Immobilized dimeric chiral Mn(III) salen complex on short channel ordered mesoporous silica as an effective catalyst for the epoxidation of non-functionalized alkenes

Article abstract of DOI:10.1016/j.tet.2012.05.022

Chiral dimeric Mn(III) salen complex with 1R, 2R-(-)-diaminocyclohexane collar was immobilized on short channel large pore sized silica through a long linker of {(CH₂)₃-NH-melamine-piperazine} to investigate its performance in enanti

Full text of DOI:10.1016/j.tet.2012.05.022

Tetrahedron 68 (2012) 6314e6322
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Immobilized dimeric chiral Mn(III) salen complex on short channel
ordered mesoporous silica as an effective catalyst for the epoxidation
of non-functionalized alkenes
*
Tamal Roy, Rukhsana I. Kureshy , Noor-ul H. Khan, Sayed H.R. Abdi, Arghya Sadhukhan, Hari C. Bajaj
Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSMCRI), Council of Scientific and Industrial Research (CSIR), G. B. Marg,
Bhavnagar 364 002, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Chiral dimeric Mn(III) salen complex with 1R, 2R-(ꢀ)-diaminocyclohexane collar was immobilized on short
Received 7 March 2012
Received in revised form 4 May 2012
Accepted 6 May 2012
2 3
channel large pore sized silica through a long linker of {(CH ) eNHemelamineepiperazine} to investigate
its performance in enantioselective epoxidation of chromenes, indene, styrene and cis
b
-methyl styrene in
ꢁ
the presence of pyridine N-oxide (PyNO) as an axial base using aqueous NaOCl as an oxidant at 0 C. The
immobilized catalyst system showed high turnover frequency (TOF) and enantioselectivity for the smaller
and bulkier alkenes like styrene, indene, 2,2-dimethylchromene and 6-cyano-2,2-dimethylchromene (ee
up to 98%). These results are the best reported for heterogeneous catalyst under biphasic reaction condi-
tions and were comparable to the dimeric Mn(III) salen system under homogeneous condition. The per-
formance of the immobilized catalyst was retained for six reuse experiments. This protocol was extended to
the synthesis of an antihypertensive drug (S)-Levchromakalim (ee 98%) at 1 g level.
Available online 14 May 2012
Keywords:
Ordered mesoporous silica
Dimeric chiral Mn(III) salen complex
Enantioselective
Chiral epoxides
Non-functionalized alkenes
Levcromakalim
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
linked with the support.17 In an alternative approach, recyclability
of chiral metal complexes under homogeneous condition is repor-
Asymmetric epoxidation of non-functionalized alkenes is an
important area of current interest due to its ability to engender up
ted by means of fine tuning in the solubility of the catalyst resulting
in precipitation of the catalyst by the addition of a solvent, in which
1
18
to two stereogenic centres at one go with construction of a highly
the catalyst is insoluble in a post catalytic work up step. This tactic
reactive three membered oxirane ring,2 consequently make them
immensely valuable as intermediates for several bioactive com-
is particularly interesting due to the creation of more than one
reactive centre that often worked in co-operation to improve
product yield and enantioselectivity with the catalyst recyclability.
However, under homogeneous condition still the separation of the
catalyst and the product from the reaction medium is still not
hassle-free vis- aꢀ -vis heterogeneous condition where the catalyst
3
,4
pounds as well as in pharmaceuticals. In early 90s Jacobsen and
Katsuki got the noteworthy breakthrough by designing of chiral
Mn(III) salen complexes, which are still most handy catalysts for the
production of epoxides from non-functionalized alkenes in high
5
e7
yield and optical purity.
However, for the sustainability of the
and the product can be separated conveniently by filtration or
centrifugation. In the past few years mesoporous organ-
iceinorganic hybrids have attracted much attention due to their
potential application as a good support for immobilization of op-
tically pure metal complex and hence their successive application
process, efficient catalyst recovery and recyclability is required that
can be achieved by supporting these catalysts on solids in order to
harness the virtues of homogenous as well as heterogeneous ca-
talysis. In this direction, immobilization of chiral Mn(III) salen
complexes have been reported by anchoring them on inorganic
19
in chiral catalysis. Keeping the above findings in observance we
have designed a melamineepiperzaine based dinuclear Mn(III)
supports,8
e12
encapsulation in zeolite cages,
8,13
vander wall’s
14
0
wrapping in a polysiloxane membrane, immobilization on or-
salen Complex 1 immobilized on aminopropyl functionalized
ganic polymer,8 use of an ionic liquid etc. Still in many cases the
activity and enantioselectivity are compromised and invariably
leaching of active metal complex was observed if it is not covalently
,15
16
mesoporous silica (AFMS) and successfully used the same in
asymmetric epoxidation reaction of non-functionalized alkenes.
