Self-supported chiral polymeric MnIII salen complexes as highly active and recyclable catalysts for epoxidation of nonfunctionalized olefins

Article abstract of DOI:10.1002/cplu.201402460

Abstract A series of self-supported chiral polymeric Mn^III N,N′-ethylenebis(salicylimine) (salen) complexes were synthesized through metalation of the corresponding salen ligands obtained by condensation of several bis/tris-aldehydes with (1R,2

Full text of DOI:10.1002/cplu.201402460

DOI: 10.1002/cplu.201402460
III
Full Papers
Self-Supported Chiral Polymeric Mn Salen Complexes as
Highly Active and Recyclable Catalysts for Epoxidation of
Nonfunctionalized Olefins
[
a]
[a, b]
[a, b]
[a, b]
Tamal Roy, Rukhsana I. Kureshy,*
Noor-ul H. Khan,
Sayed H. R. Abdi,
and
[
a, b]
Hari C. Bajaj
III
A series of self-supported chiral polymeric Mn N,N’-ethylene-
bis(salicylimine) (salen) complexes were synthesized through
metalation of the corresponding salen ligands obtained by
condensation of several bis/tris-aldehydes with (1R,2R)-1,2-dia-
minocyclohexane. Upon employment in the asymmetric epoxi-
dation reaction of nonfunctionalized olefins, all complexes
showed enhanced activity and enantioselectivity relative to the
classical Jacobsen’s monomeric salen complex. However, 1,3,5-
III
triazole-based polymeric Mn salen complex 7 was noticeably
preferred over others owing to its ability to render higher
enantioselectivity at the expense of lower catalyst loading. Fur-
thermore, complex 7 was recycled and reused in eight recy-
cling experiments with marginal loss in catalytic activity.
Introduction
Chiral epoxides are of great industrial worth as stereospecific
ring opening of these leads to the synthesis of numerous bio-
with the addition of a nonpolar solvent such as hexanes and
could subsequently be reused. Ionic liquids also featured as an
alternative solvent medium to ensure better recyclability of the
[1]
logically and pharmaceutically significant compounds. Amid
the numerous synthetic methods described in the literature for
[8]
catalyst under homogeneous reaction conditions only. Alter-
[
2]
III
III
the procurement of chiral epoxides, the Mn N,N’-ethylene-
natively, immobilization of the homogeneous Mn salen com-
bis(salicylimine) (salen) complex-catalyzed asymmetric epoxida-
plex was accomplished by using several solid supports, such as
[
3]
[4]
[9]
[10]
[9a,11]
tion developed by Jacobsen et al. and Katsuki et al. at its in-
fancy is one of the most significant ones. Despite the high cat-
alytic activity obtained with the former catalytic systems for
asymmetric epoxidation of nonfunctionalized olefins, there is
still much room for improvement in terms of catalyst loading,
reaction time, and recyclability. The expensive nature of the
chiral catalysts enforced the scientific community worldwide to
be certain about the maximum utilization of a good catalytic
system. In this direction substantial research work has been
performed for the design and synthesis of recyclable chiral
mesoporous silica, zirconium phosphate, zeolite,
poly-
[12]
[13]
[14]
oxometalate, polymers, and membranes. Both of these
techniques for catalyst recyclability have their own merits and
demerits. Heterogeneous complexes, despite showing prefer-
ence over homogeneous recyclable catalytic systems in terms
of ease of recycling and product separation, still lack the reac-
tivity and enantioselectivity obtained for the product epoxide,
which are often less than those for the monomeric Jacobsen
[15]
system possibly because of the confinement effect. Hence,
a catalytic system that can improve on the catalytic activity as
well as rendering higher enantioselectivity at the expense of
lower catalyst loading, along with a very high degree of reusa-
bility without sacrificing its catalytic activity, is always desira-
ble.
