Chiral macrocyclic salen Mn(III) complexes catalyzed enantioselective epoxidation of non-functionalized alkenes using NaOCl and urea H 2O2 as oxidants

Article abstract of DOI:10.1016/j.jcat.2010.10.002

Two new chiral Mn(III) macrocyclic salen complexes 1a and 1b were prepared for the enantioselective epoxidation of non-functionalized alkenes. A 5 mol% loading of these catalysts in the presence of pyridine N-oxide as an axial base and sodium hypochlorite

Full text of DOI:10.1016/j.jcat.2010.10.002

Journal of Catalysis 277 (2011) 123–127
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Research Note
Chiral macrocyclic salen Mn(III) complexes catalyzed enantioselective
epoxidation of non-functionalized alkenes using NaOCl and urea H2O2 as oxidants
Nabin Ch. Maity a, Sayed H.R. Abdi a, , Rukhsana I. Kureshy a, Noor-ul H. Khan a, E. Suresh b, Ganga P. Dangi a,
⇑
Hari C. Bajaj a
a Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSMCRI), Council of Scientific & Industrial Research (CSIR), Bhavnagar 364 021,
Gujarat, India
b Analytical Discipline, Central Salt and Marine Chemicals Research Institute (CSMCRI), Council of Scientific & Industrial Research (CSIR), Bhavnagar 364 021, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new chiral Mn(III) macrocyclic salen complexes 1a and 1b were prepared for the enantioselective
epoxidation of non-functionalized alkenes. A 5 mol% loading of these catalysts in the presence of pyridine
N-oxide as an axial base and sodium hypochlorite or urea hydrogen peroxide adduct as oxidant worked
well to give respective epoxides in high yields and ee (up to >95% in selected cases). The catalyst 1b with
urea hydrogen peroxide adduct as an oxidant was recovered by precipitation with hexane and was reused
up to four times with retention of enantioselectivity.
Received 26 August 2010
Revised 2 October 2010
Accepted 4 October 2010
Available online 27 November 2010
Keywords:
Enantioselective
Ó 2010 Elsevier Inc. All rights reserved.
Chiral Mn(III) macrocyclic complexes
Chiral epoxides
Non-functionalized alkenes
1. Introduction
are known to be very sensitive towards the enantioselectivity and
any change from t-butyl group has invariably resulted in the loss of
Method development for the preparation of enantiomerically
pure epoxides continues to be one of the most exciting fields of
asymmetric catalysis [1–5]. Among several catalytic strategies for
the synthesis of optically active epoxides, the asymmetric epoxida-
tion of non-functionalized alkenes catalyzed by chiral Mn(III) salen
complexes, initially developed by Katsuki and Jacobsen, are consid-
ered to be the most effective catalysts discovered in the last
25 years [2–4]. Although the classic chiral Mn(III) salen catalyst
flaunts high enantioselectivity for the asymmetric epoxidation of
Z and tri-substituted prochiral alkenes under homogeneous condi-
tion where catalyst recyclability is a major concern. To make
Mn(III) salen catalyst recyclable, strategies like its immobilization
on organic polymers [6], inorganic supports [6–14], encapsulation
in zeolite cavities [15] and physical entrapment in siloxane mem-
branes [16] have been reported. Very recently, Martinez et al.
[17,18] reported multi-step synthesis of chiral Mn(III) salen com-
plexes incorporated in a macrocycle bearing aliphatic polyether
linker at 3,30 positions of salicylidene moieties for enantioselective
epoxidation of cis-olefins. However, 3,30 positions of the salen unit
enantioselectivity [19].
In continuation of our earlier strategy [20–23] of making the
catalyst recyclable by fine-tuning the solubility of the catalyst,
we have developed a simple scheme for the synthesis of new mac-
rocyclic recyclable chiral salen ligands 1a0, 1b0 (Scheme 1) linked
through 5,50 positions of salen unit in fewer steps. Mn(III) salen
complexes 1a, 1b, derived from these ligands were used as cata-
lysts for the enantioselective epoxidation of styrene (STR), indene
(IND) and chromenes using oxidants e.g., sodium hypochlorite
(NaOCl) and urea hydrogen peroxide (UHP) in the presence of pyr-
idine N-oxide (PyNO) as an axial base at 0–5 °C. These catalysts
were recovered by precipitation with hexane after each catalytic
run and were reused several times with retention of enantioselec-
tivity when UHP was used as an oxidant.
2. Experimental
The complexes 1a and 1b were prepared by the reaction of
Mn(II) acetate with respective macrocyclic ligands 1a0 and 1b0,
which were synthesized by the reaction of chiral diamine with a
dialdehyde 3 as shown in Scheme 1 (experimental details are given
in Supporting information).
