selectivityb (%)
Sulfoxide ee (Congn)c
Temp. (ꢁC)
TONd
Solvent effect
1
2
3
4
1-SBA15/CH3Cl
2 : 1
2 : 1
2 : 1
2 : 1
2
20
34
35
36
98 (R)
99 (R)
100 (R)
98 (R)
25
25
25
25
14
1-SBA15/EtOH
1-SBA15/MeOH
1-SBA15/CH3CN
59
69
70
407
476
483
Temperature effect
5
6
7
1-SBA15/CH3CN
1-SBA15/CH3CN
1-SBA15/CH3CN
2 : 1
2 : 1
2 : 1
67
69
70
24
40
19
99 (R)
98.5 (R)
51 (R)
40
60
80
462
476
483
H2O2 amount effect
8
9
1-SBA15/CH3CN
1-SBA15/CH3CN
1 : 1
3 : 1
46
52
29
33
100 (R)
27 (R)
25
25
317
359
Control experiments
10
11
12
13
14
1-SBA15/CH3CN
No Cat./CH3CN
SBA15/CH3CN
SBA15-HBtar/CH3CN
1/CH3CN
no H2O2
2 : 1
2 : 1
2 : 1
2 : 1
7
100
0 (40)
50
53
58
88 (R)
—
—
100 (R)
100 (R)
25
25
25
25
25
48
—
—
—
575
0 (26)e
30
30
63
a
Reaction conditions: catalyst 1-SBA15 10 mg (contain 2.9 mmol Mn) or homogeneous 1 1.0 mg (contain 2.19 mmol), methyl phenyl sulde
b
(thioanisole) 1 mmol, solvent 3 mL, aqueous 30% H2O2 2.0 mmol, temperature 25 ꢁC, reaction time 2 min. Conversions and yields are based
c
on the starting substrate. ee, % enantiomeric excess (enantioselectivity) ¼ (|R ꢀ S|/|R + S|) ꢃ 100. Enantioselectivity was determined by chiral
GC-FID (ID-CYDEX-B column). The absolute conguration of the major isomer was determined by comparing the GC data with those observed
for R-(+)-limonene. d TON: turnover number ¼ number of moles of product formed per mole of the catalyst. reaction time 60 min.
e
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a
Table 3 Oxidation of hydrocarbons by 1-SBA15/H2O2
H2O2/sub.
molar ratio
Substrate
Catalyst/solvent
1-SBA15/CH3CN
1/CH3CN
Conv.b %
Product/selectivity %
RSO2/54 RSO/46
RSO2/3 RSO/97
Sulfoxide ee (Congn)c
100 (R)
Temp. (ꢁC)
TON
269
2 : 1
39
73
25
25
2 : 1
100 (R)
503
1-SBA15d/CH3CN
3 : 1
55
Epoxide/>99
—
80
379
a
Reaction conditions: 1-SBA15 10 mg (contain 2.9 mmol Mn), substrate 1 mmol, acetonitrile 3 mL, time 5 h. b Conversions and yields are based on
the starting substrate. c ee, % enantiomeric excess (enantioselectivity) ¼ (|R ꢀ S|/|R + S|) ꢃ 100. Enantioselectivity was determined by chiral GC-FID
(ID-CYDEX-B column). The absolute conguration of the major isomer was determined by comparing the GC data with those observed for
R-(+)-limonene. d 1-SBA15 20 mg, n-octane 0.1 g.
overcome the over-oxidation reaction to sulfone. This formation
Extension of the above oxidation reaction to other thiol also
ꢁ
may suggest the existence of a kinetic resolution process during gave high ees at room and 80 C temperatures (Table 3). The
the course of the reaction. A reduction or increase in H2O2 oxidation rate of 4-chlorobenzenethiol is lower than that of
content resulted also to the lower conversion and sulfoxide thioanisole. The activity and selectivity of homogeneous
selectivity (Table 2, entries 8 and 9).