III
Mn salen complexes, which was achieved in both homogene-
ous and heterogeneous conditions. For recyclability of the cat-
alyst under homogeneous reaction conditions, several dimer-
[
5]
[6]
[7]
ic, oligomeric, and polymeric versions of Jacobsen’s mono-
meric salen system have been tested, which could be precipi-
tated out from the reaction mass in a postcatalytic workup
Recently, chiral metal–organic polymers have gathered a lot
of attention as an alternative strategy for catalyst recyclability
owing to the enhanced activity and enantioselectivity relative
to their monomeric counterparts. In this direction, we have
also reported several polymeric chiral ligands based on some
[
a] Dr. T. Roy, Dr. R. I. Kureshy, Dr. N.-u. H. Khan, Dr. S. H. R. Abdi, Dr. H. C. Bajaj
Discipline of Inorganic Materials and Catalysis
Central Salt and Marine Chemicals Research Institute (CSMCRI)
Bhavnagar, Gujarat 364 021 (India)
[16]
chiral as well as nonchiral bridging precursors. Herein, we
III
have employed Mn salen complexes with various ditopic/tri-
Fax: (+91)0278-2566970
E-mail: rukhsana93@yahoo.co.in
topic linkers, namely, methylene a, piperazine b, trigol c, (R)/(S)/
racemic 1,1’-bi-2-naphthol (BINOL, d–f), and triazine-piperazine
linker g (Figure 1), on the ligands in the asymmetric epoxida-
tion of nonfunctionalized olefins in the presence of an oxygen
donor to give chiral epoxides in high yield and enantioselectiv-
ity with eight times catalyst recyclability.
[
b] Dr. R. I. Kureshy, Dr. N.-u. H. Khan, Dr. S. H. R. Abdi, Dr. H. C. Bajaj
Academy of Scientific and Innovative Research (AcSIR)
Central Salt and Marine Chemicals Research Institute (CSMCRI)
Council of Scientific & Industrial Research (CSIR)
G. B. Marg, Bhavnagar, Gujarat 364 021 (India)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201402460.
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Figure 1. General structures of the bis- and tris-aldehydes (A–G).
Results and Discussion
In the quest for developing highly recyclable self-supported
III
chiral polymeric Mn salen complexes for asymmetric epoxida-
tion of nonfunctionalized olefins, initially we have synthesized
a group of bis- and tris-aldehydes (A–G; Figure 1) by the treat-
ment of 3-tert-butyl-5-chloromethyl salicylaldehyde with sever-
al diamines/diols/triamines as per previously reported proce-
[
16]
dures. After the successful synthesis of different bis/tris-alde-
hydes, these were condensed with (1R,2R)-1,2-diaminocyclo-
hexane to obtain the polymeric salen ligands 1’–7’ that have
linker molecules of dissimilar length and geometry (see the
Supporting Information).
III
Figure 2. General structures of the polymeric (1–7) and monomeric Mn
salen (8) complexes.
Subsequently, all of these ligands were characterized by suit-
able spectroscopic methods, namely UV/Vis, IR, and NMR, to-
gether with elemental analysis and gel permeation chromatog-
raphy (see the Supporting Information), and were subjected to
metalation with Mn(OAc) ·4H O under a nitrogen atmosphere
2
2
Table 1. Screening of catalysts for the asymmetric epoxidation of inden-
followed by aerobic oxidation in the presence of LiCl to isolate
complexes 1–7 as brown solids (Figure 2). The successful syn-
theses of these complexes were further confirmed by IR spec-
troscopy, elemental analysis, and inductively coupled plasma
analysis for absolute detection of metal content (see the Sup-
porting Information).
[
a]
e.