⇑
Corresponding author. Fax: +91 0278 2566970.
E-mail address: shrabdi@csmcri.org (S.H.R. Abdi).
0021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2010.10.002

124
N.Ch. Maity et al. / Journal of Catalysis 277 (2011) 123–127
O
OH
NaH, THF
O
O
O 0
C
OH
OH
Cl
O
O
OH
OH
1
2
3
(1 ,2 )-(+)-1,2-cyclohexanediamine,
S S
THF, RT, 2 h (1
R
,2 )-(+)-1,2-diphenylethylenediamine,
R
THF, RT, 5 h
2O, MeOH,LiCl
4H
Mn(OAc)2.
Mn(OAc)2.4H2O, MeOH, LiCl
O
O
O
O
H
H
H
N
H
H
H
H
O
O
Cl Mn
O
O
Mn
O
O
Cl Mn
N
N
N
N
N
Cl
Mn Cl
N
N
O
O
H
O
O
O
O
n
n
1a (n = 0, 1)
1b (n = 0, 1)
Scheme 1. Synthesis of chiral macrocyclic Mn(III) salen complexes 1a and 1b.
2.1. General procedure for enantioselective epoxidation of non-
functionalized alkenes
adduct (70 mg, 0.725 mmol, in six equal portions) as an oxidant
at 3 °C, and the reaction was monitored on GC. After completion
of the reaction, the solvent was removed and the residue was ex-
tracted with CH2Cl2, washed with water, and dried over anhydrous
sodium sulfate. The catalyst was separated from the product epox-
ide by precipitation with hexane (2 ml) and was used as such for
further catalytic runs. Epoxides were purified by flash chromatog-
raphy through neutral alumina using ethyl acetate and hexane
(9:1) as eluent.
2.1.1. NaOCl oxidant system
Enantioselective epoxidation reaction of different alkenes
(0.625 mmol) viz. 2,2-dimethylchromene (CHR), 6-cyano-2,2-dim-
ethylchromene (CN-CHR), 6-nitro-2,2-dimethylchromene (NO2-
CHR), 6-methoxy-2,2-dimethylchromene (MeO-CHR), spiro[cyclo-
hexane-1,2-[2H][1]chromene] (Cy-CHR), IND, and STR was per-
formed by using complex 1a/1b (5 mol%) in CH2Cl2 (1 ml) in the
presence of PyNO (11.5 mg, 0.12 mmol) as an axial base and buf-
fered NaOCl (pH 11.3; 1.5 mmol added in four equal parts) as an
oxidant at 0 °C. The epoxidation reaction was monitored by GC
with n-tridecane (0.1 mmol) as GLC internal standard for product
quantification. After the reaction, the product was extracted with
CH2Cl2, washed with water, and dried over anhydrous sodium sul-
fate. The catalyst was separated from the product epoxide by pre-
cipitation with hexane (2 ml) and used as such for further catalytic
runs. Epoxides were purified by flash chromatography through
neutral alumina using ethyl acetate and hexane (9:1) as eluent.
3. Results and discussion
The synthesis of catalysts 1a and 1b is presented in Scheme 1.
The reaction of dimethanol benzene 1 with aldehyde 2 yielded
the desired dialdehyde 3 in rather low yield (maximum 32%) after
chromatographic purification because of the presence of free OH
group in aldehyde 2 resulting in the formation of side products.
The macrocyclic chiral Schiff base ligands 1a0 and 1b0 were synthe-
sized by reacting stoichiometric amount of dialdehyde 3 with cor-
responding diamines. Remarkably, the dialdehyde
3 reacted
completely (within 2 h) with (1S,2S)-(+)-1,2-diaminocyclohexane
in THF to give mostly monomeric macrocyclic ligand 1a0 (n = 0).