[{Mn(H2O)2Cl2}2(H2Btar)] (1) were also higher than those of
Control experiments proved the essential presence of the immobilized complex 1. The signicant catalytic activity of 1-
catalyst 1-SBA15 and H2O2 in catalytic oxidation of thioanisole SBA15 is obviously seen in the successful oxidation of cyclo-
(Table 2, entries 10 and 11). The catalyst support SBA-15 by itself hexene (conversion 55%), Table 3.
is able to oxidize thioanisole up to 30% and the presence of the
The heterogeneity of 1-SBA15 was conrmed by the reus-
immobilized ligand H2Btar does not add extra activity (Table 2, ability of 1-SBA15 in the oxidation of thioanisole by H2O2. Aer
entries 12 and 13). Under the optimized condition homogeneous the oxidation reaction was complete, the catalyst could be
catalyst [{Mn(H2O)2Cl2}2(H2Btar)] (1) showed the conversion and recovered simply by ltration followed by washing with CH3CN.
sulfoxide selectivity of 63 and 58%, respectively (Table 2, entry The absence of Mn in the reaction solution through leaching of
14). Comparison of the immobilized and free complex 1 activity 1 was also studied by measuring manganese. Investigation of
(Table 2, entries 14 and 4) proves that the incorporation of the the ltrate aer separation of the catalyst by atomic absorption
complex mainly inside the mesoporous channels of SBA-15 method at wavelength 279.5 nm corresponding to trace
increases its activity to the expense of sulfoxide selectivity. amounts, showed only 0.003% manganese. This nding
Reduction, however, in enantioselectivity is only 2%. Although ensured that leaching of the active supported Mn(II)-compo-
immobilization can sometimes enhance the activity of homoge- nents occur in trace amounts. Thus, the obtained catalytic
neous catalysts, a common problem encountered in asymmetric results derive mainly from the heterogeneous catalyst. Table 4
catalysis is a decrease of enantioselectivity upon heterogenisa- gives the results obtained for reusing 1-SBA15 up to four times.
tion.47 Similar to our ndings, all the heterogenised catalysts Notably, this catalyst could be reused at least four times with a
developed for epoxidation of unfunctionalised alkenes exhibit total turnover number of 2408 without a decrease in enantio-
considerably lower48–51 or no higher52,25 enantioseletivity than selectivity. In contrast, a reported Mn–MCM-41–salen catalyst
their homogeneous counterparts.
suffered from a loss of enantioselectivity of 40% ee when reused
a
Table 4 Catalytic oxidation of methyl phenyl sulfide by used catalyst and H2O2
Catalyst
Conv.b (%)
Selectivityb (%) sulfoxide/sulfone
ee (Congn)c
Fresh
70
70
69
70
70
36 : 64
37 : 63
36 : 64
35 : 65
36 : 64
98% R
Reused-1
Reused-2
Reused-3
Reused-4
100% R
100% R
100% R
100% R
a
Reaction conditions: 1-SBA15 10 mg (contain 2.9 mmol Mn), methyl phenyl sulde (thioanisole) 1.0 mmol, CH3CN 3 mL, aqueous 30% H2O2 2.0
mmol, temperature 25 ꢁC, reaction time 2 min. b Conversions and yields are based on the starting substrate. c ee (% enantiomeric excess), see Table
2 for detail.
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for the rst time.55 Li and co-workers53 reported the immobili- functionalized SBA-15 material, SBA15-Cl were synthesized
sation of a variant of the Sharpless epoxidation catalyst on according to the procedure described already.59
MCM-41 and silica. This was achieved by reacting tartaric acid
The reaction products of oxidation were determined and
with 3-aminopropyltriethoxysilane using a number of steps and analyzed by HP Agilent 6890 gas chromatograph equipped with
the triethoxysilane moiety used to attach the modied tar- a HP-5 capillary column (phenyl methyl siloxane 30 m ꢃ 320 mm
tramide to the silica surface. The nal step was addition of the ꢃ 0.25 mm) with ame-ionization detector. The enantiomeric
titanium centre in the form of titanium tetraisopropoxide to excess (ee) was determined by chiral GC (HP 6890 GC) using a
give the catalyst. The turnover numbers were 10–14 (15 for the SGE-CYDEX-B capillary column (25 m ꢃ 0.22 mm ID ꢃ
homogeneous reaction) and the ees 78–86 (83% for the homo- 0.25 mm). Temperatures: column 50 ꢁC, step 10 ꢁC minꢀ1, nal
geneous reaction). Unfortunately no catalyst reuse data and the 150 ꢁC, injector 250 ꢁC, detector 250 ꢁC; mobile phase nitrogen,
catalyst stability were reported. Various homogeneous and rate 0.7 mL minꢀ1. Retention time for sulfoxide: Rt (S)-
heterogeneous mono- and dinuclear Mn-catalysts have been enantiomer: 17.9 min; Rt (R)-enantiomer: 19.1 min. Sulfone:
used for the oxidation of cyclohexene to cyclohexene oxide54–57 Rt: 26.5 min.
