Reusable chiral macrocyclic Mn(III) salen complexes for enantioselective epoxidation of nonfunctionalized alkenes

Article abstract of DOI:10.1016/j.jcat.2011.10.011

A series of new chiral monomeric and dimeric macrocyclic Mn(III) salen complexes 1-4 with trigol linker were synthesized, characterized (by microanalysis, IR spectroscopy, UV-vis. spectroscopy, optical rotation, and mass spectrometry), and used as catalys

Full text of DOI:10.1016/j.jcat.2011.10.011

Journal of Catalysis 286 (2012) 41–50
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Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Reusable chiral macrocyclic Mn(III) salen complexes for enantioselective
epoxidation of nonfunctionalized alkenes
⇑
Rukhsana I. Kureshy , Tamal Roy, 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), Bhavnagar 364 012, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 July 2011
Revised 11 October 2011
Accepted 15 October 2011
Available online 21 November 2011
A series of new chiral monomeric and dimeric macrocyclic Mn(III) salen complexes 1–4 with trigol linker
were synthesized, characterized (by microanalysis, IR spectroscopy, UV–vis. spectroscopy, optical rota-
tion, and mass spectrometry), and used as catalysts in the enantioselective epoxidation of styrene, cis
b-methyl styrene, indene, and chromenes in the presence of several N-oxides as an axial base and NaOCl
as an oxidant at 0 °C. With the use of chiral dimeric macrocyclic catalyst 3 (2.5 mol%), enantio-pure epox-
ides were achieved in excellent yields (>99%) and enantioselectivities (ee up to 98% in selected cases). The
recycling was demonstrated with complex 4 (recyclable up to six cycles studied with retention of enanti-
oselectivity) in the asymmetric epoxidation of styrene. The kinetic investigation with complex 4 for the
epoxidation of styrene as the representative substrate showed the first-order dependence on the catalyst
and the oxidant but independent on the initial concentration of the substrate.
Keywords:
Enantioselective
Chiral Mn(III) macrocyclic salen complexes
Chiral epoxides
Nonfunctionalized alkenes
Kinetic investigation
Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction
recyclable [41–43] by manipulating the solubility of the catalyst
such that the catalysis is performed under the homogeneous con-
The design and development of chiral catalysts that can lead to
enantioselectivity in the epoxidation of nonfunctionalized alkenes
for the formation of enantio-pure epoxides constitutes a major
challenge in asymmetric synthesis [1–20]. In this context, consid-
erable success has been achieved in the asymmetric epoxidation
of nonfunctionalized alkenes catalyzed by various chiral catalysts
[13–17], noteworthy among them are chiral Mn(III) salen com-
plexes initially developed by Jacobsen and Katsuki [6,7,9–11,18–
20]. Meunier and co-workers reported multi-step synthesis of
macrocyclic version of the chiral Mn(III) salen complexes to cata-
lyze enantioselective epoxidation of cis-olefins [21,22] and also at-
tempted catalyst recycling with limited success. Moreover, the
creation of macrocycle was through polyether linker at 3,30 posi-
tions of salicylidine moiety that made the synthesis of macrocycle
complicated. Although excellent activity and enantioselectivity
was achieved with Mn(III) salen complexes, recyclability of the
expensive chiral catalysts remains an important and challenging
goal to cut down the high production cost of desired enantio-pure
epoxides. To address this issue, heterogeneous chiral catalysts
were developed by anchoring chiral Mn(III) salen complexes on
polymer supports [23,24], inorganic solid supports of varied poros-
ity [23,25–37], polysiloxane membranes [38], and in ionic liquids
[39,40]. Alternatively, homogeneous catalysts were also made
dition, but in a post-catalytic step, the catalyst is precipitated out
by the addition of a solvent in which the catalyst is insoluble. In
this context, our group has earlier reported the straightforward
synthesis of chiral macrocyclic salen ligands linked through 1,3-
phenylene-bis-(methylene)-bis-(oxy) moiety at 5,50 positions of
salen unit [44]. The Mn(III) complexes of these macrocyclic salen
ligands performed well in enantioselective epoxidation of non-
functionalized alkenes under biphasic reaction condition in the
presence of PyNO an axial base at 0 °C. Still, only 2 recycle runs
were possible with these catalysts. It would not be out of context
to mention here the high solubility of Jacobsen’s Mn(III) salen com-
plex (soluble even in hexanes) due to moderate molecular weight
and highly hydrophobic character. Keeping these facts in mind,
we have designed new monomeric and dimeric macrocyclic Mn(III)
salen complexes 1–4 of relatively high molecular weights incorpo-
rating a flexible and hydrophilic trigol linker through 5,50 positions
of salen. These catalysts were used for enantioselective epoxida-
tion of representative nonfunctionalized alkenes viz., styrene, cis
b-methyl styrene, indene, and chromenes using NaOCl as an oxi-
dant in the presence of PyNO as an axial base at 0 °C. Excellent
yield (up to 99%) of epoxides with high chiral induction (ee 98%)
was achieved in the case of 6-cyano-2,2-dimethylchromene. In
order to understand the mechanism of epoxidation reaction,
kinetic investigations were carried out using complex 4 as a
catalyst and styrene as a representative substrate with NaOCl as
an oxidant in the presence of PyNO at 0 °C. The kinetic profile
⇑
Corresponding author. Fax: +91 0278 2566970.
E-mail address: rukhsana93@yahoo.co.in (R.I. Kureshy).