[
b]
[c,d]
À3 À1 [e]
Complex
t [h]
Yield [%]
ee [%]
TOF [ꢁ10 s ]
1
2
3
4
5
6
7
8
8
8
6
12
12
12
5
>99
>99
>99
95
92
92
78
79
88
79
80
70
92
75
1.73
1.73
2.31
1.10
1.06
1.06
2.78
1.04
To determine the effect of the linker molecule on the activity
III
of the above synthesized chiral polymeric Mn salen com-
plexes, all of them were screened in the asymmetric epoxida-
tion of nonfunctionalized olefins by using indene as a model
substrate under identical reaction conditions. Preliminary re-
sults indicated a better reactivity and enantioselectivity with all
of the complexes 1–7 than with classical Jacobsen’s system 8
>99
90
12
[
a] Reaction conditions: indene (1 mmol), pyridine N-oxide (PyNO;
.1 mmol), catalyst (0.02 mmol), NaOCl (2.5 mmol), CH Cl (1 mL), 08C.
b] Determined by GC analysis. [c] Determined by HPLC on a chiral sta-
tionary phase (Chiralcel OB column). [d] Epoxide configuration is 1R,2S.
(Table 1) under the present reaction conditions.
0
2
2
The greater activity of the polymeric salen complexes was
[
perhaps governed by the creation of a supramolecular assem-
bly by each of the monomeric building units responsible for
the polymeric network. Further, when we assessed the results
[
[
e] Turnover frequency (TOF) is calculated by the expression [product]/
À1
catalyst]ꢁtime (s ).
&
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obtained for asymmetric epoxidation of indene with com-
plexes 1–6, which have ditopic bridging linkers, a general
trend in reactivity and enantioselectivity was obtained with the
length and the extension angle of the linker molecule. For in-
III
stance, in the case of polymeric Mn salen complexes with
linear bridging linkers, an increase in the length of the linker
led to higher activity and enantioselectivity (Table 1, entries 1–
III
3). This also resulted in better activity for the polymeric Mn
salen complex based on trigol linker (complex 3) relative to
the complexes obtained with the use of a methylene (com-
plex 1) or piperazine (complex 2) group as the linker. However,
the use of the BINOL moiety as the bridging linker led to a de-
crease in the reactivity of the corresponding manganese com-
plexes and the reaction required a much longer time for com-
pletion, which may be attributed to the deleterious effect on
the electronic environment of one of the active metal centers
on the other (Table 1, entries 4–6). A similar type of phenomen-
[
17]
on was also observed by Ding et al. with the use of a supra-
molecular version of Shibasaki’s La–BINOL complex, in which
reduction of the extension angle in the bridge led to lower ac-
tivity and enantioselectivity. However, differing from their
report, we have not observed any drop in enantioselectivity
except in the case of the achiral BINOL moiety (Table 1,
entry 6), in which the existence of a random twist in the supra-
molecular structure owing to the presence of an equimolar
mixture of both enantiomers of BINOL could be the decisive
Figure 3. Comparison of the catalytic activity of the polymeric complexes in
the formation of indene oxide (top) and formation of indene oxide with re-
spect to time using complex 7 (bottom).
[
16a]
factor.
Nevertheless, unlike our former report on asymmetric
[
16a]
aminolytic kinetic resolution of terminal epoxide,
no assis-
in more access to each and every single active metal center of
the polymer network, unlike in the former report in which the
tive effect of the external chirality from the BINOL moiety was
shown here, which might be a result of the absence of a bimet-
allic reaction mechanism in the case of asymmetric epoxida-
tion of nonfunctionalized olefins in the presence of an oxygen
donor.
III
cross-linked polymeric Mn complex was not soluble in the re-
action medium.