with H2O2 and t-BuOOH. However, none of the reported cata-
1H NMR spectra were recorded by use of a Bruker 250 MHz
lysts shows such a high activity or/and selectivity as complex 1. spectrometer. UV-Vis spectra of solution were recorded on a
In addition to the chemical simplicity and easy synthesis of Shimadzu 160 spectrometer. Chemical analyses of the catalysts
complex 1 and 1-SBA15, they have also the ability to limit the were performed by atomic absorption (Varian spectrometer
H2O2 disproportionation reaction to a great extent relative to AAS-110) upon chemical attack of the material using concen-
most Mn(II)/Mn(III) compounds, the H2O2/acetic acid/[LMn trated HNO3. Microanalyses were carried out using a Heraeus
(m-O)3MnL][PF6]2 system (L ¼ trimethyl-1,4,7-triazacyclononane) CHN–O– Rapid analyzer. IR spectra were recorded using Perkin-
which needs 4.6 to 13.9 equiv. of H2O2(ref. 58) and [Mn Elmer 597 and Nicolet 510P spectrophotometers. Powder X-ray
(m-terph) (H2O)2]n ([H2O2]/[substrate] ¼ 10) (terph ¼ tere- diffraction patterns were collected at the Bruker, D8
phthalate).26 Despite their power as a synthetic tool and abun- ADVANCE, Germany, wavelength 1.5406 A (Cu Ka), voltage:
ꢀ
dant use in academic research, oxidation reactions as a whole 40 kV, current, 40 mA. The size and morphology of solid
comprise as little as 3% of the reactions used on a preparative compounds were recorded by using a Hitachi F4160 scanning
scale in the pharmaceutical industry.58 Perhaps the greatest electron microscope (SEM) operated at an accelerating voltage
factor inuencing the hesitation to employ oxidation reactions of 10 KV and the textural properties determined from N2
on a large scale is the safety of these processes. Using less amount adsorption isotherms measured on a Belsorp mini II (Japan)
of H2O2 improves oxidation process safety in large scale instrument. The electron paramagnetic resonance (EPR)
syntheses.
measurements were carried out using Bruker EMX spectrometer
working at xed frequency 9.25 GHz (X-band) with Oxford
Instruments helium-ow cryostat operating in the temperature
range from 3.8 K to 300 K. A 100 kHz magnetic eld modulation
Conclusions
The highly efficient syntheses of [{Mn(H2O)2Cl2}2(H2Btar)] (1) and phase sensitive detection were used to record the derivative
and 1-SBA15 have been developed. The catalytic oxidation of of the absorbed microwave power.
thioanisole indicates that the asymmetric oxidation using
homogeneous and SBA-15 immobilized chiral manganese
Synthesis of (ꢀ)-diethyl-D-tartrate
complex as catalysts and hydrogen peroxide as oxidant is
A 50 mL round-bottomed ask equipped with a condenser was
possible, with good results. The asymmetric sulfoxidation and
charged with 0.15 g (1 mmol) of D-(S,S)-(ꢀ)-tartaric acid, 7.5 mL
epoxidation of alkenes were achieved successfully. The use of
of 96% ethanol, 4.9 mL of chloroform and 0.123 mL of HCl
chiral heterogeneous catalysts under ambient conditions offers
(1 M). The stirred mixture was heated at reux for 60 h so that
several advantages compared with their homogeneous counter-
no more water separated. A dark yellow colored oily product was
parts, such as ease of recovery and recycling and enhanced
obtained. Yield: 99.6% (0.205 g). IR (KBr, cmꢀ1): 3467 (s, vb),
stability. Immobilization of complex 1 onto SBA-15 increased
1
2983 (s), 1747 (vs.). H NMR (250 MHz, CDCl3): d ¼ 1.27, 1.30,
the selectivity toward sulfone in the oxidation.
(2 t, 6H, J ¼ 7.1, 2 CH2CH3), 4.25, 4.52 (2 q, 4H, J ¼ 7.1,
2 CH2CH3), broad 4.90 (s, 2H, 2 OHCH), 6.5 (s, 2H, OH). 13C
Experimental
NMR (62.90 MHz, CDCl3) d: 13.9, 14.1 (2C, CH3), 62.5, 62.6 (2C,
CH2), 72.1 (2C, CH–OH), 171.6, 171.7 (2C, C]O).