0021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2011.10.011

42
R.I. Kureshy et al. / Journal of Catalysis 286 (2012) 41–50
obtained has shown first-order dependence on concentrations of
catalyst and oxidant and independent on initial concentration of
the substrate. Based on kinetic, catalytic, and experimental evi-
dence, a probable mechanism of the epoxidation reaction is
suggested.
(3 ꢁ 50 mL) and finally dried over anhydrous Na2SO4. After
removal of dichloromethane under reduced pressure, the chiral
ligands 10 and 20 purified by silica gel column chromatography
(100–200 mesh) in 20% (EtOAc: Hexane) resulted in yellowish solid
monomeric macrocyclic ligands, 10 and 20.
10: Yield 85%. m.p. 104 °C. 1H NMR (500 MHz, CDCl3): d 1.44 (18
H, s), 1.73–1.91 (8H, m), 3.23–3.25 (2H, m), 3.31–3.33 (4H, m),
3.51–3.62 (8H, m), 4.19 (2H, d, J = 11), 4.43 (2H, d, J = 11), 6.72
(2H, s), 7.27 (2H, s), 8.07 (2H, s), 11.78 (2H, br) ppm. 13C NMR
(125 MHz, CDCl3): 24.3, 29.4, 32.7, 34.8, 68.5, 69.2, 70.7, 72.3,
2. Experimental
2.1. Methods and materials
76.4, 118.3, 127.3, 129.8, 137.4, 160.0, 166.2 ppm. FT-IR (KBr):
3432, 2942, 2863, 2359, 1629, 1558, 1442, 1387, 1259, 1212,
m
Manganese acetate (SD Fine Chem. Ltd.), 1S,2S-(ꢀ)-1,2-diphen-
ylethane-1,2-diamine and 1S,2S-(+)-1,2-diaminocyclohexane (Sig-
ma Aldrich) were used as received. Indene and styrene (Fluka)
were passed through a bed of neutral alumina before use. All
chromenes were synthesized according to the reported procedures
[45,46]. All the solvents were purified prior to use [47]. 3-t-Bu-5-
chloromethyl-2-hydroxy benzaldehyde was synthesized by the re-
ported procedure [48]. Trigol bis-aldehyde c was prepared by our
earlier reported method [49].
1099, 970, 845, 768, 728, 668, 594 cm–1. ½a D27
ꢂ
¼ þ195ꢃ (c = 0.052,
CH2Cl2). Anal. Calcd. for C36H52N2O6: C, 71.02; H, 8.61; N, 4.60.
Found C, 71.0; H, 8.58; N, 4.58. TOF-MS (ESI+): m/z Calcd. for
[C36H52N2O6] 608.81, Found 610.2 [M+H].
20: Yield 90%. m.p. 98 °C. 1H NMR (200 MHz, CDCl3): d 1.46
(18H, s), 3.30–3.36 (4H, m), 3.57–3.68 (8H, m), 4.19 (2H, d,
J = 10), 4.47 (2H, d, J = 10), 4.56 (2H, s), 6.72 (2H, d, J = 1.8),
7.18–7.30 (12H, m), 8.24 (2H, s), 13.86 (2H, br) ppm. 13C NMR
(50 MHz, CDCl3): 29.5, 33.9, 69.0, 70.8, 72.5, 78.6, 118.3, 127.5,
Microanalysis of the intermediates, ligands, and catalysts was
carried out on a Perkin Elmer 2400 CHNS analyzer. 1H and 13C
NMR spectra were recorded on Bruker 200 MHz or 500 MHz spec-
trometer at ambient temperature using TMS as an internal stan-
dard. FTIR spectra were recorded on a Perkin Elmer Spectrum
GX spectrophotometer as KBr pellet. Electronic spectra of chiral
macrocyclic Mn(III) salen complexes were recorded in methanol
and dichloromethane on a Varian Cary 500 Scan UV–vis.–NIR
spectrophotometer. Optical rotations of chiral intermediates and
chiral complexes were recorded on an automatic polarimeter
(digipol 78, Rudolph) instrument. All the melting points reported
here were determined on a Mettler Toledo-FB62 and were uncor-
rected. High-resolution mass spectra were obtained with a LC–MS
(Q-TOFF) LC (Waters), MS (Micromass), MALDI-TOF, Model make
Ultra flex TOF/TOF, Burker Daltonics, Germany instruments. For
product purification, flash chromatography was performed 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 determined by gas
chromatography (GC) on Shimadzu GC 14B instrument with a
stainless-steel column (2 m long, 3 mm inner diameter, 4 mm out-
er diameter) packed with 5% SE30 (mesh size 60–80) and
equipped with an FID detector. Ultrapure nitrogen was used as
carrier gas (rate 30 mL/min). Injection port and detector tempera-
ture was kept at 200 °C. For the product analysis of styrene and
indene, the column temperature was programmed at 70–140 °C,
while for chromenes, it was kept at 140 °C (isothermal). Synthetic
standards of the products were used to determine the conversions
by comparing the peak height and area. The ee of styrene oxide
was determined on GC using a chiral capillary column (Chiraldex
GTA). For the chromenes and indene epoxides, the ees were
determined on HPLC (Shimadzu SCL-10AVP) by using a Chiralcel
column (OD and OB).
128.3, 128.4, 129.2, 137.4, 139.8, 160.1, 166.8 ppm. IR (KBr):
m
3452, 2929, 2865, 1626, 1553, 1440, 1263, 1096, 848, 726, 585,
464 cm–1
.