The purposeful design of such kind of catalyst allowed us to check
the well-established fact of co-operative interaction in heteroge-
neous condition and also the advantage of recyclability protocols.
*
Corresponding author. Fax: þ91 0278 2566970; e-mail addresses: rukhsana93@
yahoo.in, rukhsana93@yahoo.co.in (R.I. Kureshy).
2 3
The use of long linker {(CH ) eNHemalamineepiperazine} to hook
0
040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.05.022

T. Roy et al. / Tetrahedron 68 (2012) 6314e6322
6315
0
the Jacobsen type dimeric Mn(III) salen complex also helped in
hinging out the catalytically active site in a reasonably good dis-
tance from the silica support. This immobilized catalyst was used
for enantioselective epoxidation of non-functionalized alkenes viz.,
complex 1 (derived from 1S,2S-(þ)-1,2 diaminocyclohexane collar)
was used to catalyze asymmetric epoxidation of 6-cyano-2,2-
dimethylchromene (1 g scale) for the synthesis of (S)-Levchroma-
kalim (antihypertensive drug) in good yield and 98% ee.
2
0
styrene, cis
b
-methyl styrene, indene and chromenes using NaOCl
ꢁ
as an oxidant in the presence of PyNO as an axial base at 0 C.
Excellent yield (up to >99%) of enantio-pure epoxides along with
high chiral induction (ee, up to 98%) in the case of indene and 6-
cyano-2,2-dimethylchromene were achieved. The present immo-
bilized complex 1 worked very well for six cycles without any
significant loss of activity and enantioselectivity. The immobilized
2. Results and discussion
Non-C symmetrical Mn(III) salen complex covalently bounded
2
on ordered mesoporous silica 1 was synthesized according to the
Scheme 1.
Scheme 1. Synthesis of chiral dimeric Mn(III) salen complex immobilized on ordered mesoporous solid support.

6316
T. Roy et al. / Tetrahedron 68 (2012) 6314e6322
Thisstrategygivesmanyfoldadvantagesoverphysicallyentrapped
bilized inside the channels of mesoporous silica material without
disturbing its periodic nature.
10c
salen complexes (1) covalent linkage prevents leaching of active
complex, (2) presence of melamine-piperazine linker provides suffi-
cient space between the wall of silica material and the active complex,
thereby providing flexibility to the active complex akin to homoge-
nous system, (3) allows two Mn(III) salen units close enough to show
cooperative interaction, (4) large pore sized silica material with well-
defined channels of shorter length minimizes the diffusional con-
strains for the reactants and product, and (5) the presence of active
complex inside the channels may provide confinement effect for
higher enantioselectivity in the product. The immobilized chiral
Mn(III) salen complex 1 and its precursors AFMS, AFMS-MP were
characterized by appropriate physicoechemical methods, such as
The PXRD pattern of the mesoporous support material AFMS
shows peaks corresponding to the reflection along 100, 110 and 200
planes indicating a uniform hexagonal lattice structure as reported
2
2
earlier. The intensities of these peaks however, gradually di-
minished on the attachment of linker AFMS-MP followed by Mn(III)
salen complex (see supplementary data, Fig. S2), which further
supported successful loading of the Mn(III) salen complex 1 through
linker inside the channels of the mesoporous material keeping the
mesoporosity intact.
SEM images of AFMS show hexagonal disc like morphology (1 to
<2
mm), which remain intact on further functionalization with
13
solid state UVevis spectroscopy (DRS), IR-, solid state NMR ( C and
linker and Mn(III) salen in AFMS-MP and complex 1, respectively,
implying thereby that the silica material has robust mechanical,
thermal and chemical stability (Fig. 2, AeC). TEM images (Fig. 2,
DeF) for all these samples distinctly show the presence of hexag-
onal pore structure when viewed along the short axis (average
2
9
2
Si), LC-MS, SEM, TEM, XRD, N adsorption and desorption, ICP and
TGA.