Whereas the reaction of dialdehyde 3 with (1R,2R)-(+)-1,2-diphen-
ylethylenediamine in THF was tricky as longer reaction time (>5 h)
resulted in the formation of polymeric compound. After several at-
tempts on its synthesis in various solvents, it was realized that at
2.1.2. UHP oxidant system
Enantioselective epoxidation of CHR, CN-CHR, NO2-CHR, MeO-
CHR, Cy-CHR, IND and STR (0.625 mmol) was performed by using
1a/1b (5 mol%) as catalyst in 1:1 dichloromethane:methanol
(1.0 ml) in the presence of PyNO (11.5 mg, 0.12 mmol) with UHP

N.Ch. Maity et al. / Journal of Catalysis 277 (2011) 123–127
125
about 5-h reaction time in THF, formation of the product 1b0 is
epoxidation of STR as representative substrate was carried with
catalysts 1a and 1b under identical reaction conditions (Table S1,
entries 1, 8–10, 17, 24–26). Among these, CH2Cl2 (Table S1, entries
1 and 17) was found to be the best solvent. It is reported in the lit-
erature [20,22] that pyridine N-oxide derivatives greatly influence
catalyst turnover and enantioselectivity in Mn(III) salen catalyzed
epoxidation of alkenes with NaOCl. According to these studies, N-
oxides prevent the formation of catalytically inactive Mn–O–Mn
species [26] as well as improve the transportation of HOCl from
aqueous phase to the organic phase. Hydrophobic N-oxides e.g.,
4-phenylpyridine N-oxide (4-PPyNO) and 4-(3-phenylpropyl) pyr-
idine N-oxide (4-PPPyNO) have been reported to stabilize the reac-
tive Mn@O intermediate [27]. In our optimized condition for the
epoxidation of styrene, the use of 4-PPyNO and 4-PPPyNO as axial
bases with catalysts 1a and 1b (Table S1, entries 11, 12, 27, 28) had
no obvious advantage over the simple and inexpensive PyNO. This
finding is in contrast to the results obtained with the use of Mn(III)
salen complex [27] but is in line with the use of built-in phase
transfer Mn(III) salen complexes reported by us [28,29]. A control
experiment for the epoxidation of STR conducted without PyNO
gave only 40–48% styrene oxide in 3 h with low enantioselectivity
(Table S1, ee 18–22%; entries 13, 29), demonstrated the positive
role of PyNO as an axial base.
We also explored the oxidant-UHP in the epoxidation of STR
with catalysts 1a and 1b in the presence of different N-oxides
viz., PyNO, 4-PPyNO, and 4-PPPyNO at 3 °C in CH2Cl2:CH3OH 1:1
(Table S1, entries 14–16, 30–32). Excellent conversion (>99%) to
styrene oxide was achieved in 5–6 h with both the catalysts, how-
ever, catalyst 1b imparted better ee (ꢀ59%) when compared to the
catalyst 1a (ee, ꢀ41%), which fared better with UHP than NaOCl.
The optimized epoxidation condition (as per entries 1 and 2; Ta-
ble 1) was applied for other non-functionalized alkenes, to see the
general applicability of catalysts 1a and 1b using PyNO as an axial
base with NaOCl as an oxidant. Excellent conversion to epoxides
(>99%) and ee (Table 1) was observed for all the substrates (entries
1–14) in 3–8 h at 0 °C. As far as enantioselectivity is in concern, the
catalyst 1b worked much better for STR (ee, 59%) and IND (ee, 91%)
than the catalyst 1a, which gave ee 33% and 75%, respectively.
However, in the case of chromenes both the catalysts imparted
1
optimum (ꢀ75%). The H NMR and LCMS data for ligand 1b0 show
the presence of monomer (n = 0), dimer (n = 1) and trimeric species
in 80%, 16% and 4%, abundance, respectively. These mono-, di- and
trimeric salen species were separated by neutral alumina column
chromatography with benzene and acetone (7:3) as eluent. Ab-
sence of aldehyde peak in NMR indicates that all the three species
were cyclic. This mixture of species was used as such for making
the chiral macrocyclic Mn(III) salen complex, as complexes of
above separated ligands gave nearly similar performance. The
monomeric 1a0 gave single crystals suitable for X-ray structure
determination from acetone in 2–3 days, which further established
its cyclic structure (Fig. S1) [24]. The ligand 1a0/1b0 on reaction
with Mn(OAc)2ꢁ4H2O and subsequent air oxidation yielded the de-
sired complex 1a/1b in quantitative yield. Numerous attempts to
get single crystals of complexes 1a and 1b failed; hence, we ex-
plored the probable structure of 1a using energy minimization op-
tion (Fig. S2) in Material Studio (version 4.2). The crystallographic
data of ligand 1a0 were used for simulating the structure of com-
plex 1a, which lacked C2-symmetry (or distorted C2-symmetry)
as against stepped conformation [25] of the Jacobsen’s Mn(III)
salen complex. The cause of this distortion is due to the shorter
length of the linker group. Both the complexes 1a and 1b
(5 mol%) were used to catalyze the enantioselective epoxidation
of STR as a representative substrate using NaOCl as an oxidant in
the presence of PyNO as an axial base, and the results are given
in Table S1. Excellent yield (>99%) of styrene oxide was achieved
in 3 h with both the catalyst. However, the ee of styrene oxide
was superior with catalyst 1b (Table S1, entry 17). Epoxidation of
STR was conducted with 2.5–10 mol% catalyst loading and the re-
sults indicated that 5 mol% (Table S1, entries 1–3, 17–19) is opti-
mum at 0 °C. Epoxidation of STR over a temperature range of ꢂ5
to 20 °C at 5 mol% catalyst loading concluded that 0 °C is optimum
reaction temperature (Table S1, entries 5–7, 21–23). Having opti-
mized catalyst loading and reaction temperature next, we opti-
mized reaction medium, as solvent is known to play a crucial
role in the enantioselective epoxidation of non-functionalized al-
kenes [17,22]. In view of this, the effect of solvents viz. CH2Cl2,
dichloroethane, CHCl3 and mixture of (CH2Cl2 + CH3CN) on the
Table 1
Asymmetric epoxidation of alkenes catalyzed by 1a and 1b with NaOCl as an oxidant.