½
a 2D7
ꢂ
¼ ꢀ136ꢃ (c = 0.206, CHCl3). Anal. Calcd. for
C44H54N2O6 C, 74.76; H, 7.70; N, 3.96. Found C, 74.73; H, 7.68;
N, 3.93. TOF-MS (ESI+): m/z Calcd. for [C44H54N2O6] 706.91, Found
708.45 [M+H].
2.3. Synthesis of chiral dimeric macrocyclic salen ligands 30 and 40
Bis-aldehyde c (0.53 g,1.1 mmol) in dry THF (1.2 mL) was taken
in a single-necked 50 mL round-bottom flask to which the solution
of 1S,2S-(+)-1,2-diaminocyclohexane (0.14 g,1.2 mmol)/1S,2S-(ꢀ)-
1,2-diphenylethane-1,2-diamine (0.27 g, 1.2 mmol) in dry THF
(0.6 mL) was added slowly and the resultant solutions were stirred
at room temperature. After completion of the reaction (2 h)
checked on TLC, the solvent was removed completely under re-
duced pressure. The bright yellow solids were extracted with
dichloromethane (50 mL), and the organic layer was washed with
water (3 ꢁ 50 mL), brine (3 ꢁ 50 mL) and finally dried over anhy-
drous Na2SO4. After removal of dichloromethane under reduced
pressure, the chiral dimeric macrocyclic ligands 30 and 40 were
purified by silica gel chromatography (100–200 mesh) with a
EtOAc-to-Hexane of 3:2.
30: Yield 96%. m.p. 76 °C. 1H NMR (500 MHz, CDCl3): d 1.38
(36H, s),1.67–1.93 (16H, m), 3.32 (4H, m), 3.55 (8H, t, J = 5),
3.61 (16H, t, J = 7), 4.37 (8H, s), 6.97 (4H, s), 7.20 (4H, s), 8.26
(4H, s), 13.86 (4H, br) ppm. 13C NMR (125 MHz, CDCl3): 25.0,
31.0, 34.7, 70.7, 72.2, 74.0, 74.8, 79.0, 119.8, 128.7, 131.2, 138.8,
161.6, 167.0 ppm; FT-IR (KBr):
m 3424, 2934, 2863, 2361, 1628,
1537, 1446, 1384, 1317, 1239, 1098, 940, 868, 785, 671, 563,
420 cm–1
.
½
a 2D7
ꢂ
¼ þ171ꢃ (c = 0.052, CH2Cl2). Anal. Calcd. for
C72H104N4O12 C, 71.02; H, 8.61; N, 4.60. Found C, 71.05; H, 8.63;
2.2. Synthesis of chiral monomeric macrocyclic salen ligands 10 and 20
N, 4.62. MALDI-TOF: m/z Calcd. for [C72H104N4O12] 1217.62, Found
1218.19 [M+H].
In a single-necked 50 mL round-bottom flask bis-aldehyde c
(0.72 g, 1.5 mmol) was taken in dry MeOH (10 mL) and was stirred
40: Yield 93%. m.p. 95 °C. 1H NMR (200 MHz, CDCl3): d 1.40 (36H,
s), 3.53–3.63 (24H, m), 4.37 (8H, s), 4.71 (4H, s), 6.97 (4H, s), 7.19–
7.29 (24H, m), 8.32 (4H, s), 13.78 (4H, br) ppm. 13C NMR (50 MHz,
CDCl3): 29.3, 34.8, 69.1, 70.6, 73.1, 80.0, 118.1, 127.6, 128.1, 128.3,
at 0 °C, and
a solution of 1S,2S-(+)-1,2-diaminocyclohexane
(0.18 g,1.6 mmol)/1S, 2S-(ꢀ)-1,2-diphenylethane-1,2-diamine (0.34
g,1.6 mmol) in dry MeOH (5 mL) was added drop wise to the above
solution. After complete addition, the resulting solution was fur-
ther stirred at room temperature. After an interval of 12 h, solvent
was completely removed under reduced pressure, and the bright
yellow solid was extracted with dichloromethane (50 mL). The
organic layer was washed with water (3 ꢁ 50 mL) and with brine
129.8, 137.3, 139.5, 159.9, 166.7 ppm. FT-IR (KBr):
m 3452, 2952,
2865, 2361, 1626, 1446, 1386, 1357, 1320, 1266, 1208, 1100,
1035, 936, 871, 801, 775, 573 cm–1. ½a D27
ꢂ
¼ ꢀ305ꢃ (c = 0.108,
CHCl3). Anal. Calcd. for C88H108N4O12 C, 74.76; H, 7.70; N, 3.96.
Found C, 74.75; H, 7.73; N, 3.98. MALDI-TOF: m/z Calcd. for
[C88H108N4O12] 1413.82, Found 1414.19 [M+H].

R.I. Kureshy et al. / Journal of Catalysis 286 (2012) 41–50
43
2.4. Synthesis of chiral monomeric and dimeric macrocyclic Mn(III)
salen complexes (1–4)
used as such for further catalytic runs. Epoxides were purified by
flash chromatography through a bed of neutral alumina using ethyl
acetate and hexane (9:1) as eluent, and their enantiomeric excess
(ee) was determined.