2
.1. Characterization of the support and the catalyst
2
2
length of the channels >100 nm to <300 nm ) of the material and
equidistant parallel fringes from side view. The presence of these
equidistant parallel fringes and uniform hexagonal lattice struc-
2
(
.1.1. Textural and chemical properties of the supported material
AFMS), AFMS with melamine piperazine linker (AFMS-MP) and im-
2
3
ture is in agreement with the finding of PXRD.
mobilized Mn(III) salen complex 1. The N adsorptionedesorption
2
The loading of the homogeneous Mn(III) salen complex in the
supported complex 1 was found to be 21 mg/100 mg as calculated
by UVevis spectroscopic measurement, ICP and TGA analysis.
isotherms (Fig. 1) of the AFMS exhibited representative type IV
isotherms,21 with hysteresis loops typically identical to that of or-
dered mesoporous materials with narrow pore size distribution
centred around 10.5 nm similar to periodic hexagonal mesoporous
silica material. Upon functionalization with linker followed by
the attachment of unsymmetrical Mn(III) salen complex D, BET
surface area and pore diameter (Fig. 1) decreased in the order
AFMS>AFMS-MP>complex 1 (Table 1), but isotherm properties
remained unchanged (see supplementary data, Fig. S1). This clearly
suggests that Mn(III) salen complex has been successfully immo-
2.2. FTIR studies onAFMS, AFMS-MPandimmobilizedcatalyst1
FTIR spectra of the AFMS show characteristic bands at 2931 and
ꢀ
1
1630 cm corresponding to
2
n(CH ) of the propyl arm of the sily-
lating agent and OeH bending vibration band, respectively. The
ꢀ
1
additional bands at 1546, 1449 and 1361 cm
for melamine
0
0
.25
.20
1000
800
600
400
200
0.15
0
0
0
.10
.05
.00
0
20
40
60
80 100 120 140 160 180 200
Pore Width (nm)
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P )
2
Fig. 1. N adsorptionedesorption isotherm and pore width distribution profile (inset) of mesoporous support material (AFMS).
ꢀ
1
functionality and at 2952 cm
2
for piperazine n(CH ) indicate
successful attachment of linker in AFMS-MP. These peaks were
accentuated when AFMS-MP was allowed to react with D showing
the successful immobilization of Mn(III) salen complex 1 (Fig. 3).
Table 1
Textural parameters for AFMS, AFMS-MP and complex 1
Compound BJH pore Total pore BET surface Homogeneous Mn(III)
ꢁ
3
2
width (A) volume (cm /g) area (m /g) salen complex loading
mg/100 mg)
2.3. UVevis. DRS studies on AFMS, AFMS-MP and immobilized
catalyst 1
(
AFMS
75
1.21
1.04
0.85
603
543
433
d
d
21
AFMS-MP 66
Complex 1 55
Fig. 4 shows the UVevis. DRS spectra of AFMS (a), linker
modified silica AFMS-MP (b) and the immobilized complex 1 (c).

T. Roy et al. / Tetrahedron 68 (2012) 6314e6322
6317
Fig. 2. SEM and TEM images of AFMS (A, D), AFMS-MP (B, E) and complex 1 (C, F).
1983
1863
681
2359
a
3789
1630
2
931
2360
798
b
c
959
3419
2361
2866
2952
1361
1449
803
804
462
%T
3
425
421
2866
953
2
1361
1
546
1447
1504
3
1081
1545
458
1
076
1
078
459
4
000.0
3600
3200
2800
2400
2000
1800
1600
1400
1200
1000
800
600
400.0
cm-1
Fig. 3. FTIR spectra of AFMS (a), AFMS-MP (b) and complex 1 (c).