D-(ꢀ)-tartaric acid, tetraethyl orthosilicate (TEOS), poly
(ethylene oxide), poly(propylene oxide), poly(ethylene oxide),
Synthesis of diethyl-2,3-O-benzylidene-D-tartrate
Pluronic P123 [EO20–PO70–EO20], 3-chloropropyl triethoxysilane
(CPTES), benzaldehyde, 4-hydroxy-benzoic acid hydrazide and This compound was prepared following a similar procedure
all other reagents were obtained from commercial sources reported in the literature.60 A 50 mL, round-bottomed ask was
(Merck or Fluka) and were used as received without further charged with (ꢀ)-diethyl-D-tartrate (206 mg, 1 mmol), benzal-
purication. Aqueous 30% hydrogen peroxide was used and its dehyde (0.106 g, 1 mmol), cyclohexane (10 mL) and p-toluene-
exact concentration was determined before use by titration with sulfonic acid monohydrate (5.6 mg, 0.029 mmol). The
standard KMnO4. Mesoporous SBA-15 and chlorine- homogeneous mixture was reuxed for 16 h. The solution was
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allowed to cool to room temperature and then was concentrated H2Btar. Then to a suspension of the resulting solid (1 g) in
by rotary evaporation. The residual yellow oil was dissolved in absolute ethanol (40 mL) a solution of MnCl2$4H2O (0.8 g) in
10 mL of diethyl ether, transferred to a separatory funnel and absolute ethanol (25 mL) was added. This was reuxed for 3 h.
washed with saturated aqueous potassium bicarbonate (5 mL) The white solid, 1-SBA15, was separated by ltration, was dried
and water (10 mL). The ethereal layer was dried over magne- and Soxhlet extracted with absolute ethanol to remove any
sium sulfate and ltered. The solution was concentrated by unreacted manganese from the surface. Analyses for SBA15-Cl:
rotary evaporation, followed by removal of solvent. The solid C, 2.97; H, 0.75%. Anal. found for 1-SBA15: C 8.64, H 1.42, N,
product was collected by suction ltration through a sintered- 1.61, Mn 3.24%.
glass funnel to give diethyl 2,3-O-benzylꢁidene-D-tartrate as
yellow crystals. Yield: 87% (0.256 g). Mp 45 C. IR (KBr, cmꢀ1):
3368 (vb, w), 3237 (w), 2982 (s), 2931 (m), 1754 (vs.), 1599 (s). 1H
NMR (250 MHz, CDCl3) d: 1.32, 1.35 (2 t, 6H, J ¼ 7.1, 2 CH2CH3),
4.23, 4.28 (2 q, 4H, J ¼ 7.1, 2 CH2CH3), 4.83, 4.95 (2 d, 2H,
J ¼ 4.0, 2 CHO), 6.16 (s, 1H, CHPh), 7.40, 7.58 (2 m, 5H, C6H5).
Acknowledgements
The authors are grateful to the University of Zanjan, Iran
National Science Foundation (INSF 92036382) and Polish
Academy of Sciences (PAN) for nancial support of this study.
Synthesis of 2,3-O-4-hydroxybenzhydrazidebenzylidene-D-
tartrate (H2Btar)
Notes and references
A mixture of 2.56 g (8.7 mmol) diethyl-2,3-O-benzylidene-D-
tartrate, 2.65 g (17.4 mmol) 4-hydroxybenzhydrazide and 0.02 g
(0.105 mmol) p-toluenesulfonic acid monohydrate in benzene
(15 mL) was reuxed for 5 days. Aer evaporation of the solvent,
it was again reuxed for 24 h with absolute ethanol. The product
2,3-O-4-hydroxybenzhydrazidebenzylidene-D-tartrate (H2Btar)
was separated as a white solid. Yield: 46.7% (2.06 g). IR (KBr,
cmꢀ1): 3460 (w), 3316 (m), 3269 (m), 3196 (s), 3043 (m), 1641
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(vs.), 1615 (vs.). H NMR (250 MHz, DMSO) d: 4.39 (s, Ar–OH),
6.77, 6.85 (2 d, 2H, J ¼ 8.0, 2 O–CH–CO), 7.41–7.82 (m, 13H,
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Synthesis of complex [{Mn(H2O)2Cl2}2(H2Btar)] (1)
H2Btar (0.020 g, 0.04 mmol) and manganese(II) chloride tetra-
hydrate (0.016 g, 0.08 mmol) were dissolved in absolute ethanol
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solvent was removed under vacuum and a dark yellow colored
product resulted. Yield: 68% (0.0237 g). IR (KBr, cmꢀ1): 3456
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1647 (s, C]O), 1609 (s, C]O), 1569 (s), 1507 (vs., C]C). Anal.
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830.21): calc. C 36.17, H 3.64, Mn 13.23%; found C 36.00, H
3.14, Mn 13.25%
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48834 | RSC Adv., 2014, 4, 48827–48835
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