The chiral monomeric/dimeric macrocyclic salen ligands 10/20
(1 mmol) or 30/40 (0.5 mmol) were dissolved in 30 mL methanol
to which Mn(CH3COO)2ꢄ4H2O (294 mg; 1.2 mmol) was added in
an inert atmosphere, and the reaction mixture was refluxed for
6 h. After completion of the reaction (checked on TLC), the reaction
mixture was cooled to room temperature, lithium chloride
(127 mg; 3 mmol) was added, and the mixture was further stirred
for 3 h exposed to air and 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 Na2SO4. After complete removal of the solvent, the de-
sired complexes 1–4 were obtained as brown solid.
2.6. Recycling of the catalyst 2 and 4
At the end of the catalytic run (checked on GC), the organic layer
was separated and the aqueous layer extracted with CH2Cl2
(3 ꢁ 50 mL). The combined organic layer was washed with water
(3 ꢁ 50 mL) and brine (3 ꢁ 50 mL) and dried over anhydrous
Na2SO4. After partial removal of CH2Cl2, the catalysts 2 and 4 were
precipitated out from the solution by the addition of 20 mL hexane
(Table 1, entry 17), dried and kept in a desiccator for further use.
1: Yield 0.66 g, 95%. m.p. > 200 °C. IR (KBr): cmꢀ1 3434(br),
2928(s), 2859(s), 1616 (s), 1540 (s), 1435 (sh), 1386 (s), 1340
(w), 1304 (w), 1263 (w), 1234 (w), 1203 (m), 1162 (m), 1071
(w), 1024 (w), 826 (w), 785 (w), 663 (w), 563 (w), 483 (w), 420
2.7. Kinetic study
The catalyst 4 (0.20 ꢁ 10ꢀ2–1.0 ꢁ 10ꢀ2 M) in 1.5 mL CH2Cl2 was
stirred with PyNO (2 ꢁ 10ꢀ2 M) and styrene (10 ꢁ 10ꢀ2
–
(w). UV–vis. (MeOH), kmax (nm) 402, 315, 256, 230. ½a D27
ꢂ
¼ 880ꢃ
25 ꢁ 10ꢀ2 M) at 0 °C; the resulting solution was treated with NaOCl
(40 ꢁ 10ꢀ2–90 ꢁ 10ꢀ2 M) and stirred constantly. To determine the
rates of epoxidation, aliquots were drawn from the reaction mix-
ture on a regular interval, quenched with triphenylphosphine, and
analyzed on GC.
(c = 0.011, CHCl3). Anal. Calcd. for C36H50MnN2O6Cl: C, 62.02, H,
7.23, N, 4.02, Found: C 62.04, H 7.26, N, 4.05. LC–MS: m/z Calcd.
for [C36H50ClMnN2O6] 696.27, Found 661.38 [MꢀCl].
2: Yield 0.73 g, 92%. m.p. > 200 °C. IR (KBr): cmꢀ1 3434 (br),
2931 (s), 2860 (s), 1615 (s), 1540 (s), 1439 (sh), 1386 (s), 1340
(w), 1304 (w), 1265 (w), 1237 (w), 1201 (m), 1165 (m), 1069
(w), 1026 (w), 826 (w), 783 (w), 667 (w), 570 (w), 483 (w), 420
3. Results and discussion
(w). UV–vis. (MeOH), kmax (nm) 402, 291, 255, 228. ½a D27
ꢂ
¼ 630ꢃ
(c = 0.011, CHCl3). Anal. Calcd. for C44H52MnN2O6Cl: C, 66.45, H,
6.59, N, 3.52, Found C 66.42, H 6.56, N, 3.49. LC–MS: m/z Calcd.
for [C44H52ClMnN2O6] 794.29, Found: 759.60 [MꢀCl].
Chiral monomeric and dimeric macrocyclic Mn(III) salen com-
plexes 1–4 were prepared as shown in Scheme 1. Thus, in a step-
wise manner, 3-t-butyl salicylaldehyde was chloromethylated to
give a [48], which on reaction with trigol b gave trigol bis-aldehyde
c in the presence of NaH [49]. The compound c on condensation
with equimolar quantities of diamines (1S,2S-(+)-1,2-diaminocy-
clohexane/1S,2S-(ꢀ)-1,2-diphenylethane-1,2-diamine) gave the li-
gands 10–40 in good yields with sufficient purity. It is worth
mentioning that when condensation reaction was carried out in
MeOH exclusively monomeric ligands 10 and 20, while in THF en-
tirely dimeric macrocyclic ligands 30 and 40 were formed. Complex-
ation of the ligands 10–40 was done with Mn(II) acetate under
refluxing condition in an inert atmosphere in MeOH. The addition
of LiCl under aerobic reaction condition at RT to the above reaction
mixture provided monomeric and dimeric macrocyclic Mn(III)
salen complexes 1–4 in excellent yields.
Styrene (0.625 mmol) was used as a test substrate to evaluate
the efficacy of catalysts 1–4 (5–2.5 mol%) in the asymmetric epox-
idation reaction using PyNO as an axial base and NaOCl as an oxi-
dant at 0 °C. It can be seen from entries 1–4 of Table 1 that dimeric
complexes 3 and 4 are more reactive and enantioselective than
complexes 1 and 2 at similar metal loading. This increase in reac-
tivity and enantioselectivity can be attributed to the increase in ac-
tive catalytic sites, which may be working in cooperation. This
phenomenon of cooperation has been reported earlier by us and
other groups for epoxidation [41–43,50] and other catalytic reac-
tions [49,51]. However, among these the dimeric macrocyclic salen
complex 4 performed better (conversion, >99% in 4 h; ee, 70%) than
rest of the complexes 1–3. Therefore, next we used this catalyst to
optimize the reaction parameters such as catalyst loading,
temperature, and solvent variations for asymmetric epoxidation
of styrene as representative substrate with NaOCl as an oxidant.