UVevis spectrum of AFMS showed no distinct band in the entire
UV-visible region, while AFMS-MP showed bands around 248
2.4. NMR spectra of AFMS-MP
and 309 nm due to the
p/p* and n/p* transitions of mela-
Due to the paramagnetic nature of Mn(III) salen complex a di-
mine functionality. In catalyst 1, additional ligand to metal charge
transfer (LMCT) band centred at 436 nm and d/d transition
band around 509 nm demonstrated successful immobilization of
rect confirmation for its grafting on silica material by NMR analysis
13
1
was not possible. However, its precursor AFMS-MP in C{ H} CP-
MAS (Fig. 5a) showed a peak at 166.7 ppm due to the aromatic
carbons of the melamine units, and 44.3 ppm due to the piperazine
10e,12c
Mn(III) salen complex onto mesoporous silica material.

6318
T. Roy et al. / Tetrahedron 68 (2012) 6314e6322
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
2.5. LC-MS spectra of AFMS-MP and complex 1
To confirm the presence of dimeric salen complex onto the solid
mesoporous support through the linker molecule, LC-MS was taken
for both AFMS-MP and complex 1 by means of HF treatment fol-
lowed by extraction of the organic compound with the help of DCM.
After evaporation of DCM the solid obtained was tested on LC-MS.
Peak at m/z value of 391.46 and 1394.29 confirms the presence of
melamineepiperazine linker AFMS-MP and dimeric salen ligand C
on solid support (see supplementary data, Fig. S3 and Fig. S4).
c
b
a
2.6. Enantioselective epoxidation of non-functionalized
alkenes using NaOCl as an oxidant
2
00
300
400
500
600
700
800
After successful characterisation of the dimeric Mn(III) salen
complex 1 immobilized on mesoporous silica support, was used as
heterogeneous catalyst for asymmetric epoxidation of non-
Wavelength (nm)
Fig. 4. UVevis (DRS) spectra of AFMS (a), AFMS-MP (b) and complex 1 (c).
functionalized alkenes viz, styrene, cis
b-methyl styrene, indene
and various chromenes in the presence of PyNO as an axial base and
ꢁ
NaOCl as an oxidant at 0 C in dichloromethane.
The data in Table 2 show that the immobilized catalyst 1 effi-
ciently promoted the enantioselective reaction of styrene (STR), cis
b
(
-methyl styrene (MSTR), indene (IND), 2,2-dimethylchromene
CHR), 6-cyano-2,2-dimethylchromene (CN-CHR), 6-nitro-2,2-
dimethylchromene (NO -CHR), 6-methoxy-2,2-dimethylchromene
MeO-CHR) and spiro[cyclohexane-1,2-[2H][1]chromene] (Cy-
2
(
CHR) (conversion, 90/99%; Table 2) to give respective epoxides in
good to excellent enantioselectivity (ee, 70e98%) in 8e9 h. Signifi-
cantly, with catalyst 1, the enantioeinduction and TOF values (Table
2) were high with all the alkenes used in the present study, which is
comparable tothe results obtained with our earlier reported dimeric
salen in a macrocycle that showed cooperativity under homoge-
1
8a,18b
neous condition
and better than earlier immobilized mono-
8
,9c,10e,12a,12b,20
meric Mn(III) salen complexes.
These results suggests
that the two Mn(III) salen units in the catalyst 1 are in close prox-
imity but dangling reasonably away from the support walls, thereby
creating an environment akin to homogeneous system. The salen
v
units in the catalyst 1 still require PyNO to stabilize Mn ]O in-
termediate in order to show high conversions and enantioselecti-
10e,12b
vities
as in its absence, the catalyst 1 epoxidized indene (Table
2, entry 3) with low conversion (45%) and ee (72%). A distinct ad-
vantage of the present mesoporous silica over other mesoporous
silica reported earlier to support Mn(III) salen for the epoxidation of
non-functionalized alkenes lies in higher conversions and ees even
8
,9c,10e,12a,12b,20
for bulkier alkenes (e.g., indene and chromenes),
possibly due to large space available inside the short channels.
Control experiments with mesoporous silica supported material
AFMS and AFMS-MP with indene as representative substrate
showed negligible catalytic activity to give epoxide in trace amount
(Table 2, entry 4, 5). Hence, it can be safely concluded that the cat-
alytic activity is entirely due to the chiral dimeric Mn(III) salen
complex 1 immobilized on the mesoporous silica. To study the
leaching of the chiral metal complex or de-coordinated Mn from the
immobilized catalyst 1, the post epoxidation reaction mixture was
analyzed by ICP, which showed no trace of manganese. Moreover,
after the first catalytic run of epoxidation reaction of indene as
representative substrate, the catalyst was separated by centrifuga-
tion. Fresh reactants are added to the supernatant. Gas chromato-
graphic analysis of the reaction mixture showed no further
formation of the product, indene epoxide.