R
R'
catalyst (5 mol%), NaOCl (1.5 mmol)
PyNO (0.12 mmol), 0 oC
R
R'
*
*
H
H
O
Entrya
Catalyst
Substrate
STR
% Yieldb
Time (h)
ee (%)
Configuration
TOF ꢃ 10ꢂ3 sꢂ1f
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1a
1b
1a
1b
1a
1b
1a
1b
1a
1b
1a
1b
1a
1b
>99
>99
>99
3
3
4.5
4
6
5
6
5
8
7
6
5
6
5
9
2
33c
59c
75d
91d
86e
84e
93e
92e
73e
77e
90e
91e
93e
95e
92
S
R
1.83
1.83
1.22
1.38
0.92
1.10
0.92
1.10
0.69
0.79
0.92
1.10
0.92
1.10
0.37
4.17
IND
1S,2R
1R,2S
3S,4S
3R,4R
3S,4S
3R,4R
3S,4S
3R,4R
3S,4S
3R,4R
3S,4S
3R,4R
3S,4S
3S,4S
Cy-CHR
CHR
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
60
MeO-CHR
NO2-CHR
CN-CHR
Jacobson’s catalyst
Katsuki’s catalyst
CN-CHR
CHR
75
99
a
b
c
d
e
f
Reaction condition: catalyst (5 mol% in 1 ml CH2Cl2), substrate (0.625 mmol), pyridine N-oxide (0.12 mmol), NaOCl (1.5 mmol).
Determined on GC.
Chiral capillary column GTA-type.
Chiral HPLC OB column.
Chiral HPLC OD column.
Turn over frequency is calculated by the expression [product]/[catalyst] ꢃ time (sꢂ1).

126
N.Ch. Maity et al. / Journal of Catalysis 277 (2011) 123–127
Table 2
Epoxidation of alkenes catalyzed by 1a and 1b with UHP adduct as an oxidant.
Entrya
Catalyst
Substrate
STR
% Yieldc
Time (h)
ee (%)
Configuration
TOF ꢃ 10ꢂ3 sꢂ1g
1
1a
1b
1b
1b
1b
1a
1b
1a
1b
1a
1b
1a
1b
1a
1b
1a
1b
>99
>99
>92
>80
>75
93
99
95
90
>99
>99
90
98
48
96
44
35
6
8
8
8
8
41d
59d
59d
58d
59d
93e
62e
84f
86f
90f
93f
89f
93f
91f
92f
91f
80f
S
R
R
R
0.91
0.69
0.63
0.55
0.52
0.65
0.55
0.24
0.23
0.25
0.25
0.62
0.54
0.12
0.24
0.10
0.08
2b
3h,b
4i,b
5j,b
6
R
IND
8
1S,2R
1R,2S
3S,4S
3R,4R
3S,4S
3R,4R
3S,4S
3R,4R
3S,4S
3R,4R
3S,4S
3R,4R
7
8
9
10
22
22
22
22
8
10
22
22
24
24
Cy-CHR
CHR
10
11
12
13
14
15
16
17
MeO-CHR
NO2-CHR
CN-CHR
a
b
c
d
e
f
Reaction condition: catalyst (5 mol% in 1 ml CH2Cl2), substrate (0.625 mmol), pyridine N-oxide (0.12 mmol), UHP (1.5 eq. with respect to substrate) at 3 °C.
Reaction condition: catalyst (5 mol% in 2 ml CH2Cl2), substrate (1.15 mmol), pyridine N-oxide (0.23 mmol), UHP (1.5 eq. with respect to substrate) at 3 °C.