A decrease in catalyst loadings (entries 5 and 6) from the test-
loading (entry 4) under similar reaction condition resulted in
lowering of reaction rates and enantioselectivity. Whereas, an in-
crease in the catalyst loading (5 mol%, entry 7) led to no observable
improvement in the catalyst performance except for a decrease in
reaction time. Hence, 2.5 mol% catalyst loading was taken as
3: Yield 1.3 g, 93%. m.p. > 200 °C. IR (KBr): cmꢀ1 3438 (br), 2935
(s), 2862 (s), 1614 (s), 1541 (s), 1437 (sh), 1387 (s), 1341 (w), 1306
(w), 1265 (w), 1238 (w), 1201 (m), 1162 (m), 1090 (w), 1026 (w),
824 (w), 781 (w), 663 (w), 564 (w), 484 (w), 421 (w). UV–vis.
(MeOH), kmax (nm) 405, 297, 259, 230. ½a D27
ꢂ
¼ 1356ꢃ (c = 0.01,
CHCl3). Anal. Calcd. for C72H100Mn2N4O12Cl2: C, 62.02, H, 7.23, N,
4.02, Found C 62.00, H 7.19, N, 4.00.LC–MS: m/z Calcd. for
[C72H100Cl2Mn2N4O12] 1392.55, Found: 1357.96 [MꢀCl].
4: Yield 1.48 g, 93%. m.p. > 200 °C. IR (KBr): cmꢀ1 3438 (br),
2919 (s), 2865 (s), 1613 (s), 1541 (s), 1456 (sh), 1387 (s), 1342
(w), 1306 (w), 1265 (w), 1235 (w), 1202 (m), 1163 (m), 1090
(w), 1023 (w), 824 (w), 775 (w), 671 (w), 572 (w), 484 (w), 421
(w). UV–vis. (MeOH), kmax (nm) 418, 341, 259, 230. ½a D27
ꢂ
¼ 749ꢃ
(c = 0.075, CHCl3). Anal. Calcd. for C88H104Mn2N4O12Cl2: C66.45,
H, 6.59, N, 3.52, Found: C 66.4 3, H 6.52, N, 3.5 4. LC–MS: m/z Calcd.
for [C88H104Cl2Mn2N4O12] 1588.58, Found 1553.61 [MꢀCl].
2.5. General procedure for enantioselective epoxidation of
nonfunctionalized alkenes using NaOCl as an oxidant
Enantioselective epoxidation reactions of different alkenes viz.
styrene (STR), cis b-methyl styrene, indene (IND), 2,2-dimethyl-
chromene (CHR), 6-cyano-2,2-dimethylchromene (CN-CHR),
6-nitro-2,2-dimethylchromene (NO2-CHR), 6-methoxy-2,2-dim-
ethylchromene (MeO-CHR), and spiro[cyclohexane-1,2-[2H][1]
chromene] (Cy-CHR) (0.625 mmol) were performed using com-
plexes 1–4 (1–5 mol%) as catalyst in dichloromethane (1 mL) in
the presence of PyNO (6 mg, 0.063 mmol) as an axial base and buf-
fered (pH 11.3) NaOCl (1.5 mmol added in 4 equal parts) as an oxi-
dant at 0 °C. The progress of the reaction was monitored by GC
analysis of the products using n-tri-decane (0.1 mmol) as the GLC
internal standard. After completion of the reaction, the product
chiral epoxide of the respective alkene was extracted with CH2Cl2,
washed with water, and dried over Na2SO4. The catalyst was
separated from the product by precipitation with hexane and was

44
R.I. Kureshy et al. / Journal of Catalysis 286 (2012) 41–50
Table 1
Screening of the catalysts 1–4a for enantioselective epoxidation of styrene and optimization of the reaction conditions.
Entry
Catalyst
Catalyst loading
Axial base
Solvent
Time (h)
Yield (%)b
ee (%)c
TOF ꢁ 10ꢀ3 sꢀ1e
1
2
3
4
5
6
7
1
2
3
4
4
4
4
4
4
4
4
4
4
4
4
4
4
5
5
PyNO
PyNO
PyNO
PyNO
PyNO
PyNO
PyNO
PyNO
PyNO
PyNO
PyNO
PyNO
PPyNO
PPPyNO
NMO
–
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
6
7
3
4
4
4
3
3
1
3.5
8
6
3
>99
>99
>99
>99
72
54d
62d
62d
70d
58d
63d
70d
52d
28d
65d
52d
54d
70d
70d
55d
32d
70d
0.93
0.79
3.70
2.78
5
3.70
1.85
3.70
11.11
3.17
1.18
1.85
3.70
3.70
4.27
0.29
2.22
2.5
2.5
1
1.5
5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
80
>99
>99
>99
>99
85
>99
>99
>99
96
8¥
9–
10
11
12
13
14
15
16
17f
Toluene + CHCl3
MeOH
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
3
2.5
12
5
31
>99
PyNO
a
Reaction condition: catalyst (1–5 mol% in 1 ml CH2Cl2), substrate (0.625 mmol), pyridine-N-oxide (0.063 mmol), NaOCl (1.5 mmol), reaction temp. 0 °C (¥ reaction temp.