Having identified complex 7 as the catalyst of choice, the
optimization of reaction conditions, such as choice of oxidants,
axial bases, catalyst loading, solvents, and temperature, was in-
vestigated for the asymmetric epoxidation of indene as
a model substrate, as these parameters are known to influence
the yield and enantioselectivity of the product. First, we
screened several oxidants, namely NaOCl, urea–hydrogen per-
oxide (UHP), tBuOOH, meta-chloroperbenzoic acid (mCPBA), io-
dosylbenzene (PhIO), and oxone, for the enantioselective epox-
idation of indene (Figure 4). In the case of UHP, tBuOOH, and
oxone, a similar level of enantioselectivity was observed; how-
Next, on moving from a ditopic bridging linker to a tritopic
one, that is, salen ligand derived from a tris-aldehyde grown
on a triazine-piperazine linker, we found a significant increase
in both reactivity and enantioselectivity for the production of
chiral epoxides (Table 1, entry 7). This observation was in con-
[
17]
cordance with the report of Ding et al. in which it was wit-
nessed that a polymeric complex based on a tritopic ligand
with large linker was also able to produce high reactivity and
enantioselectivity. A similar type of effect, namely an increase
in catalytic activity with an increase in the length of the linker
[
18]
molecule, was also observed by our group as well as by Fu
[
9e]
III
et al. for immobilized Mn salen complexes on solid porous
materials. In our present study, complex 7 has a fairly large
linker of triazine-piperazine, which separates the two reactive
metal centers by a distance that might be suitable for both
catalyst activity and the formation of a supramolecular assem-
bly. Besides, distinct from the previous report by Gothelf
[
7c]
et al. on the asymmetric epoxidation of nonfunctionalized
III
olefins by using a polymeric Mn salen complex grown on
a tris-aldehyde, we have found greater activity and enantiose-
lectivity with our polymeric complex 7 (Figure 3) at the ex-
pense of lower catalyst loading. The reason behind the en-
hanced activity of the present complex is perhaps owing to its
better solubility in the reaction medium, which in turn results
Figure 4. Screening of oxidants for the catalytic oxidation of indene using
complex 7 as catalyst.
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ever, there was a significant drop in the formation of epoxide
when tBuOOH and oxone were used as oxidants. On conduct-
ing the epoxidation reaction of indene with mCPBA and PhIO,
both enantiomeric excess and yield were inferior compared
with those of NaOCl and UHP.
Thereafter, we varied the catalyst loading from 0.5 to
2
1
mol% with respect to indene (Figure 5) and found that
mol% of the catalyst is sufficient to give 99% yield with 92%
Figure 5. Variation of catalyst loading in the asymmetric epoxidation of
indene using complex 7.
Figure 7. Variation of the axial base (top) and solvent (bottom) in the asym-
metric epoxidation of indene using complex 7. Reaction conditions: indene
(
1 mmol), axial base (0.1 mmol), catalyst (0.01 mmol), NaOCl (2.5 mmol), sol-
enantioselectivity of indene epoxide with NaOCl as the oxi-
dant. Temperature plays a crucial role in the enantiomeric
excess (ee) of the products in most of the asymmetric organic
transformations including the asymmetric epoxidation reac-
tion. Hence, the enantioselective epoxidation reaction was
studied in a temperature range from 08C to room temperature
vent (1 mL), 08C. NMO=N-methylmorpholine N-oxide.
dation reaction. Under the present reaction conditions we
were able to attain good catalytic activity using inexpensive
water-soluble PyNO as an axial base (Figure 7, top). The use of
an expensive and hydrophobic version of PyNO, such as 4-
phenyl PyNO (PPyNO) or 4-phenyl propyl PyNO (PPPyNO), gave
similar activity and enantioselectivity, albeit in less reaction
time owing to better transportation of active oxidant HOCl,
generated in situ from the aqueous layer to the organic layer.
Henceforth, for the subsequent catalytic studies we zeroed
upon PyNO as an optimal axial base (Figure 7, top). Different
solvents were explored for the epoxidation reaction of indene
using dichloromethane (CH Cl ), chloroform, toluene, metha-
(Figure 6), and 08C was found to be optimum in terms of yield
and enantioselectivity of the product.
It is well known that axial bases have a pronounced effect
on the reactivity and selectivity of the enantioselective epoxi-
2
2
nol, acetonitrile, and CH Cl /methanol mixture. The data in
2
2
Figure 7 (bottom) clearly show that CH Cl is the solvent of
2
2
choice.