The recyclability of the immobilized catalyst 1 was carried for the
epoxidation of indene as a representative substrate. After the first
run of the epoxidation reaction, the catalyst was separated by cen-
trifugation. The separated catalyst was washed thoroughly with
dichloromethane, dried, and subjected to another cycle with fresh
reactants under similar epoxidation conditions. The recovered
Fig. 5. Solid state CP-MAS 13C NMR (a) and 29Si-NMR (b) spectra of AFMS-MP.
units. The two peaks at 23.17 and 10.06 ppm were assigned to the
aliphatic carbon atom of the aminopropyl linker while its third
peak due to CeN was merged with CeN peaks of piperazine at
w44 ppm24 These signal clearly provided evidence for the in-
corporation of the linker group on silica matrix.
2
9
The Si-MAS spectra of AFMS-MP shows three sets of broad
peak in between ꢀ102 and ꢀ112 ppm consisting of Q2, Q3 and Q4
silicon centres, which indicates that the silane is successfully
grafted to the surface (Fig. 5b).

T. Roy et al. / Tetrahedron 68 (2012) 6314e6322
6319
Table 2
Product yield and ee data for epoxidation of non-functionalized alkenes catalysed by complexes 1e4 in presence of PyNO as an axial base with NaOCl as an oxidanta
ꢀ1j
Entry
Catalyst
Substrate
STYf
% Yieldb
Time (h)
Ee (%)
TOFꢂ10ꢀ3
1.74
1.54
0.69
d
s
c
1
2
3
Complex 1
Complex 1
Complex 1
AFMS
AFMS-MP
Complex 1
Complex 1
Complex 1
Complex 1
Complex 1
Complex 1
>99
>99
45
2
4
99
92
90
8
9
9
9
9
9
9
9
9
70
92
72
d
g
d
IND
i
INDg
IND
IND
d
4
5
6
7
8
9
d
96e
d
h
CHR
1.52
1.42
1.39
1.51
1.54
1.25
h
e
Cy-CHR
90
98
h
e
CN-CHR
h
MeO-CHR
98
91
85
98
1
1
0
1
MSTYg
CN-CHR
>99k
90
9
10
0
l
ꢁ
Reaction was performed using (S,S)-form of the catalyst in CH
2
Cl
2
(40 mL) with catalyst (0.2 mmol), substrate (10 mmol), PyNO (1.3 mmol), aqueous NaOCl (27.5 mmol) at 0 C.
a
ꢁ
Reactions were performed in CH
Based on GC.
Chiral capillary column GTA-type.
Chiral HPLC OB column.
Chiral HPLC OD Column.
Epoxide configuration R.
2
Cl
2
(4 mL) with catalyst (0.02 mmol), substrate (1.00 mmol), PyNO (0.13 mmol), aqueous NaOCl (2.75 mmol) at 0 C.
b
c
d
e
f
g
h
i
Epoxide configuration: 1R,2S.
Epoxide configuration 3R,4R.
Reaction conducted in absence of PyNO.
j
Turn over frequency is calculated by the expression [product]/[catalyst]ꢂtime (sꢀ1).
k
l
Ratio of cis and trans epoxide is 85:15.
Epoxide configuration 3S,4S.
catalyst was used for six repeated cycles with no observable loss in
its activity and enantioselectivity (Fig. 6). This indicates that the
chiral dimeric Mn(III) salen complex is strongly bonded to the or-
dered short channel mesoporous silica through the apical
coordination.
3. Conclusions
Chiral dimeric Mn(III) salen complex immobilized into disc like
mesoporous silica of short channel was used for the enantiose-
lective epoxidation of various non-functionalized alkenes, which
showed activity and enantioselectivity comparable with dimeric
macrocyclic homogeneous catalysts. The present support due to
large pore size and short channel length was apparently re-
sponsible for least diffusional constrains in accessing the sup-
ported Mn(III) salen sites, thereby giving high activity and
enantioselectivity even for relatively bulkier substrates, such as
indene and Cy-CHR and 6eCNe2,2-dimethylchromene and the
catalyst was easily recycled six times without any loss in its per-
0
formance. This protocol with catalyst 1 , derived from 1S,2S-(þ)-1,2
diaminocyclohexane collar was used for the synthesis of (S)-lev-
chromakalim, a potent hypertensive drug in single step in good
yield and enantioselectivity by using 6-cyano 2,2 dimethylchro-
mene oxide.