10 °C, – reaction temp. RT).
b
Determined on GC.
Chiral capillary column GTA-type.
c
d
Epoxide configuration S.
e
Turnover frequency is calculated by the expression [product]/[catalyst] ꢁ time (sꢀ1).
f
Reaction condition: catalyst 4 (2.5 mol% in 20 ml CH2Cl2), substrate (10 mmol), pyridine-N-oxide (1 mmol), NaOCl (25 mmol), reaction temp. 0 °C.
optimum for varying reaction temperature from 0 °C to RT. As ex-
pected, there was a marked drop in the product ee on increasing
the reaction temperature (entries 8 and 9). Having optimized cat-
alyst loading (2.5 mol%) and reaction temperature (0 °C), variation
in solvent was attempted as its effect on the reactivity and enanti-
oselectivity of epoxidation reaction is well documented [23,52].
Accordingly, the epoxidation reaction was conducted in different
solvents viz., CHCl3, toluene + CHCl3, and MeOH (entries 10–12).
However, best results were obtained with the use of CH2Cl2 as
solvent, which was used in our subsequent studies as preferred
solvent (Fig. 1).
It is well-known in literature that the addition of pyridine-N-
oxide derivatives in Mn(III) salen-based catalysts, employing NaO-
Cl as an oxidant, improves both catalyst turnover and enantioselec-
tivity [41]. The rationale behind this improvement is based on dual
role of N-oxide, (a) N-oxides help in stabilizing the reactive
MnV = O species and prevents the formation of Mn–O–Mn species,
which is catalytically inactive [53] and (b) hydrophobic N-oxides
viz., 4-phenylpyridine-N-oxide (4-PPyNO), and 4-(3-phenylpropyl)
pyridine-N-oxide (4-PPPyNO) help in transporting HOCl from
aqueous phase to the organic phase [54] where the catalyst and
substrate actually reside. This led us to study the above mentioned
axial bases and hydrophilic pyridine-N-oxide (PyNO) and N-methyl
morpholine-N-oxide (NMO) for the epoxidation of styrene in
dichloromethane with catalyst 4 (Table 1, entries 13–15) and NaO-
Cl as an oxidant at 0 °C. Both 4-PPyNO and 4-PPPyNO worked well
(reaction took 3 h to give the similar results); however, the results
showed that there is no apparent advantage over the simple and
inexpensive PyNO (Fig. 1). The reason behind this finding may be
attributed to the introduction of trigol linkage in these macrocyclic
Mn(III) complexes, which makes them relatively more hydrophilic
and thus residing at the interface of organic and aqueous phase. In
the event of this, the requirement of HOCl transportation to the or-
ganic phase is minimized, consequently, even hydrophilic PyNO is
good enough for this catalytic system. This finding is in consonance
with the earlier reported Mn(III) salen complexes [41,42]. Never-
theless, the use of PyNO in the epoxidation reaction is essential
as its absence gave the product styrene oxide in 31% yield and
32% ee in 12 h (Table 1, entry 16).
The above optimization study was carried out by using styrene
as a model substrate with catalyst 4 having 1,2-diphenylethane-
1,2-diamine collar, which is known to give better ee in the product
styrene oxide as compared to the catalysts with 1,2-diaminocyclo-
hexane collar. However, for the substrates like chromenes [48] and
indene [42], the salen catalysts with 1,2-diaminocyclohexane col-
lar often work better. Due to this reason, we used the above opti-
mized reaction condition in the epoxidation of chromenes and
indene with all the catalysts 1–4 (Table 2). In all the cases, nearly
quantitative yield of epoxides was obtained in 5.5–15 h, where
the reactions were significantly faster and more enantioselective
for dimeric complexes 3 and 4 as compared to their monomeric
counterparts 1 and 2. However, difference in the enantioselectivi-
ties obtained with the catalysts having 1,2-diaminocyclohexane
and 1,2-diphenylethane-1,2-diamine collar remained within 1–
4%. This shows that there is no significant role of the diamine collar
in imparting enantio-induction in the product for the present cat-
alyst design (Fig. 2). In terms of product configuration, the trend
obtained with catalyst 1–4 followed the similar trend obtained
for Jacobsen’s catalyst. However, dimeric catalysts 3 and 4 were
significantly better (entries 19 and 20; yield, >99%; ee, 96–98%)
in terms of reactivity and enantioselectivity than Jacobsen’s
catalyst (entry 21; yield, 62%; ee, 92%) as exemplified with the
cyano-chromene epoxidation reaction under similar metal loading
and other reaction conditions.
To better demonstrate the applicability of this protocol at high-
er scale (ꢅ15 times than catalyst screening scale), styrene
(10 mmol) epoxidation was conducted with catalyst 4 under the
optimized reaction conditions (Table 1, entry 17). The reaction at
this scale gave similar product yield and ee in 5 h as compared to
4 h taken at 0.625 mmol scale of styrene (Table 1, entry 4).

R.I. Kureshy et al. / Journal of Catalysis 286 (2012) 41–50
45
Scheme 1. Schematic representation for synthesis of catalyst 1–4.
Catalyst recyclability experiments were conducted on
a
protocol was also applied for the monomeric catalyst 2. However,
the recovery of the monomeric catalyst after the first cycle was
only ꢅ80% possibly due to its relatively higher solubility in n-
hexane. Besides, after 3rd catalytic cycle, there was a change in
catalyst color (from reddish brown to light brown) with marked
decrease in product yield and ee, suggesting the relatively poor
stability of monomeric complex under the epoxidation reaction
condition than its dimeric counterpart.