Once all the reaction parameters governing the asymmetric
epoxidation reaction were established, next we performed
asymmetric epoxidation of other nonfunctionalized olefins,
that is, styrene and simple or substituted chromenes, by using
III
polymeric Mn salen complex 7 under optimized reaction con-
ditions (Table 2, entries 1–6). All the above substrates except
styrene performed well with quantitative yields and 90–99%
enantioselectivity for the product epoxides in 5–8 hours. De-
spite enantioselective epoxidation of styrene with complex 7
giving styrene oxide in quantitative yield, still only a moderate
enantioselectivity (60% ee) was obtained in this case (Table 2,
Figure 6. Variation of reaction temperature in the asymmetric epoxidation of
indene using complex 7.
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Table 2. Variation of substrate in the asymmetric epoxidation of nonfunc-
[
a]
tionalized olefins using complex 7.
[
b]
À3 À1 [c]
Entry Substrate
1
t [h] Yield [%]
ee [%] TOF [ꢁ10 s ]
[
d]
4.5
6
>99
>99
60
95
6.17
4.63
[
[
e]
e]
2
3
8
>99
94
3.47
[
[
e]
e]
4
5
8
5
>99
>99
99
92
3.47
5.55
[
f]
f]
6
7
6
6
92
95
52
54
4.26
4.40
[
[
g]
8
8
5
90
90
33
3.12
5.00
[
h]
[i]
[j]
9
91
[
(
a] Reaction conditions: substrate (1 mmol), PyNO (0.1 mmol), catalyst 7
0.01 mmol), NaOCl (2.5 mmol), CH Cl (1 mL), 08C. [b] Determined by GC
2
2
analysis. [c] Turnover frequency is calculated by the expression [product]/
À1
[
catalyst]ꢁtime (s ). [d] Determined by HPLC analysis on a chiral station-
Figure 8. Study of catalyst recyclability in the asymmetric epoxidation of
indene with complex 7 using NaOCl (top) and UHP (bottom) as oxidant.
ary phase (chiral capillary column GTA type). Epoxide configuration is R.
e] Determined by HPLC analysis on a chiral stationary phase ( Chiralcel
[
OD column). Epoxide configuration is 3R,4R. [f] Determined by HPLC anal-
ysis on a chiral stationary phase (Chiralcel AS-H column). Epoxide configu-
ration R. [g] Determined by HPLC analysis on a chiral stationary phase
ther there was a visible drop in both yield and enantioselectivi-
ty of the product epoxides (Figure 8). This is probably a result
of oxidative degradation of the complex, which was also re-
flected in the change of catalyst color from dark brown to
light brown. On the contrary, when we utilized the same cata-
lytic system for the study of recyclability under single-phase re-
action conditions using UHP adduct as an oxidant, the catalyst
could be recycled eight times without any significant loss in
catalytic activity or enantioselectivity (Figure 8).
(
1
Chiralcel IA column). Epoxide configuration is R. [h] Reaction scale of
0 mmol. [i] Yield of isolated product. [j] Determined by HPLC analysis on
a chiral stationary phase (Chiralcel OB column). Epoxide configuration is
R,2S.
1
entry 1). The ee values were particularly encouraging for bulki-
er alkenes such as chromene and its derivatives (Table 2, en-
tries 2–5). A better enantioselectivity was obtained when there
was an electron-withdrawing group present in the chromene
moiety at the 6-position (Table 2, entry 4), whereas an electron-
donating group at the same place resulted in little decrease in
ee value (Table 2, entry 5). To show the practical applicability of
the catalytic system, we performed asymmetric epoxidation of
indene at a larger reaction scale (10 mmol) to obtain the corre-
sponding product indene oxide in high yield of isolated mate-
rial (90%) and enantioselectivity (91%), which further supports
the robustness of the present catalytic system (Table 2,
entry 6).
After obtaining better recyclability of complex 7 using UHP
as an oxidant, next we explored the efficacy of complex 7 for
the asymmetric epoxidation of all the above tested nonfunc-
tionalized olefins by using the same oxidant in the presence of
PyNO as an axial base at 38C and 1:1 CH Cl /MeOH as reaction
2
2
medium; the results are summarized in Table 3 (entries 1–6).