4
4
. Experimental methods
Fig. 6. Recycling of complex 1 using indene as the test substrate.
.1. General
Pluronic P123 (Aldrich), sodium metasilicate (Qualigens, India),
The characterization of the recycled catalyst (after one cycle) was
accomplished by FTIR, ICP, and CHN analysis, which suggested no
significant changes in its structure barring some entrapment of re-
actants within the silica matrix. In all the catalytic reaction the
configuration of the product epoxide was the same as that of the
catalyst.
HCl (Ranbaxy, India), APTES (Fluka), aqueous NaOCl (12%) (National
Chemicals, India), Manganese acetate (SD Fine Chemicals, India)
and PyNO (Fluka) were used as received. Indene and styrene were
passed through a pad of neutral alumina before use. All the chro-
2
0
2
6,27
menes were synthesized by reported method.
A highly ordered
hexagonal disc like aminopropyl functionalized silica (AFMS) was
2
2
As a synthetic relevance of asymmetric epoxidation protocol, we
synthesized as per reported procedure using a MAS-II microwave.
0
have synthesized immobilized complex 1 with 1S,2S-(þ)-1,2 dia-
All of the solvents used in the present study were purified by
2
8
minocyclohexane collar and used it for the synthesis of valuable
known methods. The product epoxides were purified by carrying
out flash chromatography using silica gel 60e200 mesh purchased
from SD Fine Chemicals Limited, India. The purity of the solvents,
alkenes and the product epoxide were checked on gas
2
5
antihypertensive drug (S)-Levchromakalim via asymmetric ep-
oxidation of 6-cyano-2,2 dimethylchromene with overall good yield
(
54%) (Scheme 2) (For details please see the supplementary data).

6320
T. Roy et al. / Tetrahedron 68 (2012) 6314e6322
Scheme 2. Synthesis of (S)-levcromakalim.
chromatography (GC) using a Shimadzu GC 14B instrument
equipped with a stainless-steel column (2 m long, 3 mm inner di-
ameter, 4 mm outer diameter) packed with 5% SE 30 (mesh size,
electron microscopy (SEM) analysis of the sample was done with
a LEO 1430 VP.
6
0e80) and having a flame ionization detector (FID). Ultrapure N
2
4.2. Synthesis of aminopropyl functionalized mesoporous
silica AFMS
was used as a carrier gas (rate 30 mL/min) with the injection port
temperature set at 200 C. The column temperature for analysis of
ꢁ
epoxidation product of styrene, cis
was in the range of 70e140 C, whereas for chromenes the iso-
thermal temperature condition was maintained at 140 C. The ra-
b
-methyl styrene and indene
Highly ordered aminopropyl group functionalized mesoporous
ꢁ
silica (AFMS) was synthesized according to the procedure reported
ꢁ
22
by Park et al. In a typical synthetic procedure, 10 g of triblock,
cemic epoxides were prepared by the epoxidation of respective
alkenes with 3-chloroperbenzoic acid in CH Cl and were used to
2 2
poly(ethylene oxide)epoly(propylene oxide)epoly(ethylene oxide)
(EO20ePO70eEO20) (Pluronic P123, mw 5800) was dissolved in 156 g
of double distilled water. Tothis solution, a solution of Na SiO , 9H O
determine conversions by relating the height and area of the GC
peaks. The ees of styrene epoxide was determined by GC on a chiral
capillary column (Chiraldex GTA), whereas epoxides obtained from
epoxidation of indene and chromenes were determined on HPLC
2
3
2
(27.34 g in 100 g of double distilled water) was added in a drop-wise
ꢁ
manner. The mixed solution was stirred at 40 C until the entire
solution became homogenous and transparent that followed the
addition of 3-aminopropyl triethoxysiliane (APTES) (1.73 g). This
mixture was stirred further for 30 min, to which conc. HCl (81 g) was
added all at once. The mixture after stirring for 1 h was subjected to
1
13
(
Shimadzu SCL-10AVP) using a Chiralcel column (OB/OD). H &
C
NMR spectra were recorded on Bruker 200 MHz or 500 MHz
spectrometer at ambient temperature. The IR spectra were recor-
ded on a PerkineElmer Spectrum GX spectrophotometer in KBr/
Nujol mull. High-resolution mass spectra were recorded using a LC-
MS (Q-TOFF) LC (Waters), MS (Micromass) instrument. Diffuse re-
flectance spectra (DRS) were obtained from UVevis-NIR scanning
spectrophotometer UV-3101 PC. Microanalysis of the complex, in-
termediates was done on a PerkineElmer 2400 CHNS analyzer.