In order to understand the mechanism of the epoxidation reac-
tion, the kinetics of epoxidation of styrene as a representative
substrate was investigated using catalyst 4 in the presence of PyNO
as an axial base with NaOCl, as a function of the concentrations of
catalysts, oxidant, and styrene at 0 °C. In all the kinetic runs, the
plots for formation of styrene epoxide with time were found to
be linear in the initial stage of the reaction, which attained
10 mmol scale of styrene with catalyst 4 (2.5 mol%) with NaOCl
as an oxidant in the presence of PyNO as an axial base in dichloro-
methane at 0 °C. After the catalytic run, the solvent was reduced to
ca. one-fourth and the complex 4 was precipitated out by the
addition of excess of n-hexane. The precipitated catalyst was
washed thoroughly with hexane, dried in vacuum, stored under
dry and inert atmosphere, and used as such for the subsequent
catalytic run without further purification. The recovered catalyst
worked well for the epoxidation of styrene in the same manner
as that of the fresh catalyst. The recycling data as given in Table 3
suggest that the recovered catalyst 4 is stable and worked well for
six cycles without significant loss in performance. However, there
was some physical loss (ꢅ1–2% per recycle) of catalyst, and the
amount of styrene was adjusted accordingly. This recycling

46
R.I. Kureshy et al. / Journal of Catalysis 286 (2012) 41–50
Fig. 1. Optimization of reaction conditions for enantioselective epoxidation of styrene with complex 4.
Table 2
Product yield and ee data for the epoxidation of nonfunctionalized alkenes catalyzed by complexes 1–4.a
Entry
Catalyst
Substrate
INDf
% Yieldb
Time (h)
ee (%)
TOF ꢁ 10ꢀ3 sꢀ1e
1(2)
3(4)
5(6)
7(8)
9(10)
11(12)
13(14)
15(16)
17(18)
19(20)
21
1(2)⁄
3(4)–
1(2)⁄
3(4)–
1(2)⁄
3(4)–
1(2)⁄
3(4)–
1(2)⁄
3(4)–
Jacobsen⁄h
4#
>99(>99)
>99(>99)
>99(>99)
>99(>99)
>99(>99)
>99(>99)
>99(>99)
>99(>99)
>99(>99)
>99(>99)
62
10(11)
7(7.5)
14(14)
10(10)
15(15)
10(10)
15(15)
10(10)
10(10)
5.5(6)
10
84(82)c
89(92)c
92(88)d
94(90)d
85(87)d
93(91)d
85(84)d
93(95)d
93(89)d
98(96)d
92d
0.56(0.51)
1.59(1.48)
0.40(0.40)
1.11(1.11)
0.37(0.37)
1.11(1.11)
0.37(0.37)
1.11(1.11)
0.56(0.56)
2.02(1.85)
0.34
CHRg
Cy-CHRg
MeO-CHRg
CN-CHRg
CN-CHRg
22
cis-b-Me styj
>99i
8
93b
1.39
a
b
c
d
e
f
Reaction condition: ⁄ catalyst loading (5 mol%), – catalyst loading (2.5 mol%) in 1 ml CH2Cl2, substrate (0.625 mmol), pyridine-N-oxide (0.063 mmol), NaOCl (1.5 mmol).
Determined on GC.
Chiral HPLC OB column.
Chiral HPLC OD column.
Turnover frequency is calculated by the expression [product]/[catalyst] ꢁ time (sꢀ1).
Product configuration: 1S,2R.
g
h
i
Product configuration: 3S,4S.
With (1S,2S)-1,2-diaminocyclohexane collar.
Ratio of (E/Z) epoxide 17:83.
j
cis-b-Me sty = cis-b-methyl styrene.
saturation near completion giving >99% styrene epoxide (Fig. 3).
Based on these results, the initial rate constants kobs (up to the lin-
ear portion of the graph) were determined by the amount of epox-
ide formed with respect to time.
3.1. Dependence of the reaction rate on catalyst concentration
The epoxidation of styrene was studied by conducting the
epoxidation reaction at different concentrations (over a range of

R.I. Kureshy et al. / Journal of Catalysis 286 (2012) 41–50
47
Fig. 2. Yield [%] and ee [%] of chiral epoxides with the catalysts 1–4.
Table 3
Recyclability data of catalyst 4 in the enantioselective synthesis of styrene oxide using
NaOCl as an oxidant and PyNO as an axial base.a
Run
Time (h)
Yield (%)
ee (%)
1
2
3
4
5
6
5
5
5
5
5
5
>99
>99
98
98
96
70
70
70
70
70
70
95
a
Reaction condition: catalyst 4 (2.5 mol% in 20 ml CH2Cl2), substrate (10 mmol),
pyridine-N-oxide (1 mmol), NaOCl (25 mmol), reaction temp. 0 °C.
Fig. 4. Plot of catalyst concentration versus kobs at 0 °C, [styrene] = 0.2 M,
[oxidant] = 0.8 M.
Table 4
Dependence of the catalyst concentration for the epox-
idation of styrene at 0 °C, [styrene] = 0.2 M, [oxi-
dant] = 0.8 M, [PyNO] = 0.02 M.