From turnover frequency (TOF) calculations it is apparent that
the asymmetric epoxidation reaction was relatively slower with
UHP as an oxidant than with NaOCl, with almost parallel enan-
tioselectivity values obtained in both cases, which was in
[19]
Chiral catalysts are expensive in nature so it is desirable to
check for their recyclability if possible. We also subjected our
best catalytic system, that is, complex 7, to the study of reusa-
bility and found that the activity of the catalyst remained con-
stant for four consecutive cycles. However, on proceeding fur-
tandem with our earlier report.
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[
22]
cylaldehyde was synthesized by the previously known method.
Table 3. Enantioselective epoxidation of nonfunctionalized olefins with
[16a,c]
[16b]
The bis-
and tris-aldehydes
(A–G), used for the synthesis of
[
a]
complex 7 using urea–hydrogen peroxide (UHP) as an oxidant.
polymeric ligands, were prepared as per our earlier reports. Micro-
analysis of the intermediates, ligands, and catalysts was performed
1
13
on a PerkinElmer 2400 CHNS analyzer. H and C NMR spectra
were recorded on Bruker 200 MHz or 500 MHz spectrometer at am-
bient temperature with reference to TMS. FTIR spectra were re-
corded on a PerkinElmer Spectrum GX spectrophotometer with
[
b]
À3 À1 [c]
Entry Substrate
1
t [h] Yield [%]
ee [%] TOF [ꢁ10 s ]
III
samples as KBr pellets. Electronic spectra of the polymeric Mn
[
d]
8
8
>99
>99
96
58
3.47
3.47
2.67
salen complexes were recorded in dichloromethane on a Varian
Cary 500 Scan UV/Vis–near-IR spectrophotometer. Optical rotations
of chiral intermediates and chiral complexes were recorded on an
automatic polarimeter (Digipol 78, Rudolph). High-resolution mass
spectra were obtained with LC-MS (Q-TOF) LC (Waters), MS (Micro-
mass), MALDI-TOF, and Ultraflex TOF/TOF instruments (Bruker Dal-
tonics, Germany). The product epoxides were purified with flash
chromatography using silica gel 60–200 mesh purchased from SD
Fine Chemicals Limited, Mumbai (India). The purity of the solvents
and alkenes and the analysis of the epoxide product were deter-
mined by gas chromatography (GC) on a Shimadzu GC 14B instru-
ment with a stainless-steel column (2 m long, 3 mm inner diameter,
[
e]
2
3
90
[
f]
f]
10
92
92
[
4
10
90
2.50
[
[
f]
f]
5
6
10
10
88
92
94
90
2.44
2.55
4
mm outer diameter) packed with 5% SE30 (mesh size 60–80) and
equipped with a flame ionization detector. Ultrapure nitrogen was
À1
used as carrier gas (rate 30 mLmin ). The injection port and detec-
tor temperature was kept at 2008C. For the product analysis of sty-
rene and indene, the column temperature was programmed at 70
to 1408C, whereas for chromenes it was kept at 1408C (isother-
mal). Synthetic standards of the products were used to determine
the conversions by comparing the peak height and area. The ee
value of styrene oxide was determined by GC using a chiral capilla-
ry column (Chiraldex GTA). For the chromenes and indene epox-
ides, the ee values were determined by HPLC (Shimadzu SCL-
[
(
a] Reaction conditions: substrate (1 mmol), PyNO (0.1 mmol), catalyst
0.01 mmol), UHP (1.5 mmol), CH Cl (1 mL), 38C. [b] Determined by GC
2
2
analysis. [c] Turnover frequency is calculated by the expression [product]/
À1
[catalyst]ꢁtime (s ). [d] Determined by HPLC analysis on a chiral station-
ary phase (Chiral capillary column GTA type). Epoxide configuration is R.
e] Determined by HPLC analysis on a chiral stationary phase (Chiralcel
[
OB column). Epoxide configuration is 1R,2S. [f] Determined by HPLC anal-
ysis on a chiral stationary phase (Chiralcel OD column). Epoxide configu-
ration is 1R,2S.