Determined on GC.
Chiral capillary column GTA-type.
Chiral HPLC OB column.
Chiral HPLC OD column.
g
h
i
Turn over frequency is calculated by the expression [product]/[catalyst] ꢃ time (sꢂ1).
2nd run with the recovered catalyst.
3rd run with the recovered catalyst.
4th run with the recovered catalyst.
j
nearly similar enantioselectivites (Table 1). Nevertheless, ee with
substrates CHR and chromenes having electron-withdrawing
groups (NO2-CHR and CN-CHR) (entries 11–14) were better than
Cy-CHR and electron-rich MeO-CHR (entries 5, 6, 9, 10). For com-
parison, epoxidation of CN-CHR was conducted under the above
optimized reaction condition with the monomeric Jacobsen Mn(III)
salen complex in the presence of PyNO, which gave the corre-
sponding epoxide in 60% yield with 92% ee in 9 h (entry 15, Ta-
ble 1), while complexes 1a and 1b gave >99% epoxide yield with
93–95% ee in 5–6 h (entries 13 and 14). Further, Katsuki Mn(III)
salen catalyst gave 75% conversion to 2,2-dimethylchromene oxide
with 99% ee in presence of 4-PPyNO as an axial base [2], while cat-
alysts 1a and 1b gave better conversion (>99%) to epoxide with
ꢀ93% ee. Additionally, catalysts 1a and 1b have the advantage of
recyclability over classical Jacobsen and Katsuki catalysts.
hexane, dried in vacuum, and used for the subsequent catalytic
run without further purification. The enantioselectivity of the
recovered complex did not change (entries 2–5, Table 2) during
four reuse experiments. However, a decrease in epoxide yield
was observed, due to minor physical loss of the catalyst during
its recovery process. The same reaction when conducted with NaO-
Cl, the recovered catalyst turned light brown in color from original
dark brown possibly due to its oxidative degradation hence failed
to further catalyze the epoxidation reaction. An indication of this
is apparent in the IR and UV–Vis data of recovered catalyst, which
is correlated for the degradation of imine and phenoxide moieties
in the catalyst (Figs. S3 and S4).
4. Conclusions
Enantioselective epoxidation of above-mentioned substrates
were also conducted with catalysts 1a and 1b using UHP as an oxi-
dant in the presence of PyNO as an axial base at 3 °C, and results
are presented in Table 2. Turn over frequency (TOF) values calcu-
lated for these reactions indicate that UHP took longer reaction
time than NaOCl to give high conversions to their respective epox-
ides (entries 1–17, Table 2). Nevertheless, styrene, indene and elec-
tron-rich chromene (MeO-CHR) reacted readily (entries 1–2, 6–7,
12–13, Table 2) to give respective epoxides in 41–93% ee. On the
other hand, Cy-CHR and CHR (entries 8–11) and electron-depleted
chromenes like CN-CR and NO2-CHR reacted sluggishly (22–24 h;
entries 14–17) but the enantioselectivity was reasonably high
(ee, 84–93%). In all catalytic runs, both the catalysts 1a and 1b, irre-
spective of the oxidant used (NaOCl and UHP) the configuration of
the dominant enantiomer of the product was the same as that of
the catalyst. These results indicate that transition species (via
LꢄMn = O, Lꢄ = chiral macrocyclic ligand) with both oxidants are
common during oxygen atom transfer to substrate [22].
In conclusion, we have demonstrated a simple synthesis meth-
od for the preparation of two new Mn(III) salen complexes 1a and
1b incorporated in macrocyclic framework having benzylic ether
bridge linked to 5,50 positions of the salicylidene moieties. These
complexes worked well as catalyst in the enantioselective epoxida-
tion of non-functionalized alkenes with NaOCl and UHP as oxidants
in the presence of inexpensive PyNO as an axial base at 0–3 °C. The
catalyst 1b could be recycled couple of times with the retention of
ee for the epoxidation of styrene with UHP as an oxidant, however,
with NaOCl as an oxidant, catalyst degraded after one cycle.
Acknowledgments
Authors thank CSIR-Network project and DST for financial sup-
port. N.C.M. thanks CSIR for the award of JRF. We thank Dr. Santosh
Agrawal for valuable discussions.
For reusability experiments, epoxidation of STR with 1b was
conducted under the optimized reaction condition using UHP as
an oxidant. After first catalytic run (entry 2, Table 2), the macrocy-
clic complex was precipitated from the reaction mixture by the
addition of hexane. The precipitated complex was washed with
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jcat.2010.10.002.

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