[Catalyst] ꢁ 104
M
kobs ꢁ 104 M minꢀ1
20
30
50
20.00
30.67
50.67
96.00
100
Fig. 3. Time-dependence plot of the formation of epoxide at 0 °C,
[catalyst] = 0.005 M, [oxidant] = 0.8 M, and [styrene] = 0.2 M.
condition. On the other hand, the plot of the log kobs versus log[cat-
alyst] were found to be linear with unit slopes (d log kobs/d log[cat-
alyst]) ꢅ1) that clearly indicated the first-order dependence of the
rate of the epoxidation reaction with respect to the concentration
of the catalyst 4. The kinetic data of the catalyst dependence for
the epoxidation of styrene are given in Table 4.
0.002–0.01 M) of catalyst 4, keeping the concentrations of oxidant
and styrene constant. A linear increase in the styrene epoxide for-
mation was observed with an increase in the initial concentration
of the catalyst 4. The plot of the rate (kobs) of the styrene epoxide
formation versus the concentration of the catalyst 4 passed
through the origin (Fig. 4), indicating the absolute necessity of
the catalyst for the reaction to proceed under the experimental
3.2. Dependence of the reaction rate on the concentration of oxidant
The effect of concentration of the oxidant on the rate of epoxi-
dation of styrene was evaluated by varying oxidant concentration

48
R.I. Kureshy et al. / Journal of Catalysis 286 (2012) 41–50
Fig. 5. Plot of oxidant concentration versus kobs at 0 °C, [styrene] = 0.2 M,
[catalyst] = 0.005 M.
Fig. 6. Plot of substrate concentration versus kobs at 0 °C, [catalyst] = 0.005 M,
[oxidant] = 0.8 M.
Table 5
3.3. Dependence of the reaction rate on the concentration of alkene
Dependence of the oxidant concentration for the epox-
idation of styrene at 0 °C, [styrene] = 0.2 M, [cata-
lyst] = 0.005 M, [PyNO] = 0.02 M.
Kinetic experiments were conducted at different initial concen-
trations of styrene (Table 6) ranging from 0.1 M to 0.25 M, by keep-
ing the other reaction parameters constant indicated a zero-order
dependence of rate of the reaction with initial concentration of sty-
rene (Fig. 6). The tendency of following zero-order kinetics during
catalytic epoxidation of alkenes in terms of higher alkene concen-
trations has been reported earlier in Mn(III) salen-catalyzed epox-
idation of nonfunctionalized alkenes using NaOCl [55], HOCl [56],
and UHP as oxidants [57]. Possibly, at higher concentrations of
the alkenes, the major contribution of the epoxidation reaction is
toward the formation of epoxide from the alkene and contribution
toward the side reaction is negligible. It is only at lower concentra-
tions of the alkenes, the side reaction becomes significant. Under
our experimental conditions, styrene epoxide was the only prod-
uct, further supporting the zero-order dependence in styrene con-
centration at reasonably high concentrations.
On the basis of kinetics and experimental results, a probable
mechanism for the epoxidation of alkene is proposed (Scheme 2)
where the catalyst first gets oxidized by the oxidant, NaOCl to form
an oxo complex LMnV = O (intermediate a) at the rate-determining
step. According to Scheme 2, the epoxidation reaction may proceed
via concerted oxygen atom attack (intermediate b, path A) or
oxametalla-oxetane formation (intermediate c, Path B) to give
selectively epoxide and the catalyst LMnIII back in its original form.
The radical pathway C via an intermediate d has been proposed to
be absent or insignificant (for the test substrate styrene) as there
was no aldehyde formation [58] under our epoxidation reaction
conditions. To further verify this statement, cis-b-methyl styrene
[Oxidant] M
kobs ꢁ 104 M minꢀ1
24.00
37.33
50.67
57.33
0.4
0.6
0.8
0.9
Table 6
Dependence of the kobs on the concentration of styrene
at
0 °C,
[catalyst] = 0.005 M,
[oxidant] = 0.8 M,
[PyNO] = 0.02 M.
[Styrene] M
kobs ꢁ 104 M minꢀ1
50.67
49.33
50.67
50.67
0.10
0.15
0.20
0.25
over a range of 0.4–0.9 M, whereas the substrate and the catalyst
concentration was kept fixed at 0.2 M and 0.005 M, respectively.
Under the employed reaction concentration rate of the epoxidation
increased linearly with an increase in initial concentration of oxi-
dant (Fig. 5). The plot of the rate constant versus oxidant concen-
tration further confirmed the first-order dependence of rate
constant with oxidant concentration (d log kobs/d log [oxidant]
ꢅ1) (Table 5).
Scheme 2. Probable mechanisms for the chiral macrocyclic Mn(III) salen-catalyzed epoxidation: (A) concerted reaction; (B) reaction via metalla-oxetane intermediate;
(C) reaction via a radical intermediate.

R.I. Kureshy et al. / Journal of Catalysis 286 (2012) 41–50
49
1.0
0.8
0.6
0.4
0.2
0.0
Appendix A. Supplementary material
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.jcat.2011.10.011.
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In conclusion, we have designed chiral monomeric and dimeric
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positions of the salen unit. These complexes worked very well as
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alkenes with NaOCl as oxidants in the presence of inexpensive
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Authors are thankful to the CSIR Network project for supporting
this work. Tamal Roy is thankful to CSIR for the award of JRF. We
are thankful to one of the referee for suggesting the use of cis-b-
methyl styrene as a substrate for the epoxidation in order to check
the stereospecificity of the reaction.

50
R.I. Kureshy et al. / Journal of Catalysis 286 (2012) 41–50
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