1
0AVP) on a Chiralcel column (OD and OB).
General method for preparation of polymeric salen ligands
Conclusion
The polymeric salen ligands based on several bis/tris-aldehydes
[16a–c]
(A–G) were prepared as per earlier reported procedure.
In
III
We have synthesized a series of self-supported polymeric Mn
a typical procedure, a solution of (1R,2R)-1,2-diaminocyclohexane
and a solution of a suitable bis/tris-aldehyde were taken together
in an appropriate molar ratio and the resulting mixture was heated
at reflux until the disappearance of the starting aldehyde, as ob-
served by TLC. Subsequently, the solvent was evaporated and the
salen complexes based on several bis/tris-aldehydes and em-
ployed them for the asymmetric epoxidation of nonfunctional-
ized olefins with different oxidants under homogeneous condi-
tions. A systematic evaluation has been done on the effect of
the linker molecule joining the salen units on the outcome of
yellow mass thus obtained was dissolved in CH Cl2 and washed
2
thoroughly with water and brine. The organic layer was separated
and removal of the solvent under reduced pressure gave the poly-
meric salen ligands 1’–7’ as yellow solids.
III
the asymmetric epoxidation reaction. The Mn salen complex
based on triazole linker was found to be most effective in
terms of activity and enantioselectivity when NaOCl was used
as oxidant. Further, this complex was recycled several times
under the described reaction conditions without any loss in ac-
tivity and enantioselectivity in the presence of urea–hydrogen
peroxide and NaOCl as oxidants.
III
General method for preparation of polymeric Mn salen
complexes
Mn(CH COO) ·4H O (490 mg, 2 mmol) was added to a solution of
3
2
2
a polymeric salen ligand (1’–7’; 1 mmol with respect to monomeric
unit) in CH Cl /methanol mixture (30 mL, 1:1) under an inert atmos-
2
2
Experimental Section
phere, and the resulting mixture was heated at reflux for 6–8 h.
After completion of the reaction, the mixture was cooled to room
temperature, lithium chloride (170 mg, 4 mmol) was added, and
the mixture was further stirred for 3 h with exposure to air and
then filtered. The solvent was removed from the filtrate and the
residue was extracted with dichloromethane. The organic layer was
washed with water and brine, and dried over anhydrous Na SO .
Methods and materials
Manganese acetate (SD Fine Chem. Ltd.) and (1R,2R)-(À)-1,2-diami-
nocyclohexane (Sigma Aldrich) were used as received. Indene and
styrene (Fluka) were passed through a bed of neutral alumina
before use. All the chromenes used in the present study were syn-
thesized according to literature reports. All the solvents were pu-
rified by distillation prior to use. 3-tert-Butyl-5-chloromethyl sali-
2
4
[20]
After complete removal of the solvent the desired complexes 1–7
were obtained as brown solids.
[21]
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General procedure for enantioselective epoxidation of non-
functionalized olefins using NaOCl as an oxidant
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(
0.1 mmol) as the GLC internal standard.
After completion of the reaction, the product chiral epoxide of the
respective alkene was extracted with CH Cl , washed with water,
2
2
and dried over Na SO . The catalyst was separated from the prod-
2
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anes/ethyl acetate (90:10), dried, and reused for further catalytic
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Published online on && &&, 0000
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FULL PAPERS
T. Roy, R. I. Kureshy,* N.-u. H. Khan,
S. H. R. Abdi, H. C. Bajaj
&& – &&
III
Self-Supported Chiral Polymeric Mn
Salen Complexes as Highly Active and
Recyclable Catalysts for Epoxidation
of Nonfunctionalized Olefins
In the long run: Self-supported chiral
polymeric Mn complexes have been
of catalyst recyclability (see scheme;
PyNO=pyridine N-oxide). A correlation
between linker length and catalyst ac-
tivity was established from experimental
evidence.
III
evaluated in the asymmetric epoxida-
tion of nonfunctionalized olefins, to
obtain epoxides in good yield and
enantioselectivity with a high degree
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