Estimation of Mn content in the loaded complex was done by using
inductively coupled plasma (ICP) spectrometer (PerkineElmer,
Optima 2000 DV) and by using TGA (Mettler Toledo). Powder X-ray
ꢁ
microwave irradiation under static condition at 100 C temperature
and 300 W operating power for 2 h. The white crystalline product
thus obtained was filtered off, washed with warm deionized water
ꢁ
(4ꢂ500 mL) and dried at 60 C under vacuum. The template was
removed by soxhlet extraction with ethanol over a period of 24 h.
ꢁ
Finally the solid was dried at 60 C under vacuum. The character-
ization was accomplished by means of IR-, diffuse reflectance
UVevis spectroscopy, XRD, microanalysis, SEM, TEM and N2
adsorptionedesorption studies.
diffraction (XRD) patterns of the samples were recorded on a Phi-
ꢁ
lips X’Pert MPD diffractometer using Cu-K
a
(
l
¼1.5405 A) radiation
4.3. Synthesis of AFMS-MP
ꢁ
with a step size of 0.02 2
q
and a step time of 5 s of curved Cu-K
a
monochromator under identical conditions. BET surface area was
determined using N sorption data measured at 77 K using a volu-
metric adsorption setup (Porous Materials; PMI system DET sorp-
tometer). The pore diameter of the samples were determined from
the desorption branch of the N adsorption isotherm using the BJH
2
method. Transmission electron microscopy (TEM) images were
taken with the help of a Philips Tecnai 20 instrument. Scanning
Melamineepiperazine-mesoporous silica composite (AFMS-
MP) was prepared according to the procedure reported by Shantz
2
2
4
et al. In a typical synthetic procedure 1 g of AFMS was added to
a solution comprising of cyanuric chloride (CC) (1.25 g, 6.78 mmol)
and diisopropylethylamine (DIPEA) (8 mmol) in THF (25 mL) under
an inert atmosphere. The above slurry was stirred for a period of
24 h and filtered. The residue thus obtained was washed

T. Roy et al. / Tetrahedron 68 (2012) 6314e6322
6321
sequentially with 50 mL portions of methanol, dichloromethane
(1 mmol) as substrates in dichloromethane (3 mL) in the presence
of PyNO (0.13 mmol) as an axial base with aqueous buffered NaOCl
(12%, 2.75 mmol; pH¼11.3) as an oxidant. The NaOCl was added in
(
(
DCM) and THF and transferred to a RBF containing piperazine
0.86 g, 10 mmol) in dry THF (25 mL) under inert atmosphere. The
ꢁ
resulting mass was stirred for 24 h, filtered and washed in the same
five equal portions at 0 C, and the reaction mass was stirred using
manner as described above. Finally the functionalized silica mate-
a mechanical stirrer at 900ꢄ20 rpm. The course of the epoxidation
reaction was monitored by GC with n-tridecane (0.1 mmol) as a GLC
internal standard for product quantification. After completion of
the reaction, the immobilized catalyst 1 was separated by centri-
fugation, washed thoroughly with dichloromethane, and dried in
vacuum for subsequent catalytic runs.
rial was dried at RT under vacuum for 6 h (Scheme 1) (1.1 g); n
max
ꢀ1
(
KBr) 458, 803, 1076, 1361, 1449, 1546, 2952, 3425 cm
;
lmax 248,
3
09 nm; d {d } NMR (125 MHz) 10.1, 11.6, 23.2, 25.4, 44.3, 166.7.
C H
4
.4. Synthesis of unsymmetrical Mn(III) salen complex D
The synthesis of a non-C
was accomplished by our previously reported method.
2
-symmetrical Mn(III) salen complex D
Acknowledgements
12a
The
synthetic procedure involved 1:1 condensation of 3,5-di-tert-butyl
salicylaldehyde wih (1R,2R)-(-)-cyclohexane diamine to get chiral
half unit N-(2-hydroxy-3,5-di-tert-butylbenzaldehyde)-1-amino-2-
cyclohexaneimine B, which (331 mg, 1 mmol) was allowed to react
with 5-chloromethyl 3-tert-butylsalicylaldehyde (227 mg, 1 mmol)
in dry methanol to get unsymmetrical chiral salen ligand C. The
ligand C (431 mg, 0.8 mmol) subsequently on complexation with
Mn(CH COO) .4H O (294 mg, 1.2 mmol) followed by aerobic oxi-
3 2 2
dation in the presence of LiCl (102 mg, 2.4 mmol) gave the un-
symmetrical Mn(III) salen complex D (Scheme 1).
Authors are thankful to the CSIR-Network project for supporting
this work. T.R. is thankful to CSIR for the award of SRF.
Supplementary data
2
Additional characterization data including PXRD, N adsorp-
tionedesorption profile, IR-, LC-MS, characterization data for
product epoxides and synthetic procedure of (S)-levcromakalim
along with characterization data are included in Supplementary
data. Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.tet.2012.05.022.
Ligand C: (0.458 g, 85%); [Found: C, 73.48; H, 8.76; N, 5.17.
C
33
H47ClN
2 2
O requires C, 73.54; H, 8.73; N, 5.20%]; nmax (KBr) 3411,
2
957, 2863,1629,1597,1470,1441,1390,1361,1272,1251,1204,1172,
ꢀ1
References and notes
1
1
6
2
2
095,1044, 878, 827, 712, 644, 590 cm
8H),1.42 (s, 9H),1.47e2.10 (m, 8H), 3.30e3.48 (m, 2H), 4.56 (s, 2H),
.85 (d,1H, J 1.9 Hz), 7.05 (d, 2.0 Hz), 7.43 (d,1H, J 2.2 Hz), 7.52 (d,1H J
.2 Hz), 8.32 (s,1H), 8.44 (s,1H),13.50 (b s,1H),14.48 (b s,1H); 24.2,
4.8, 26.2, 29.3, 29.6, 29.8, 33.9, 34.1, 35.2, 45.8, 72.3, 78.1, 116.8,
H 3
; d (200 MHz CDCl ) 1.23 (s,
1. (a) Song, F.; Wang, C.; Lin, W. Chem. Commun. 2011, 8256; (b) Faveri, G. D.;
Ilyashenko, G.; Watkinson, M. Chem. Soc. Rev. 2011, 40, 1722; (c) Song, F.; Wang,
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C
118.2,122.4,126.4,127.4,127.6,136.2,139.8,157.9,161.5,164.8,165.7.
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3
0
C
(
1
3
33
H
45Cl
2
MnN
c 0.04, CH Cl );
252, 1202, 1029, 836, 568 cm
2
O
2
requires C, 63.16; H, 7.23; N, 4.46%]; ½ ꢃ ¼þ663
a
D
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2
nmax (KBr) 3434, 2955, 2866,1613,1536,1436,1389,
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ꢀ1
;
lmax (CH
2
Cl
2
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20, 284 nm.
.5. Synthesis of complex 1
The silica composite AFMS-MP (1 g) was added to a solution of
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4
Ed.; VCH: New York, NY, 1993; ch. 4.2, and references therein.
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8
unsymmetrical Mn(III) salen complex D (314 mg, 0.5 mmol) in dry
toluene (10 mL) under inert atmosphere and the reaction mixture
was refluxed for 24 h. The immobilized complex 1 was washed with
toluene and solvent ether followed by extraction with DCM and
methanol on a soxhlet extractor until the washings become col-
ourless. All the washings were collected together, solvents were
evaporated and the residue was dissolved in 10 mL of dry toluene in
order to measure the difference between the initial and final con-
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on solid support (Scheme 1).
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545, 2953, 3421 cm 1;
ꢀ
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Complex 1 (1.04 g);
ꢀ
1
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